Coalbed Methane
Operations
Coalbed Methane
Operations
v
v
v
Copyright © 1992 by Gas Research Institute All Rights Reserved
By Vicki A. Hollub
Taurus Exploration, Inc. (Birmingham, Alabama)
Paul S. Schafer
About the Authors
LEGAL NOTICE: This publication was prepared as an account of work sponsored by Gas Research Institute (GRI) and other organizations. Neither GRI, members of GRI, nor any person acting on behalf of either:
a. makes any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this publication, nor that the use of any information, apparatus, method, or process disclosed in this publication may not infringe privately owned rights; or
b. assumes any liability with respect to the use of, or for damages resulting from the use of, any information, apparatus, method, or process disclosed in this publication. Reference to trade names or specific commercial products, commodities, or services in this publication does not represent or constitute an endorsement, recommendation, or favoring by GRI of the specific commercial product, commodity, or service.
Disclaimer
Vicki A. Hollu
b, P.E. works with Taurus Exploration, Inc. as a reservoir engineer at the GRI RockCreek research project in Alabama. She previously worked ten years with OXY USA as a drilling engineer and as a senior production engineer. Vicki holds a B.S. in Mineral Engineering from The University of Alabama and is a registered professional engineer. She is a member of the Society of Petroleum Engineers (SPE) and currently serves as chairperson of the SPE Professional Engineering Registration Committee.
Paul S. Schafer
owns and operates Schafer Associates, a consultancy that provides technical communication services to the petroleum and petrochemical industries. He previously worked ten years with Marathon Oil Company as a production and operations engineer and as an advanced reservoir engineer. Paul holds a Master of Technical and Scientific Communication from Miami University at Oxford, Ohio and a B. S. in Petroleum Engineering from Marietta College. He is a member of the Society of Petroleum Engineers and the Society for Technical Communication.About This Guide
A
Guide to Coalbed Methane Operations provides practicalinformation on siting, drilling, completing, and producing coalbed methane wells. Whether you’re an experienced coalbed methane producer or you’re exploring coalbed methane operations for the first time, this guide will give you the information you need to make informed decisions about producing this resource.
This guide is a “working reference.” It will help you in planning and performing field activities. Each chapter provides an overview of key field operations as well as specific guidelines for performing them. The chapters also describe the equipment and materials required for each operation. Though the guide focuses on developing mul-tiple coal seams in the Black Warrior Basin, you can apply many of the concepts to other coal basins as well.
You will notice an emphasis on practical applications rather than lengthy technical explanations and engineering data. However, if you want to investigate any of the topics in greater depth, the Additional Resources section at the end of each chapter will guide you to selected references. The information in this guide represents the shared knowl-edge and expertise of many specialists in the coalbed methane field. Much of this information resulted from GRI’s Rock Creek Methane from Multiple Coal Seams
Completion Project and from several operators and service company representatives in the Black Warrior Basin of Alabama. We hope this guide contributes to greater un-derstanding of coalbed methane production and more economical development of this gas resource.
About this Guide i
List of Figures and Tables iv
Conventions Used in This Guide vii Acknowledgments viii
About Producing Coalbed Methane x
Chapter I Selecting and Preparing a Field Site 1-1
Protecting Wetland Areas 1-2 Disposing Produced Water 1-3
Controlling Non-Point Source (NPS) Pollution 1-4
Preventing Spills 1-13
Safety and Operating Guidelines 1-14
Chapter 2 Drilling and Casing the Wellbore 2-1
Planning the Drilling Program 2-2
Drilling the Wellbore 2-32
Coring the Wellbore 2-36
Casing and Cementing the Wellbore 2-4
Chapter 3 Wireline Logging 3-1
Sources for Estimating Reservoir Properties 3-2
Open Hole Logging Tools 3-4
Selecting an Open Hole Logging Suite 3-35
Guidelines for Open Hole Logging 3-36
Cased Hole Logging Tools 3-37
Selecting a Cased Hole Logging Suite 3-41
Guidelines for Cased Hole Logging 3-42
Production Logging Tools 3-44
Chapter 4 Completing the Well 4-1
Reservoir Considerations in Completing Coalbed Methane Wells 4-2
Objectives of Completing the Well 4-2
Completing in Open Hole 4-4
Completing in Cased Hole 4-8
Accessing the Formation 4-10
Selecting Production Tubing 4-27
Performing a Fracture Treatment 5-35 Evaluating a Fracture Treatment 5-48
Chapter 6 Selecting Production Equipment and Facilities 6-1
Estimating the Volume of Water to be Produced 6-2 Pumping Equipment 6-3
Power Supply for Pumping Equipment 6-19 Surface Production Facilities 6-23
Gas Compressors 6-35
Gas Dehydration Equipment 6-40
Chapter 7 Operating Wells and Production Equipment 7-1
Preparing Surface Facilities for Production 7-2 Unloading the Well 7-3
Bringing the Well on Line 7-8
Troubleshooting Well and Equipment Problems 7-8
Chapter 8 Treating and Disposing Produced Water 8-1
Characteristics of Coalbed Methane Produced Water 8-2 Regulations and Permitting for Water Disposal 8-6
Considerations for Designing a Water Disposal System 8-8 Methods for Treating and Disposing Produced Water 8-10
Chapter 9 Testing the Well 9-1
Performing Pressure Transient Tests 9-2
Evaluating Production from Multiple-Seam Wells 9-21
Appendix A Summary of Permitting Requirements for Drilling a Coalbed Methane Well in Alabama
Appendix B Quality Control and Job Supervision Guidelines for Stimulation Treatments
Appendix C Procedures and Surface Equipment for Implementing the Forced Closure Fracturing Technique
Chapter 4 Completing the Well
Figure 4-1 Single-Zone Open Hole Completion 4-5 Figure 4-2 Multiple-Zone Open Hole Completion 4-8 Figure 4-3 Multiple-Zone Cased Hole Completion 4-9
Chapter 2 Drilling and Casing the Wellbore
Figure 2-1 The Planning Process for Drilling a Coalbed Methane Well 2-2 Figure 2-2 Setting Casing Through Zones with Lower Fracture Gradients 2-5 Figure 2-3 Selecting Hole Size 2-7
Figure 2-4 Casing Selection Chart 2-11
Figure 2-5 Conventional Rotary and Rotary-Percussion Drilling Techniques 2-16 Figure 2-6 Typical Cementing Manifold 2-50
Figure 2-7 Two Stage Cementing 2-52
Figure 3-1 Bulk Density Log 3-6
Figure 3-2 Comparison of Conventional and Mineral Logging Density Logs 3-9 Figure 3-3 Dual Induction/Shallow Log 3-13
Figure 3-4 Phasor Induction Log 3-14 Figure 3-5 SP Log 3-20
Figure 3-6 Compensated Neutron Log 3-21
Figure 3-7 Comparison of Cleat Orientation from Microscanner® Log & Cores 3-23
Figure 3-8 Sonic Log 3-25
Figure 3-9 Full Waveform Sonic Log 3-27
Figure 3-10 Geochemical and Carbon/Oxygen Log 3-29 Figure 3-11 VOLAN® Log 3-30
Figure 3-12 Spectral Gamma Ray Log 3-32
Figure 3-13 Computer-Processed Coal Quality Log 3-34 Figure 3-14 Cement Bond/Variable Density Log 3-40
Figure 3-15 Wellhead Configuration for Annular Logging 3-44 Figure 3-16 Flowmeter Developed for Coalbed Methane Wells 3-46 Figure 3-17 Flowmeter Log 3-47
Chapter 3 Wireline Logging
Table 3-1 Primary Non-Log Sources for Estimating Reservoir Properties 3-2 Table 3-2 Logging Sources for Estimating Reservoir Properties 3-3
Table 3-3 Matrix Densities for Common Formations 3-7
Table 3-4 Photoelectric Absorption Index for Common Formations 3-10 Table 3-5 Total Natural Radioactivity of Common Formations 3-11 Table 3-6 Responses for Logs Commonly Used to Evaluate Coals 3-16 Table 3-7 Logging Tools for Open Hole Exploration Wells 3-35 Table 3-8 Logging Tools for Open Hole Development Wells 3-36 Table 3-9 Logging Tools for Cased Hole Wells 3-42
2 - 1
3 - 1
Figure 6-1 Beam Pumping System 6-5
Figure 6-2 Top-Seating Pump Hold-Down 6-8 Figure 6-3 Bottom-Seating Pump Hold-Down 6-9 Figure 6-4 Gas Anchor 6-10
Figure 6-5 Progressing Cavity Pump 6-13 Figure 6-6 Gas Lift Installation 6-16 Figure 6-7 Electric Submersible Pump 6-18
Figure 6-8 Water Flow Path for Fields In Black Warrior Basin 6-24 Figure 6-9 Gas Flow Path for Fields In Black Warrior Basin 6-30 Figure 5-1 Instantaneous Shut in Pressure (ISIP) 5-8
Figure 5-2 Wellbore Configurations for Fracturing 5-13
Figure 5-3 "Dead String" for Measuring Bottomhole Pressure 5-16 Figure 5-4 Nolte Plot for Evaluating Fracture Pressures 5-38 Figure 5-5 Tiltmeter Sensor 5-53
Figure 5-6 Tiltmeter Installation 5-54
Figure 5-7 Tiltmeter Displays for Fractures 5-55
Figure 4-7 Limited Entry Multiple-Zone Completion 4-22 Figure 4-8 Lithology of the Well P5 Interseam Completion 4-25
Table 5-1 Minifracture Tests 5-2
Table 5-2 Information for Designing a Fracture Treatment 5-5 Table 5-3 Pumping Schedule for a Gel Fracture Treatment 5-28 Table 5-4 Pumping Schedule for a Foam Fracture Treatment 5-29
Table 6-1 Artificial Lift Methods for Coalbed Methane 6-4 Table 6-2 Comparison of Gas Flow Meters 6-32
Table 6-3 Typical Sales Gas Specifications 6-33
Figure 8-1 Water Disposal System in Black Warrior Basin 8-13
Chapter 5 Fracturing Coal Seams
Chapter 8 Treating and Disposing Produced Water
Chapter 7 Operating Wells and Production Equipment
Chapter 6 Selecting Production Equipment and Facilities
8 - 1 7 - 1 6 - 1 5 - 1
Figure 7-1 Beam Pumping System 7-10
Figure 7-2 Troubleshooting Beam Pumps (I) 7-11 Figure 7-3 Troubleshooting Beam Pumps (II) 7-12
Figure 7-4 Troubleshooting Progressing Cavity Pumps 7-16
Figure 9-1 Slug Test Equipment Configuration 9-4
Figure 9-2 Typical Coalbed Methane Production Decline Curve 9-23 Figure 9-3 Two-Seam Well Test Using the ZIP Tool 9-24
Figure 9-4 Three-Seam Well Test Using the ZIP Tool 9-25 Table 9-1 Data Frequency for Slug Tests 9-8
Chapter 9 Testing the Well
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❖This Guide
everal special elements in this guide’s text will help you quickly identify different types of information:
4. Numbered information gives step-by-step instructions for a
procedure.
n A solid box indicates general guidelines to follow before or during a particular task.
v A cut diamond highlights a list of characteristics, features,
benefits, or limitations of an object, technique, or procedure.
u A solid diamond describes a circumstance or condition you
might encounter and then explains possible ways to respond to the situation.
▲ Caution
A triangular “caution” note warns you about a situation that could be unsafe, environmentally hazardous, or damaging to equipment. ❈Important
Information that is particularly important for you to understand is
highlighted with the symbol above.
Acknowledgments
A
Guide to Coalbed Methane Operations was possible because of thegenerous contributions of experience and knowledge by the people listed below:
Dr. Richard Schraufnagel — Gas Research Institute (GRI)
Senior Project Manager, Coalbed Methane Engineering Dr. Schraufnagel generated the concept for this guide and provided important guidance and support throughout its develop-ment.
Stephen Spafford — Taurus Exploration, Inc.
Manager, Rock Creek Project
Selecting and preparing a field site, drilling, completing, fractur-ing, and treating and disposing produced water
Francis Dobscha — GeoMet, Inc.
Special thanks to Fran for his extensive contributions on selecting and preparing a field site, drilling, completing, fracturing, select-ing production equipment, operatselect-ing wells and production equip-ment, treating and disposing produced water, and testing wells
Jerry Saulsberry — Taurus Exploration, Inc.
Drilling, wireline logging, fracturing, and testing wells
Peter Steidl — Taurus Exploration, Inc.
Wireline logging
Paul Stubbs — GeoMet, Inc.
Testing wells
Randy McDaniel — Taurus Exploration, Inc.
Selecting and preparing a field site, and treating and disposing produced water
Brian Luckianow — Taurus Exploration, Inc.
Selecting and preparing a field site, and treating and disposing produced water
Michael Conway — Stim-Lab, Inc.
Completing and Fracturing
Allen Neel and Bill Lawrence — Black Warrior Drilling and Completion Company
Drilling and completing
Brad Taff and Ted Martin — Halliburton Logging Services, Inc.
Wireline logging
Daniel Felcman and Doug Womack — Tidewater Compression Services, Inc.
Selecting gas compression equipment
Brad Benge and Roger Hudson — Tidewater Compression Services, Inc.
Operating and maintaining gas compression equipment
Richard Montman, Dick Bretzke, and Robert Singleton — Halliburton Services, Inc.
Fracturing and cementing
Jerry Broadway — Black Warrior Drilling and Completion Com-pany
Selecting and operating progressing cavity pumps
Adam Olszewski — ResTech, Inc.
Wireline logging
Larry Strider — AMPCO Resources, Inc.
Drilling, completing, and selecting pumps
Gary Conner — Computalog Wireline Services, Inc.
Production logging
David Stuart — Robbins and Myers, Inc.
Selecting and operating progressing cavity pumps
Matt Hollub — Graphic Artist
Methane
oalbed methane is produced commercially in the United States, and it has attracted worldwide attention as a potential source of costcompetitive natu ral gas. Since the beginning of the coalbed methane industry in the mid1970s, operators have modified and applied petroleum industry technology to improve the operation of their fields. However, conventional oil and gas tech-nology does not always work effectively for producing coalbed methane. Because coal geology is so different from that of typical gas formations, you must use a different approach that takes into account:
■ The composition of the rock. Coal is 90 percent organic, whereas conven
tional gas formations are nearly 100 percent inorganic.
■ The different mechanical properties of coal. Coal is brittle and
weak, and it tends to collapse in the wellbore.
■ Coal’s naturally occurring fractures, or cleats. These fractures,
called face cleats and butt cleats, are extensive in coals. Most coal reservoirs, however, require hydraulic fracturing to stimulate produc tion.
■ Coal’s gas storage mechanism. Gas is adsorbed or attached onto the
internal surfaces of the coal, whereas gas is confined in the pore spaces of conventional rocks.
■ The large volumes of water present in the coal seams. Water must be
pumped continuously from coal seams to reduce reservoir pressure and release the gas.
■ The low pressure of coal reservoirs. Backpressure on the wellhead
must be kept low to maximize gas flow. And all produced gas must be compressed for delivery to a sales pipeline.
■ The modest gasflow rates from coal reservoirs. Capital outlays and
operating expenses must be minimized to produce an economical project.
To develop techniques for economically producing coalbed methane fields, Gas Research Institute (GRI) and Taurus Exploration, Inc. designed The Rock Creek Methane from Multiple Coal Seams Completion Project. This field research site is located in the Black Warrior Basin southwest of Bir mingham, Alabama.
The overall objective of this project, initiated in 1983, is to develop tech nology for more cost-effective production of methane from shallow, thin multiple coal seams using single vertical wellbores. Ile project has specifi cally focused on determining the best combination of drilling, completing, stimulating, and operating techniques to economically produce these wells. The Rock Creek project and the work of other operators in the Black War-rior Basin have produced many practical techniques and guidelines for developing coalbed methane fields. The cooperation and open communi-cation between operators and service companies in the Black Warrior Basin have been necessary to advance both basic knowledge and applied experience in producing methane from coal seams.
Chapter
1
Selecting and Preparing a Field Site
As citizens become increasingly aware of and concerned about envi-ronmental issues, the number and scope of envienvi-ronmental regulations continue to grow. Certain activities related to coalbed methane production are regulated by State and Federal agencies to help prevent damage to the environment. By incorporating sound environmental management into the planning and operation of a coalbed methane field, you will help protect the environment, minimize current regula-tory requirements, and possibly avoid costly penalties.
You should become familiar with the applicable environmental regu-lations in your area before selecting and preparing a field site. The U.S. Environmental Protection Agency (EPA) has primary jurisdiction over environmental regulations in the United States, but administra-tion of regulaadministra-tions varies from state to state. In the Black Warrior Basin of Alabama, the Alabama Department of Environmental Man-agement (ADEM) and the Army Corps of Engineers (ACOE) admin-ister most environmental regulations.
Environmental Guidelines
I
n selecting and preparing a field site, you will make some of the most important decisions about the coalbed methane project. These decisions will affect the environmental, safety and operations aspects of the project. These factors, in turn, will likely influence the project’s economic success.methane sites in the Black Warrior Basin are:
•
Protecting Wetland Areas•
Disposing Produced Water•
Controlling Non-Point Source (NPS) Pollution•
Preventing Oil Spills• Pr
otecting Historical SitesProtecting Wetland Areas
The impact of wetlands presents the single most critical regulatory issue in establishing right-of-way for pipelines, roads, and pads. Operating coalbed methane facilities often requires some activity in wetlands (e.g. an access road or a pipeline system). Coalbed methane facilities or activities which occur in wetlands are regulated and require a permit.
By knowing wetlands regulations, you can incorporate them into site planning to avoid or minimize dirt fill placed in wetlands. If you consider wetlands at the onset of planning, you can likely locate most facilities in non-wetland (upland) areas and thus avoid or minimize regulatory permitting.
To identify or verify wetlands areas within the proposed site, you should have a qualified biologist who knows the wetlands regulations conduct a field survey. Make sure this wetlands survey is conducted before completing final field development plans.
Regulatory agencies use “The Federal Manual for Identifying and
Delineating Wetlands” (“Federal Manual”) as the technical basis for
identifying and delineating wetlands. The person conducting the field investigation must be familiar with wetlands and must be trained to use this manual.
Because the ACOE makes final decisions on jurisdictional wetlands delineations, you should confirm the findings of the field survey with the ACOE. If the area is determined to be a wetland, a jurisdictional wetland boundary should be delineated. If possible, you should move the proposed facility site to avoid or minimize impacts to wetlands. If you cannot avoid impacts to wetlands, you must apply for a wetlands
permit. For more information about permits, refer to Additional
Resources at the end of this chapter.
Disposing Produced Water
The ability to dispose produced water is key to the successful operation of a coalbed methane field. Produced water must be managed to comply with the National Pollutant Discharge Elimination System (NPDES) requirements. The NPDES is governed by the U.S. Envi-ronmental Protection Agency (EPA) and is administered locally by the states. If NPDES standards are not met, production from the field could be forced to stop. Therefore, you must carefully plan for the management of produced water when selecting the field site.
The NPDES program defines the criteria for discharging water produced from coalbed methane wells into waterways. No produced water can be discharged into a river or stream without an NPDES permit. In the Black Warrior Basin, this program is administered by the Alabama Department of Environmental Management (ADEM). Your selection of a field site should be based on a thorough analysis of water treatment and disposal options (refer to Chapter Eight for more information). Begin by learning the NPDES permitting require-ments and procedures in your area. Give special attention to the questions below, which could influence your choice of a site: ■
■ ■ ■
■ What is the maximum volume of produced water which I will need to dispose?
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■ What is the chemical composition of this water?
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■ Are there waterways near the site that could be used for water discharge?
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■ Do these waterways have sufficient year-round flow to allow discharge in compliance with discharge limits?
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■ Are other operators using the same drainage basin to dis-charge produced water?
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■ What discharge limits do the regulatory agencies place on the waterway overall and on individual dischargers into the waterway?
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■ What is the life of a discharge permit?
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■ How do I renew a discharge permit?
For more information on treating and disposing of produced water, refer to Chapter Eight.
The Alabama Department of Environmental Management (ADEM) defines a pollutant as any item entering a waterbody that changes the composition of the water. A pollutant entering a waterbody through a NPDES permitted discharge is called a point source discharge. However, a pollutant that reaches a waterbody by other means that are not traceable to an identifiable facility, such as storm water runoff, seepage, percolation, etc., is called a non-point source discharge. Non-point source regulation, which is controlled in Alabama by ADEM and EPA, probably receives the highest priority of any regulation during coalbed methane development, and has increased the finding cost for methane significantly in recent years. Therefore, when planning a field site, you should consider the requirements concerning non-point source pollution.
One of the best ways to manage potential non-point source discharge is by implementing a Best Management Practices Plan (BMP) A BMP presents policies and procedures that can lessen the probability of initial causes of non-point source pollution. The Coalbed Methane Association of Alabama developed such a plan to assist operators in the Black Warrior Basin. This BMP, which is presented below, provides sound guidelines for:
•
Controlling Erosion•
Siting and Constructing Roads•
Developing Drilling Locations•
Siting and Constructing Pipelines•
Preventing Oil SpillsControlling Erosion
The major component of non-point source pollution is sedimentation from soil erosion. Sedimentation reduces stream capacities, interrupts ecosystems, carries other pollutants into a waterbody and may cause other potential environmental problems. Soil types, which vary greatly from one location to another, significantly influence soil erosion characteristics and are a factor in designing and implementing BMPs.
To minimize erosion when constructing coalbed methane facilities, practice these general erosion control techniques:
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■■ Divert runoff from well sites and roads onto level vegetated areas, terracing, riprap, or other areas that will disperse the water and prevent soil erosion.
■ ■■
■■ Install temporary erosion controls such as hay bales and/or silt fences in the natural drainage areas before or during the construction of well sites, roads, etc.
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■■ Install more permanent erosion control devices (i.e., geotextiles, riprap, matting, etc.) in areas of severe erosion.
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■■ Line, fertilize, and seed and/or mulch roadsides, drilling loca-tions and pipelines where slopes are sufficient to cause high velocity flow and erosion.
Perform this operation as soon as practical after construction and use accepted soil conservation practices.
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■ Pave and cover with gravel or plant vegetation on all disturbed areas, regardless of location.
Perform this operation as soon as practical, and maintain all erosion controls until the disturbed area is covered or permanent vegetation is re-established.
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■ Reuse onsite topsoil, if available, on the surface of each site.
This action will help maintain vegetation in disturbed areas.
Siting and Constructing Roads
Roads are necessary to provide access to each well and to facilities. Permanent access roads are usually built so that equipment can be moved in and out of the locations as needed initially and during later maintenance. Roads also provide access for monitoring wells and facilities.
When siting access roads, follow the guidelines below to the extent practical:
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■ Use existing roads, when suitable, to prevent further soil disturbance.
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■ Site roads along ridge lines to minimize road grades and to lessen the potential of disturbing a water course.
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■ Minimize road grades whenever practical.
When constructing roads, follow the guidelines below whenever practical:
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■ Construct roads and roadway drainage only under the guid-ance of a person experienced in road construction techniques and erosion control.
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■ Install velocity breakers (stabilized water bars) to control high velocity flow and potential stream erosion.
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■ Avoid constructing roads through areas having highly erodible soils, wetlands or wet meadows.
If necessary to build roads in these areas, use erosion control methods and wetland road construction techniques to minimize disturbance.
If operations are not permitted under Section 404 of the Clean Water Act (Nationwide Permit) you must obtain individual permits from the U.S. Corps of Engineers (ACOE) before disturbing any wetland area. In addition, you may need an ACOE permit under the requirements of Section 10 of the Rivers and Harbors Act of 1899 and/or section 193 of the Marine Protection, Research and Sanctu-aries Act.
■ ■ ■ ■
■ Test quarterly for pH any mine tailings (i.e., black or red rock) used in roadbed construction.
Test each source of “black or red” rock. The pH must range from 6 to 9 pH units.
Keep good records of the testing for three years.
■ ■■
■■ Never use known hazardous or toxic materials in constructing roadbeds.
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■■ Maintain vegetated filter strips of sufficient length to assist sediment deposition between streams and roads.
If terrain limitations necessitate, use other permanent methods (geotextiles, riprap, matting, etc.) instead of or in conjunction with vegetated filter strips, provided the water course is not altered or diverted.
■ ■■
■■ Take measures to prevent construction materials (dirt, boul-ders, rock, trees, etc.) from being deposited into water-bodies.
If these materials inadvertently enter the water, take environmen-tally sound measures to remove them immediately. These mea-sures should prevent further environmental damage.
Constructing Stream Crossings
Because of the topography of coalbed methane operations in many areas, you may need to cross a stream with a road. Roadways can cause
❈ ❈❈ ❈
more water course disturbance, redirect flow, and/or possibly limit movement of stream life. Through planning and careful construction, you can eliminate or significantly lessen potential environmental damage when crossing streams.
When developing roadstream crossings, follow the guidelines below whenever practical:
■ ■ ■ ■
■ Minimize stream crossings whenever practical. Use existing culverts, bridges, fords and/or other crossings whenever pos-sible.
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■ Make stream crossings at right angles to the main stream channel, when practical and/or when it will limit environmen-tal damage.
■ ■ ■ ■
■ Test quarterly for pH each source of mine tailings (black or red rock) used for fill material during construction of the stream crossing.
The pH must range from 6 to 9 pH units. Keep good records of the testing for 3 years.
■ ■■ ■
■ Never use known hazardous or toxic materials in constructing stream crossings.
■ Submit a stream crossing plan for pre-approval to the state environmental agency.
In Alabama, these plans are based on mean stream water flow of less than 10 cfs (using the best available historical data). If the crossing plan is for a stream with mean water flow of 10 cfs or greater or where there is greater than 200 cubic yards of fill below the plane of the ordinary high water mark, you must coordinate the plan with the Alabama Department of Environmental Manage-ment (ADEM) and the Army Corps of Engineers (ACOE) or the environmental agency in your state.
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Developing Drilling Locations
Drilling pads are constructed to allow movement of a drilling rig and other heavy equipment into the location. This location is usually an all-weather installation that provides access for field people to maintain and observe the well.
A drilling or reserve pit is a temporary earthen pit for storing materials used or generated in drilling or working over the well. The reserve pit may also be used as an emergency catch basin for location runoff, water produced during drilling operations, or oil from equipment which may be inadvertently spilled. This pit helps prevent environ-mental damage by eliminating discharge of liquids and solids off the drilling pad.
To eliminate or minimize environmental damage, practice the follow-ing guidelines, whenever possible, in constructfollow-ing drillfollow-ing pads: ■
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■■ Keep the size of the drilling pad as small as practical to lessen the amount of surface area disturbed.
■ ■■
■■ Minimize all slopes and use appropriate erosion control and construction techniques to lessen erosion of those slopes.
■ ■■
■■ Construct pads and/or pits at a sufficient distance from a waterbody for maintenance of a streamside management zone (SMZ).
A streamside management zone is an area along a stream bank where existing vegetation is not disturbed, which helps prevent soil from moving into the stream.
If pads and/or pits are necessarily built adjacent to water bodies, take appropriate measures to protect that waterbody and water quality.
If sufficient SMZ area is not available, use other erosion control measures in conjunction with available SMZ to lessen potential water quality and water body damage, provided the water course is not altered or diverted.
■ ■■
■■ Take measures to prevent construction materials (dirt, boul-ders, rock, trees, etc.) from being deposited into waterbodies.
environmen-tally sound measures to remove them immediately. These measures should prevent further environmental damage.
■ ■ ■ ■
■ Contour sites during construction to prevent stormwater runoff from creating erosion paths.
To eliminate or minimize environmental damage, practice the follow-ing guidelines, whenever possible, in constructfollow-ing drillfollow-ing pits: ■
■ ■ ■
■ Do not use materials that adversely affect pit wall integrity (i.e., trees, tree stumps, large boulders, etc.).
■ ■ ■ ■
■ Construct pits, if practical, in cut or non-disturbed areas instead of areas that have been dirt filled.
If necessary, to construct pits in fill, take measures to compact the pit walls to ensure structural integrity. Compact all fill areas and all containment pits built in fill material.
■ ■ ■ ■
■ Line pits with polyethylene or other non-permeable material in areas where soil types do not prevent potential contamina-tion of groundwater.
■ ■ ■ ■
■ Dispose of pit waste waters under the guidelines established by the ADEM Interim Land Application Guidelines (or your state environmental agency), and the subsequent BMP plans filed by each operator for handling these fluids.
■ ■ ■ ■
■ Do not place in or over levees or walls siphons or openings that would permit escape of contents thereby causing pollution or contamination.
■ ■ ■ ■
■ Do not allow liquid level in pits to rise within two feet of the pit levees or walls. Maintain pit levees or walls at all times to prevent deterioration, subsequent overfill, and leakage of contents to the environment.
Pipelines are also needed to collect natural gas from individual wells to compression facilities, and from compression facilities to gas sales lines. Because pipelines are usually buried, they disturb a water course for a very short time.
By applying proper erosion/sedimentation control techniques, you can limit environmental damage. When siting pipelines, follow the guide-lines below to the extent practical:
■ ■ ■ ■
■ Site gathering lines along road rights-of-way.
■ ■ ■ ■
■ Minimize stream crossings if you cannot follow roadways.
If necessary to cross streams while constructing a pipeline, mini-mize stream disturbance and use erosion control techniques to prevent sedimentation of the stream body downstream of the crossing.
■ ■ ■ ■
■ Do not place into a reserve pit any oil, trash or other materials which would increase the difficulty in cleanup of the pit or otherwise harm the environment.
Properly store or dispose such material according to applicable state or federal regulations.
Do not burn or bury garbage on site. Dispose all garbage at an approved landfill site.
■ ■ ■ ■
■ You may burn trees and stumps (not household garbage) on location after notifying the Alabama Forestry Commission and according to local, State, and Federal regulations.
■ ■ ■ ■
■ Empty and close drilling pits by burying them after drilling and fracturing operations are completed. Contour and seed the area.
Before closing the pit, drain and haul away liquids in the pit and remove or perforate the pit liner.
Siting and Constructing Pipelines
Pipelines are necessary in coalbed methane operations to collect produced water to a central facility and discharge site.
If operations are not permitted under Section 404 of the Clean Water Act (Nationwide Permit), the operator must obtain indi-vidual permits from the Army Corps of Engineers before disturb-ing any wetland area.
■ ■ ■ ■
■ Minimize pipeline grades where practical.
■ ■ ■ ■
■ Minimize rights-of-way within acceptable pipeline construc-tion techniques.
When constructing pipelines, follow the guidelines below to the extent practical:
■ ■ ■ ■
■ Construct pipelines only under the guidance of a person experienced in pipeline construction techniques and erosion control.
■ ■ ■ ■
■ Install water bars on extreme pipeline right-of-way grades to reduce runoff velocities.
■ ■ ■ ■
■ Avoid areas of highly erodible soils, wetlands and wet mead-ows.
If necessary to construct pipelines in these areas, use erosion control methods and wetland pipeline construction techniques to minimize disturbance to these areas.
■ ■ ■ ■
■ Maintain vegetated filter strips of sufficient length to assist sediment depositions between streams and pipelines.
If terrain limitations necessitate, use other permanent methods (geotextiles, riprap, matting, etc.) instead of, or in conjunction with, vegetated filter strips.
■ ■ ■ ■
■ Backfill trenches with soil according to accepted pipeline construction techniques.
■ ■ ■ ■
Preventing Spills
By properly siting a coalbed methane facility, you can greatly reduce control requirements and impacts associated with a release event (spill).
Any coalbed methane operation must prepare a Spill Prevention Control and Countermeasure Plan (SPCC) to prevent the discharge of oil from any facility into or upon any waters of the state. This plan is required under Title 40 of the Code of Federal Regulations, Part 112 (40 CFR 112), “Oil Pollution Prevention-Non-Transportation Related Onshore and Offshore Facilities”.
The basic elements of an SPCC Plan consist of the identification and description of the following:
❖ General setting of the facility
❖ Inventory of spills and potential spill sources
❖ Structures and/or equipment to prevent spills from reaching waters of the state and conformance with applicable SPCC guidelines.
The operator of a coalbed methane operation is responsible for determining which specific parts of the regulation apply to his opera-tion.
When planning a coalbed methane site, you should carefully consider where you locate potential oil spill sources such as compressor stations, bulk waste oil storage, and fuel bulk storage. For example, in most cases it is advantageous to locate compressors on top of hills or knolls. However, if a large oil spill occurred at the compressor, oil could migrate quickly down the hill and into streams.
Siting a facility away from potentially environmentally sensitive areas such as streams, rivers, and wetlands greatly reduces exposure to any mitigative action required in the event of an oil release.
Planning facilities to comply with SPCC requirements will help reduce unforeseen spill cleanup costs. If a spill should occur, effective control
measures will help reduce impacts to the environment and necessary clean-up efforts.
Protecting Historical Sites
To protect any sites having potential historical or cultural significance, you should have an historical or cultural resource assessment per-formed on the site before beginning any development. Such an assessment can identify areas that should not be disturbed and can help avoid unnecessary problems in developing the site. To find a person qualified to perform an historical or cultural resource assessment, you can contact a university or historical center in your area.
■ ■■ ■
■ Learn all applicable State and Federal environmental regula-tions before selecting and preparing a site. For more
informa-tion see “Environmental Guidelines” in this guide.
■ ■■ ■
■ Establish good relations with landowners and residents near the field site.
These people can be great allies for your project if treated with courtesy and respect. They may be instrumental in granting mineral rights and access rights-of-way and in reporting any trespassing or vandalism at the site.
Meet and talk with landowners and residents individually before conducting any site surveys or other field activities.
Explain plans for developing the field and what types of activities they could expect from a coalbed methane operation. Candidly address their questions, concerns, and fears.
Pre-Planning
In planning a coalbed methane site, you will make many important decisions that will affect the safety of workers and the efficient operation of the field throughout its life. To help ensure a sound site development plan, follow the guidelines below:
Safety and Operating Guidelines
❈ ❈❈
■ ■ ■ ■
■ Before beginning site development, delineate roads, drilling pads and pits, and facility locations with visible reference markers. Carefully review development plans with the site developers.
These preparations will minimize environmental impact and help ensure that site developers do not harm life or property of nearby landowners and residents.
■ ■■
■■ If site development will involve clearing a substantial amount of timber, you may consider contracting with a timber com-pany to cut and purchase the timber. Obtain necessary autho-rization from landowners before clearing any timber.
Contracting timbering to a qualified timber company may make site development safer and easier. In addition, revenue from selling the timber may help offset any payments to landowners for timber removed during site preparation.
Clearing Timber
■ ■■
■■ Place gravel or similar material on roadbeds to provide a stable surface for heavy equipment.
Road surfacing is especially important during the winter and wet seasons.
■ ■■
■■ Plan main access road(s) into the site with the help and cooperation of a county commissioner (or equivalent public official) to help ensure safe road design.
■ ■■
■■ Construct roads along ridge tops when practical. Attempt to design roads so drivers will have a clear line of sight.
■ ■■
■■ Avoid designing roads with sharp curves, blind spots, steep grades, or in or near streams, valleys, or severe drop-offs.
■ ■■
■■ Place state-approved caution signs on both sides of the
trance to the road(s) from any highways. Consult the state Department of Transportation for the correct specifications and placement of these signs and any other requirements.
Developing Well Sites
■ ■ ■ ■
■ Develop the well site at least several months in advance of well work.
This step will facilitate proper drainage and create a more stable surface for heavy equipment.
■ ■ ■ ■
■ Develop well sites during the dry summer months to signifi-cantly reduce costs.
■ ■ ■ ■
■ Determine the size of the well site based on the space needed to accommodate not only the drilling rig, but the fracturing equipment (fluid tanks, pumps, blenders, turbines, etc.) as well.
■ ■ ■ ■
■ Locate production equipment (separators, meters, compres-sors, tanks, etc.) around the perimeter of the site to create an open work area near the wellhead.
■ ■ ■ ■
■ Locate production equipment (separators, meters, compres-sors, tanks, etc.) near main gas and water collection lines and power lines to avoid digging up the well pad area for repairs.
Additional Resources
“Best Management Practices Plan For Non-Point Source Discharge Control, Coalbed Methane Resource Extraction Industry,” Coalbed Methane Association of Alabama and Alabama
Department of Environmental Management, 1990.
“Environmental Protection Agency Regulations on Oil Pollution Prevention,” 40 CFR 112, March 26, 1976.
Federal Interagency Committee for Wetland Delineation, 1989. “Fed-eral Manual for Identifying and Delineating Jurisdictional Wet-lands,” U.S. Army Corps of Engineers, U.S. Environmental Protection Agency, U.S. Fish and Wildlife Service, and U.S.D.A. Soil Conservation Service, Washington, D.C. Cooperative
Publication.
Federal Register, Part II Department of Defense, Corps of Engineers, Department of the Army, 33 CFR Parts 320 through 330, “Regulatory Programs of the Corps of Engineers,” Final Rule, Vol. 51, No. 219, Thursday November 13, 1986, Rules and
Regulations.
Luckianow, B.J., W.C. Burkett, and C. Bertram, “Overview of Envi-ronmental Concerns for Siting of Coalbed Methane Facilities,” Proceedings of the 1991 Coalbed Methane Symposium, The Univer-sity of Alabama, Tuscaloosa, (May 13-16).
Simpson, T.E., “Environmental Overview, Coalbed Methane Gas Development in Alabama, 1984-1989,” Dames & Moore, 1989.
2
Drilling and Casing the Wellbore
T
o successfully drill and case a coalbed methane well, you must consider several operational factors not usually encountered with conventional wells. For example, most coalbed wells in the Black Warrior Basin are drilled into relatively shallow (500-3500 feet), low-pressure coal formations. Because these formations produce very low rates of gas, project economics require an extremely efficient and cost-effective drilling program. A significant part of this drilling program will be shaped by the stimulation treatment and completion methods you select for the wells. Similarly, the unique mechanical properties of coals require that you use procedures that avoid damaging the coal formation. This chapter explains these and other important consider-ations for drilling a coalbed methane well.This chapter will guide you through:
•
Planning the Drilling Program•
Drilling the Wellbore•
Coring the WellborePlanning the Drilling Program
By carefully planning your coalbed drilling program, you can help ensure productive, economical coalbed methane wells. Figure 2-1
illustrates the steps of an effective planning process. Each of the steps is explained below.
Figure 2-1
The Planning Process for Drilling a Coalbed Methane Well
1. Collecting Information
5. Selecting Casing Weight and Grade
3. Selecting Casing Setting Depth
4. Selecting Hole Size
2. Evaluating Formations
7. Designing the Hydraulics of the Drillstring
6. Selecting a Drilling Technique
11. Complying with Regulatory Permitting Requirements
9. Designing the Cementing Program
10. Selecting the Drilling Rig and Drilling Equipment
8. Selecting the Drillbit and Drillstring
Before you can make informed decisions about a drilling program, you must learn as much as possible about coalbed drilling and production operations in your area. Begin by collecting any well information available from offset coalbed methane operators. You may also find some of this information recorded as public information at your local and state oil and gas regulatory agencies. Specifically, you should try to obtain this well information:
❖ Formation depth, pressure, and production
❖ Type of coal and non-coal formations
❖ Well logs
❖ Rig type and drilling assembly
❖ Drilling fluid specifications
❖ Casing program
❖ Drilling problems encountered
❖ Stimulation and completion methods
In addition, you should talk with drilling contractors who have substantial experience in your area of interest. You should try to find out:
❖ Types of rigs, surface and downhole equipment commonly used
❖ Drilling problems typically encountered
❖ Drilling procedures for eliminating problems
❖ Equipment cost and availability
You should also become familiar with considerations for preparing the well site for drilling operations. For information on this topic, refer to
Chapter 1 of this guide.
Finally, you should consult with your local and state oil and gas agencies and environmental agencies to learn what laws and regu-lations you must follow.
After collecting offset well information, you should evaluate any available well logs and drilling records to determine approximate depths for prospective coal intervals. You should also attempt to identify any potential problem zones, such as:
❖ Depleted zones that may cause lost circulation
❖ Sloughing shales
❖ Overpressured zones or water disposal zones
❖ Fresh water aquifers
Accurately identifying prospective coal intervals and problem zones will help you to design an effective casing and cementing program.
To select the casing string and drilling equipment, you must first determine at which depths to set casing in the wellbore. The casing setting depths will depend primarily on these factors:
❖ Fracture gradients of coal seams and adjacent f o r m a t i o n s
❖ Regulatory requirements
❖ Drilling problems
❖ Isolation of coal seams
Before selecting the casing setting depth, you first must determine the fracture gradient, or pressure per foot of depth, required to fracture the coal seams and adjacent formations. In general, you should set casing through zones that have a fracture gradient that is
2. Evaluating Formations
significantly different than the fracture gradient of deeper zones.
Figure 2-2 illustrates how an operator could prevent possible lost circulation problems by setting casing through a low-fracture-gradient coal seam before drilling ahead through a coal seam having a significantly higher fracture gradient.
Figure 2-2
Setting Casing Through Zones with Lower Fracture Gradients
You can predict fracture gradients by using various published correlations or by using a fracture gradient formula, such as Eaton’s Equation, shown below:
where:
F = fracture gradient, psi/ft S = overburden stress, psi P = wellbore pressure, psi D = depth, ft v = Poisson’s ratio
(
F = S-P D x)
v 1-v + P DFracture gradients for coal seams in the Black Warrior Basin range from as low as 0.5 psi/ft to over 1.0 psi/ft.
To determine proper casing setting depths, you must also consider the requirements of state and local regulatory agencies. For example, regulatory agencies governing the Black Warrior Basin require that you set a minimum of 300 feet of surface casing in wells up to 4000 feet deep.
You should also consider potential drilling problems when determin-ing casdetermin-ing settdetermin-ing depths. Set casdetermin-ing to isolate zones that may cause problems such as water influx, sloughing shales, or abnormal pres-sures.
Finally, when selecting casing setting depths, you should isolate prospective coal seams to optimize well completions. For example, set surface casing deep enough to eliminate drilling problems, but try not to set surface or intermediate casing across coal intervals that you plan to complete. A well completed through two strings of casing (surface and production casing) will likely be much less productive than a well completed through only one string.
4. Selecting Hole Size
Before the rest of the drilling program can be designed, you must first determine the sizes of the hole to be drilled. You should base the hole sizes on the casing program rather than selecting casing based on a pre-selected hole size. By carefully planning the hole and casing sizes, you can avoid many operational problems later in the life of the well.
This section will guide you through the steps for determining proper hole sizes. Figure 2-3 illustrates the steps in this process. Each of these steps is explained below.
Figure 2-3 Selecting Hole Size
Production Rates
Production Considerations Other Considerations
Artificial Lift Method
Completion Method
Select Production Hole Size Select Optimum Production Casing Size
Removing Drilling Cuttings Tubing Size Performing Stimulation Treatments Performing Future Workovers and Recompletions Select Optimum Surface Casing Size
Select Surface Hole Size
Production Rates
To select optimum hole size, you should begin by esti-mating the expected water and gas production rates for the well. You may be able to obtain these esti-mates from offset well data, as explained earlier in Collecting
Information.
Artificial Lift Method
Next, you must decide what method of artificial lift you will use to remove water from the wellbore. Because coalbed methane reservoirs typically have very low pressures, you must select a lift system that will main-tain a low wellbore water level to minimize bottomhole pressure and optimize gas production. For more information on selecting an artificial lift system, refer to Chapter 6.
Tubing Size
When you design the artificial lift system, you will de-termine the optimum production tubing size to install in the well. This decision is based on the type and size of lift system you select as well as the estimated produc-tion rates. For more informaproduc-tion on selecting tubing size, refer to Chapter 4.
Selecting an insufficient tubing size may pre-vent you from effectively dewatering a coalbed reservoir, and thus severely limit ultimate gas p r o d u c t i o n .
Completion Method
Next, you should consider how you will complete the
well. Your choice of an open hole or cased hole completion will influence the amount and size of production casing you run. For example, you must select casing sizes that will accommodate the diameter of completion tools (e.g., perforation guns, slotting tools, underreamers) you will need to complete the well. For more information on designing the well completion, refer to Chapter 4. After determining the optimum casing string for your tubing and completion requirements, you should consider several other factors.
❈ ❈ ❈ ❈
❈Important
Other Considerations In Selecting Hole Size
Performing Stimulation Treatments
In addition to the production considerations above, you must also consider whether you will perform a fracture stimulation on the well. If you plan to fracture the well, determine whether the fracture will be pumped down the tubing string or down the casing string. If you plan to pump the treatment down the casing, size the casing large enough to accommodate the desired treatment rates. In addition, you must determine whether you will run isolation baffles for fracturing treat-ments. If you plan to use isolation baffles, you must install them when you run the casing string. For more information on fracturing considerations, refer to Chapter 5.
Selecting an insufficient casing size can limit the injection rate or fluid type needed for an effective fracture treatment.
Removing Drilling Cuttings
You should also determine the hole size required to effectively remove cuttings from the hole. Because of the shallow, low-pressure coal formations in the Black Warrior Basin, most wells in this basin are drilled using compressed air or air mist instead of drilling mud. To effectively remove cuttings from an air-drilled hole, you must prop-erly size the hole and the air compressors. The larger the hole size you select, the greater will be the volume of air required to remove cuttings. As you increase hole size, you also increase the horsepower required to lift cuttings. Therefore, when selecting the optimum hole size for removing cuttings, you must also consider the cost for the size of compressor you will use.
Performing Future Workovers and Recompletions
When selecting hole size, you should also consider the sizes of any downhole tools that you may need to run to workover or recomplete the well in the future. Make sure casing strings have sufficient clearance to accommodate these tools. For more information on the types of tools you may need to use, refer to Chapter 4.
❈ ❈❈
Analyzing Production Considerations and Other Considerations
Next, independently evaluate the hole size requirements of each production and other consideration explained above. Then select the optimum production casing size that best satisfies all these require-ments.
For additional guidance in evaluating hole sizes for particular appli-cations, consult with drilling contractors, service company represen-tatives, and well operators who are experienced in drilling, stimulat-ing, completstimulat-ing, and producing coalbed methane wells. These people can explain the specifications and operation of their tools and discuss the requirements of your particular operation.
Selecting Optimum Production Casing Size
Select the production casing size that best satisfies all of the produc-tion and other consideraproduc-tions explained above.
Many operators in the Black Warrior Basin run 4-1/2 inch or 5-1/2 inch production casing. Most of the wells at the Rock Creek Project were cased with 5-1/2 inch production casing.
Selecting Production Hole Size
The size of the production casing you select will help determine the size of the production hole required. The hole size you select should be large enough to prevent the casing from sticking while being run. In addition, the hole size should allow sufficient annular space to provide an effective cement job. Many operators in the Black Warrior Basin drill a 7-7/8 inch production hole to accommodate a 5-1/2 inch production casing string. For additional guidance in selecting a proper hole size, refer to Figure 2-4.
1. Determine the size of the last casing to be run. 2. Enter the chart at that casing size.
3. Follow the arrows to select the hole size required to set that size pipe (e.g., 5 in. casing inside 6-1/8 in. or 6-1/2 in. hole).
Solid lines indicate commonly used bits for that size pipe. This bit size will normally provide adequate clearance to run and cement the casing (e.g., 5-1/2 in. casing inside 7-7/8 in. hole). Dashed lines indicate less common hole sizes (e.g., 5 in. casing
inside 6-1/8 in. hole). If you select a dashed path, you should carefully consider casing connections, mud weight, cementing, and doglegs. Large OD connections, thick mudcake buildup, problem cementing areas (high water loss, lost returns, etc.), and doglegs may aggravate attempts to run casing when clearance is low.
4. Follow the arrows to select a casing large enough to allow passage of a bit to drill the hole selected in step 3.
Solid lines indicate commonly required casing sizes, encompassing most weights (e.g., 6-1/2 in. bit inside 7-5/8 in. casing). You can use this chart to select the casing, hole, and bit sizes for many drilling programs. To use the chart, follow the steps below:
Selecting Optimum Surface Casing Size
The size of the production casing you select will determine the size of the surface casing string to run. You should select surface casing large enough to accommodate the bit needed to drill the hole for the production casing string. If you plan to run a cement collar on the production string, make sure the drift diameter of the surface casing is large enough to accommodate the bit required to provide the maximum hole size for the cementing collar, as specified by the cementing collar manufacturer.
Many operators in the Black Warrior Basin run an 8-5/8 inch surface casing string. Most of the wells at the Rock Creek Project were also cased with 8-5/8 inch surface casing.
Selecting Surface Hole Size
The size of the surface casing you select will determine the size of the surface hole required. Many operators in the Black Warrior Basin drill a 12-1/4 inch surface hole to accommodate an 8-5/8 inch surface casing string. For additional guidance in selecting surface hole size, refer to Figure 2-4.
Before beginning your casing and cementing program you should obtain a casing and cementing handbook from one of the major oilfield service companies. This handbook provides specifications and other useful information on casing and cementing equipment and materials.
When you design a casing string, you must consider three principal forces:
•
Burst Pressure•
Collapse Pressure•
Tensile LoadBurst Pressure
Burst pressure refers to a condition of unbalanced internal pressure. Burst pressure is probably the most important factor in designing the coalbed casing string because the pipe will likely experience the greatest pressures during fracturing stimulations, when treating pressures can exceed 5000 psi. You can estimate the treating pressures required by using the fracture gradients you predicted when determining casing setting depth (step 3 above). Once you have estimated fracture gradients for the coal seams of interest, you can select the proper casing weight and grade. For more informa-tion on casing specificainforma-tions, refer to a service company casing handbook.
Tensile Load
Tensile load is the force exerted on a joint by the weight of the joints below it. Because each joint supports all the weight below it, the greatest tension occurs at the top of the string. Most coalbed wells in the Black Warrior Basin are shallow; therefore, tensile load is not a primary consideration for this area.
Production casing is usually available in sizes ranging from 4.5 inches to 7.0 inches and in a variety of weights and grades. Casing is also Collapse pressure is the unbalanced external pressure imposed on the pipe. The worst operational case is for the pipe to be empty with a normal hydrostatic pressure gradient exerted on it from the outside. The greatest differential pressure exerted on the casing is most likely to occur during flowback of a fracture treatment or during the later stage of production when pressure inside the wellbore decreases significantly. You should design the casing string for this worst case scenario.
Typically, water levels in coalbed wells are pumped down to minimize hydrostatic pressure and optimize gas production. The collapse pressure becomes a more significant factor in deeper coalbed wells. Because of the relatively shallow wells (500-3500 feet) in the Black Warrior Basin, casing collapse has posed few problems in this area. However, the collapse strength of the casing may be reduced by mechanical operations such as slotting or high density perforating.
classified as API (American Petroleum Institute) standard casing or limited service casing. API standard meets all specifications for wall thickness, outside diameter, inside diameter, drift, collapse, internal yield, and joint yield strength ratings for its respective grade. Limited service casing is also called “mill reject” because one or more specifications does not meet API standards. However, limited service casing may also be tested to 80 percent of the minimum yield as set forth by API specifications. Therefore, to reduce cost you may choose to use limited service casing for some applications.
Typical casing grades are F-25, H-40, J-55, K-55, C-75, N-80, C-95, and P-110. These grades represent the strength of the casing. A variety of casing weights and wall thicknesses is also available for use according to well conditions. Select the size, weight, and grade of production casing based on the individual well design and completion technique. For more information on completing coalbed methane wells, refer to Chapter 4.
Before ordering casing, find out the limitations of casing weight and length for the rig you will use to run the tubulars. By ordering Range Two casing and tubing, which have lengths of 28-32 feet, you may be able to use a smaller, less costly rig.
Most Black Warrior Basin operators complete coalbed methane wells simply using a production string set through a shallow sur-face casing. They generally run 5-1/2 inch casing in a 7-7/8 inch hole. The surface casing usually consists of 300 feet of 8-5/8 inch casing set in a 12-1/4 inch hole.
Casing Used in the Black Warrior Basin
Using casing smaller than 4-1/2 inch (O.D) limits the size of production tubing you can run inside it. If the casing/tubing annulus is too small, the flow path for gas will be restricted and the annulus can easily plug.
❈ ❈ ❈ ❈ ❈ Important ▲ ▲ ▲ ▲ ▲ Caution
To select the most effective drilling technique for your area of interest, you must consider the geologic and reservoir conditions of the coal basin. Generally, wells drilled in the eastern United States target shallow coal beds (less than 4000 feet) in geologically older (Pennsylvanian) and more competent formations. Operators in this area usually employ relatively simple drilling techniques. In contrast, complex drilling techniques are used to drill wells in the western United States, which usually target younger (Cretaceous) formations that are deeper, over-pressured, and less competent. Operators in the Black Warrior Basin frequently drill coalbed wells using the rotary-percussion technique, with air or air-foam mist as the circulating fluid. Figure 2-5 shows a comparison of the con-ventional rotary and the rotary-percussion drilling techniques. Rotary-percussion drilling has become a standard technique in the Black Warrior Basin because it typically yields higher penetration rates and lower drilling costs than conventional rotary drilling. In addition, the rotary-percussion technique minimizes formation damage because it uses no drilling mud.
In the northern end of the Black Warrior Basin, where the surface formations are hard, coalbed wells are often drilled from surface to total depth using the rotary-percussion technique. In this area, drilling with a tri-cone rotary bit yields lower penetration rates because at shallow depths it is not possible to apply sufficient weight on the bit.
In the southern end of the Black Warrior Basin, however, where the softer Cretaceous formations are encountered from surface to as deep as 500 feet, the surface hole must be drilled using a tri-cone rotary bit with drilling fluid (usually water) to prevent hole col-lapse. After drilling through the Cretaceous formations and setting surface casing, drillers usually switch to rotary-percussion drilling to achieve greater penetration rates in the harder formations. Most of the coalbeds in the Black Warrior Basin are water satu-rated, low pressure, low permeability formations. In some parts of the basin, little formation water flows into the wellbore during drilling, and air circulation can easily remove not only cuttings, but any produced water as well. When the wells at the Rock Creek site
were drilled, a mixture of water and liquid soap was added to the compressed air to enhance lifting of cuttings and cleaning of the hole. For more information on removing drilling cuttings, refer to step 7, Designing the Hydraulics of the Drillstring.
Figure 2-5
Conventional Rotary and Rotary-Percussion Drilling Techniques
