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
1. Collecting Information
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
3. Selecting Casing Setting Depth
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 D
Fracture 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