Cavings Field Guide

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Cavings Field Guide

AN AID TO MANAGING WELLBORE

STABILITY IN REAL TIME

April, 2008

INTELLECTUAL PROPERTY &CONFIDENTIALITY NOTICE

This document contains confidential information, which is the exclusive property of BP America, Inc. In whole or part, this document or its attachments MAY NOT be reproduced by any means,

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CONTENTS

Exploration & Production BP Confidential

FOREWORD

BP spends over U.S. $6 billion annually drilling and completing wells. Operating in six continents of the world, BP drills wells in some of the most extreme

environments experienced anywhere – from deep water offshore Angola to the Arctic tundra. Not all these wells are as trouble-free as we would wish them to be. Drilling Non-Productive Time (NPT) cost BP approaching 4,000 days and U.S. $1 billion in 2007. Of this figure, formation-related problems account for over one-third of the NPT total.

This guide is designed for BP operations geologists, well-site geologists, mud-loggers and well-site leaders to enable them to spot early warning signs of impending downhole trouble. While BP’s geoscientists, drilling engineers, wellbore stability specialists and well-site leaders apply the best technology available in designing and executing wells, drilling surprises still occur.

Formation properties and pore pressures may differ from the pre-drill predictions, and corrections to mud weights and drilling practices may have to be

implemented at the rig site to respond to these changing conditions. Diagnosing a wellbore instability problem as it begins to develop – and

intervening appropriately to limit its severity – offers great opportunities to make a step-change in BP’s drilling performance by significantly reducing

formation-related NPT. Many times, the first indication of a wellbore instability problem starting to occur will be a small number of cavings seen at the shakers along with the regular drill cuttings. Not all cavings are the same. Their size and shape can provide diagnostic evidence that identifies the cause of the wellbore instability problem. Recognized early, the appropriate remedial actions can be put in place to manage the problem and to finish drilling the well successfully.

This cavings guide was compiled by Stephen Willson, Rock Mechanics Advisor in BP’s Exploration & Production Technology Group. Cavings photographs have been sent to Stephen by others over the years asking for help with wellbore instability problems. Many of these photographs are reproduced in this Guide, with an explanation by Stephen regarding their meaning from a stability viewpoint and suggested remedial measures to prevent the problem becoming worse.

Hopefully this Guide will be useful in your work, as we strive to make BP the world’s best driller.

Kate Baker,

Distinguished Advisor

Well Planning & Geotechnical Network Director

Tom Christopher,

Drilling Engineering Excellence Network Director

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1 INTRODUCTION

1 INTRODUCTION – THE NEED FOR CAVINGS

MONITORING

Typical mix of cavings and cuttings from a well experiencing modest instability

1.1 Cavings Analysis

Cavings analysis is key to managing wellbore stability in real time. Cavings monitoring provides:

• A warning signal that the wellbore is failing.

• An indication of which formations are unstable.

• Evidence of the mode of wellbore failure.

• Information to decide the appropriate remedial action – e.g. improve hole

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1 INTRODUCTION

1.2 Cavings Identification

Cavings recovered when drilling a fault zone

Cavings are pieces of rock from the wellbore wall that were not produced directly by the action of the drill bit. Cavings can have many shapes. They may be long and splintery, tabular or angular in shape. Cavings usually come from shales that have become unstable. The shape of the caving indicates the cause of rock failure and the appropriate remedial action to be taken.

Cavings Characteristics - Cavings have no bit marks (this distinguishes cavings from cuttings). The size of cavings can be large or small with the typical size being 1 in to 3 in (2.5 cm to 7.6 cm). Small coffee grounds size/shape cavings are usually an indication of reworked cuttings or cavings that have spent time in the wellbore before being transported to surface. Soft cavings usually indicate some adverse mud/shale chemical reaction is taking place. There are three general cavings shape types: angular, tabular and splintery.

1. Large angular cavings are typically freshly produced from borehole breakouts.

2. Large tabular cavings typically come from a fault zone, natural fracture/joint sets in relatively hard rock, or as a result of bedding parallel failure.

3. Splintery cavings are produced when drilling underbalanced in hard, strong rocks (especially those that are gas bearing). Weaker rocks may produce angular cavings when the overbalance is insufficient.

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1 INTRODUCTION

1.3 Cavings versus Cuttings

Roller cone bit (crushes rock) PDC bit (shears rock)

Cuttings are pieces of rock that are broken away by the action of the drill-bit teeth. Cuttings are screened out of the liquid mud system at the shale shakers and are typically monitored for composition, size, shape, colour, texture,

hydrocarbon content and other properties by the mud engineer, the mud logger and other rig-site personnel. The mud logger is normally responsible for

capturing samples of cuttings for subsequent analysis and well records. Polycrystalline Diamond Compact (PDC) drill bits create cuttings that have characteristic bit teeth marks on the ribbon-like cuttings.

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1 INTRODUCTION

1.4 Cuttings by Bit Type – Reamers vs. Drill Bit

Reamers can create cuttings of varying shapes, sometimes much larger than the drill bit cuttings.

Cutting from near-bit reamer with 19 mm cutters (left image) and PDC bit with 13 mm cutters (right image)

Mixture of bit and reamer cuttings from Thunder Horse MC775-1 well, 14,119 ft MD from the 14-1/2 in x 17 in hole interval

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1 INTRODUCTION

1.5 Cavings Overview

1.5.1 Borehole Cavings – Identification and Monitoring Guide

The descriptions in this section are provided to facilitate accurate identification of cavings types. This identification should lead to a correct diagnosis of the

wellbore instability mechanism being experienced and provide a decision basis for the best remedial action.

1.5.2 Cavings Definition

Cavings are rock fragments from the borehole wall caused by failure of the wellbore some time after drilling. Cavings are predominantly shales, though other lithologies may be affected depending on the wellbore failure mechanism and the rock type exposed in the borehole.

1.5.3 Cavings Size

Typical cavings size average 1 to 3 in (2.5 to 7.6 cm), but can range in size from 1/2 in to over 4 in (1 cm to over 10 cm). Very small, coffee grounds size/shape cavings are usually an indication of reworked cuttings or cavings that have spent time in the wellbore before being transported to surface. Very large cavings are produced as a result of bedding collapse or naturally fractured formations.

1.5.4 Cavings Texture

Cuttings and cavings should ideally be hard. Soft cavings or cuttings usually indicate some adverse mud/shale chemical reaction is taking place. This can be caused by a chemical or salinity concentration incompatibility between the drilling fluid and the formation.

1.5.5 Identifying the Cavings Type - Cavings Shape

The correct interpretation of the cavings shape is important to:

• determine the cause of wellbore failure.

• determine the optimal remedial action required.

Cavings can often be used to diagnose wellbore failure mechanisms. Correct cavings identification can mitigate well risk by facilitating remedial actions before the instability mechanism becomes unmanageable.

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1 INTRODUCTION

1.6 Identifying Cuttings from Cavings

Mixture of cuttings and cavings from 16,180 ft MD in a high-angle well

Comparison between cuttings and cavings generation rates from a well in Colombia

Various reporting formats are used to record the cuttings volume and cavings produced while drilling. Section 7 (Cavings Documentation) in this Guide provides detailed information on this topic. Additionally, use of the cavings mechanisms check sheet (Section 7) can aid in identifying cavings types encountered.

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1 INTRODUCTION

1.7 Quantification of the Amount of Cavings

Periods of caving are sometimes referred to as a Cavings Attack. The degree or severity of the cavings may be defined on a scale of 0 to 5. The table below may be used as a guide to assess the amount of cavings being produced.

The following points are critical to the understanding of this table.

• The Cavings Attack Severity Scale is intended only as a guide to assign a

measure to the proportion of cavings that might be seen at the shale shakers. The cavings rate can indicate the severity of borehole failure, coupled with the efficiency of hole cleaning. The cavings rate should be assessed every 30 min and is the time required to fill a calibrated (e.g. 5-litre or 2-gallon) bucket placed underneath the shakers. This method may seem crude, but it is versatile in terms of the number of different models of rig that it can be applied to and is a reliable quantitative measure of the volume of cuttings and cavings coming over the shakers.

• The tolerance of wellbore instability will depend much upon the well and

formation type being drilled. In extended reach wells, for example, the

longer lag time associated with cavings being transported to surface, and the greater challenges for hole cleaning, means that the downhole problem can be more severe than that seen at surface. Here, prompt intervention will be required much sooner than in a vertical well intersecting the same

formations.

• A small proportion of background-level angular cavings (<5% of the total

rock material) may occur when using a mud weight that is at the minimum necessary for permissible and manageable breakouts in the hole section being drilled. This cavings volume should reduce significantly when circulating more than two-times the hole volume without drilling ahead. There should be no tight-spots or overpulls when coming off-bottom with the drillpipe under this condition.

• Minor breakouts along the borehole length that produce this small volume of

angular cavings may be permissible. But monitor drilling parameters for any evidence of a more severe problem developing at a localized weak-spot or problem zone in the well. In long hole sections the use of paleontology (paleo) may help with identifying the lithologies from which the cavings have come.

• When tripping out of the hole, tight-spots or overpulls on the drillpipe indicate

a developing problem. When running back in the hole, tight-spots from bridging, taking weight on the pipe, or recording fill on bottom again indicate a developing problem. Under these situations more proactive management of hole cleaning and increases in mud weight may be necessary.

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1 INTRODUCTION

Cavings Attack Severity Scale

Degree of Severity Type of Cavings Attack Description 0 None

Normal drilling conditions. Cuttings comprise almost all the material coming over the shakers. A small amount of angular cavings (less than 5% of the total rock material) may pass over the shakers when the well is being drilled with a mud weight that results in permissible and manageable breakouts. No remedial action is necessary.

1 Light

Small amounts of cavings (less than 10% of the total rock material) passing over the shale shakers particularly after trips. The volume of caved material should ease up after a few circulations. Note the cavings shape – angular cavings are permissible, but the presence of

tabular or splintery cavings indicates that the mode of borehole failure

is changing, and greater vigilance is needed. For angular cavings – manage good hole cleaning. For tabular and splintery cavings – consider increasing mud-weight.

2 Moderate

Noticeable amounts of cavings (10% to 25% of the total rock material) passing over the shale shakers after trips or during drilling. The volume of angular cavings should decrease to less than 10% after a few circulations. If not, a small increase in mud weight (0.2 ppg / 0.02 SG) is recommended. Intervention and remediation is needed if the cavings are tabular or splintery. For splintery cavings, increase mud weight. For tabular cavings take great care to clean the hole; increase mud weight also.

3 Strong

Significant amounts of cavings (25% to 50% of the total rock material) passing over the shale shakers after trips and during drilling. The cavings form a continuous layer on the shale shakers and do not ease up after a few circulations. Intervention is required – increases in mud weight and improvements in hole cleaning are necessary.

4 Severe

The shale shakers are completely covered with large cavings,

comprising 50% to 90% of the total rock material, several layers deep. Prompt intervention is required – increases in mud weight and

improvements in hole cleaning are necessary. 5 _SevereVery

A serious problem representing total borehole collapse. Shale shakers have difficulties in coping with the massive volumes of caved material, representing over 90% of the total rock volume. Major intervention is required.

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2 COMMON DRILLING PROBLEMS

2 COMMON DRILLING PROBLEMS

There are numerous causes of NPT that must be managed when drilling a well. Different downhole conditions could be a possible cause of cavings or an

unusual flow of cuttings across the shakers - see the related figure at the bottom of each page in this section, adapted from Aldred et al. (1999).

Differential sticking – typically caused by a thick filter-cake buildup across

permeable formations when the drilling mud solids are inappropriately sized. Sticking may also be an indicator of possible chemical incompatibility with some formation types if over-pulls or high friction factors are seen when running in the hole or when tripping.

Geopressure – formations where the formation pore fluid pressure is greater

than hydrostatic (ca. 0. 45 psi/ft or 10.2 MPa/km) or drilling with a wellbore fluid pressure less than the pore pressure can result in fragments of shale popping off the borehole wall. This is the classic cause of splintery cavings.

Unconsolidated zones – typically characterized by uncemented sands and

gravels (running sands). These can result in large amounts of running sands swamping the shakers or causing borehole pack-offs, particularly around stabilizers and larger bottom hole assembly (BHA) components.

Fractured or faulted zones – rock surrounding a fault or fractured zone may be

mechanically weakened making it more susceptible to instability relative to undisturbed rock. Where the rock has a pronounced intrinsic fabric or layering, failure along these pre-existing planes of weakness often produces classic

tabular-shaped cavings. Chaotic, randomly-shaped cavings from a faulted region are also referred to as rubble zone cavings.

Differential sticking Geopressure Unconsolidated zones Fractured or faulted zones

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Undergauge hole – when re-drilling a pilot hole, or underreaming an existing

borehole to a larger hole size, cuttings may possess a different size and texture to those generated when first drilling the borehole. This is not a cause of

instability and is to be expected but should be recognized as a source of different cuttings type(s). (See also Mobile formations in this section).

Key seating – when moving the drillpipe, a slot can be worn on the high- or

low-side of the borehole. This can be a cause of additional rock material removed from the borehole. The abrasion of the formation by the drillpipe may produce coffee grounds size/shape cavings that are small in size. The enlarged borehole area may also be a cause of poor hole cleaning due to reduced flow velocity which reduces the solids-carrying capacity of the annular drilling fluid through this enlarged section.

Reactive formations - often called gumbo shales, these smectite-rich rocks tend

to swell and disperse in the drilling mud when drilled with a fluid having

inadequate inhibitive properties. This can be a problem with water-based and synthetic oil-based drilling fluids. This is a problem of mud chemistry, and not of mud weight.

Mobile formations – severe undergauge borehole conditions can occur in

naturally-mobile formations, such as salt. In certain geological settings (as in Colombia), high tectonic stresses can cause ovalization of the borehole as a result in the large stress differentials. Both situations result in tight hole problems and the possibility of packing-off. This problem is typically solved by increasing the mud weight.

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Collapsed casing – where inadequate centralization or cementation exists in

casings adjacent to a mobile formation, point-loading by the formation can result in deformed or collapsed casing. Deformed casing can also occur in

high-pressure, high-temperature (HPHT) wells due to annular pressure buildup (APB) during drilling. Both casing issues are characterized by tight-spots, particularly during the passage of drill collars and stabilizers, and by the risk of packing-off. In extreme cases, metal shavings may be observed in the returns crossing the shaker.

Junk – a term normally used to describe mechanical debris resulting from the

failure of drilling or BHA components. The heavier metal debris may be difficult to remove by conventional drilling fluid circulation or by pumping sweeps. Junk remaining in the borehole may be a cause of stuck pipe, particularly when pulling back into the casing.

Cement-related problems – cement cavings can be caused by cement

remaining in the shoe-track of the previous hole section, or coating the borehole wall of the current hole when the prior casing failed to be run to bottom. Cement cavings are distinguished from rock cavings by their arbitrary shape and colour difference. Where slabs of cement fall off the borehole wall – possibly by drill-string vibrations – this can lead to a hole cleaning and stuck pipe problems.

Drill string vibration – when drilling / reaming parameters (weight-on-bit,

rate-of-penetration (ROP), etc.) are not optimized, drill-string vibrations may result. In these instances, the drillpipe, collars or stabilizers may violently contact the

borehole wall. Drill string vibration can cause borehole wall destabilization or a disturbance in built up cuttings beds. In fragile formations – such as those surrounding faults – unwanted drill string vibration can contribute to further cavings and instability.

Collapsed casing Junk Cement-related

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Wellbore geometry – when drilling with a well trajectory deviated at more than

35° from vertical, or where harder rock layers are interbedded with weaker rock, wellbore geometry can result in ledging problems. With ledging, casing collars, BHAs or stabilizers can become hung-up on irregularities on the borehole wall. Cavings and poor hole cleaning can result when these ledges are removed by the action of the drillpipe. Another example of the well geometry affecting

wellbore instability is when the well deviation is within 20° of the bedding-parallel direction. In certain layered shale formations, breakup of the layers can occur resulting in tabular cavings falling from the top and bottom of the borehole.

Poor hole cleaning – can be confused with problems of wellbore instability. Has

the borehole packed-off because of excessive cavings caused by wellbore instability, or is the flow-rate being used insufficient to clean (or remove) the cuttings produced while drilling? Distinguishing between cavings and cuttings coming over the shakers, particularly after pumping viscous sweeps or high density pills, is an important diagnostic to identifying hole cleaning problems and/or borehole instability. Enlarged sections of the wellbore will result in a hole cleaning problem across these intervals as the flow velocity is reduced. During reductions in flow velocity, response timing is important to minimise the instability occurring and mitigate a future hole cleaning problem.

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3 SPECTRUM OF INSTABILITY MECHANISMS

3 SPECTRUM OF INSTABILITY MECHANISMS

Schematic of instability mechanisms with varying rock fabric

The cavings produced from wellbore instability can occur due to a variety of mechanisms. The fabric of the rock has a big influence on the mode of instability, as does the well trajectory relative to bedding.

In uniform rock with little bedding, or in near-vertical wells drilled almost perpendicular to bedding, wellbore breakouts are formed (see (a) in figure above). This mode of instability results in angular cavings.

In rock with more pronounced bedding, wellbore failure along planes of

weakness results in tabular cavings. This is most severe in wells drilled parallel to bedding where a roof collapse mode of failure can occur (see (b) in figure above).

In rubblized zones surrounding faults and salt bodies, fluid invasion between the rock fragments can result in runaway instability (see (c) in figure above) as the rock surrounding the borehole enlarges over time.

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3.1 Wellbore Breakouts

Wellbore breakout and examples of angular cavings – image from Ewy & Cook (1990)

Wellbore breakouts are regions of failure on the sides of a borehole created when the mud weight is not sufficient to resist the concentrated stresses that form around the borehole when it is drilled. The cavings from this mode of failure are angular in shape. One side of the cavings may be a freshly-cut borehole surface. This mode of instability can develop quickly after the formation is first drilled if stresses are high relative to the drilling mud weight. This is the clearest indicator of not having enough mud weight. In some circumstances, wellbore breakouts can develop higher up in the open-hole section as a consequence of time-dependent effects. These may include mud returns heating formations shallower in the section, or from salinity incompatibility between the drilling fluid and the formation.

Remediation: Raising mud weight in 0.2 ppg / 0.02 SG increments is usually

sufficient to overcome this mode of borehole failure. After each mud weight increase, monitor the flow of angular cavings over the shakers. Once formed, breakouts can typically become stable with modest increases in mud weight.

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3.2 Roof Collapse

Examples of roof collapse shale failure in fissile shale – image from Økland & Cook (1998)

Some shales have a slate-like, fissile fabric, with many relatively thin layers. These shales have a pronounced strength anisotropy. They can be strong perpendicular to bedding, but weak parallel to bedding.

In high-angle wells drilled at a deviation that is within 20° of the bedding-parallel direction this difference in strength can result in the sides of the borehole being stable, but the top and bottom of the borehole failing in shear. This results in the roof collapse mode of failure. The cavings from this mode of failure are

characteristically tabular. Two of the surfaces will align with the bedding. This mode of instability can be aggravated by incorrect mud chemistry or filter-cake characteristics, as the tabular rock fragments can become destabilized by mud invasion.

Remediation: There is no guaranteed remediation measure to wellbore

instability caused by this roof collapse mechanism once it has started. The key is drilling these shales initially with a mud weight sufficient to prevent this mode of failure. If this problem exists, good hole cleaning practices and controlled drilling are important. Clean the hole and try to avoid swabbing or surging the formation. A small increase in mud weight (0.2 ppg / 0.02 SG) may temporarily relieve the problems of roof collapse, but any benefit is likely to be short-lived. In future wells alter the well trajectory to avoid drilling within 20° of the bedding parallel direction.

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3.3 Rubble Zones

As the name implies, rubble zones comprise the mechanically weakened region of rock that might surround a fault or around a salt body or other diapir. It

consists of rock fragments that are held in place by the friction acting between the pieces of rock.

After drilling, drilling fluid filtrate may penetrate the network of cracks between the individual rock fragments. When this happens, the mud overbalance is removed and the drilling fluid does not provide support to the borehole wall.

There is a key stone effect as rock fragments fall into the borehole. As the

overall rock mass becomes more fragile, any surging of the borehole can lead to massive instability, frequently resulting in a stuck pipe problem.

Remediation: There is no guaranteed remediation measure to wellbore

instability through a rubble zone. Good hole cleaning practices and controlled drilling are the key. Existing rock fragments need to be cleaned from the

borehole without building up a bed of cavings that will result in a pack-off. Swab and surge pressures need to be minimised to avoid destabilizing the formation further. Adding gilsonite, SteelSeal or other fine asphaltene additives may help reduce fluid loss and plug the fine fractures. A small increase in mud weight (0.2 ppg / 0.02 SG) may help temporarily, but any benefit is likely to be short-lived. The most effective prevention measure is to plan the well path to minimise the length and time that the rubble zone section is left exposed in the open borehole. Use a flat rheology drilling fluid to reduce downhole pressure fluctuations. Treat the drilling fluid with suitable fluid loss additives prior to drilling the rubble zone.

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4 CAVINGS BY WELL TYPE

4 CAVINGS BY WELL TYPE AND DRILLING

SCENARIO

Cavings volumes from North Sea ERD Well

Certain well types and drilling situations have a higher likelihood of specific types of borehole instability. This section covers these well types and drilling

situations. The types of cavings to look for are described; the reasons for the mode of failure are discussed; and remedial actions are recommended. The well types and drilling situations covered in this section are:

• High angle wells,

• Extended Reach Drilling (ERD) wells,

• Drilling into pressure ramps,

• Underbalanced drilling,

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4.1 High Angle Wells

For the purposes of this guide, any well drilled at greater than 35° deviation from vertical is a high angle well. High angle wells require more attention to good hole cleaning, and the adoption of drilling practices that effectively remove the cuttings and cavings from the borehole. The buildup of a cuttings and cavings bed can lead to overpulls, pack-offs and lost-circulation if the pack-off causes a pressure surge that exceeds the fracture gradient in the well.

One of the greatest risks to wellbore stability occurs when drilling within 20° of the bedding-parallel direction. In steeply-dipping formations, this situation can occur even in wells with modest deviations. Under these conditions, the wellbore can fail along bedding planes on the rock, or along pre-existing cleavages in fractured or jointed rock.

Examples of bedding planes and pre-existing cleavages seen in tabular cavings Remediation: The key is drilling with a mud weight sufficient to prevent this mode of failure. If this problem occurs, good hole cleaning practices and controlled drilling are important. Clean the hole and try to avoid swabbing or surging the formation. A small increase in mud weight (0.2 ppg / 0.02 SG) may temporarily relieve the problems of failure along bedding-planes, but any benefit is likely short-lived.

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4.2 Extended Reach Wells (ERD)

Examples of cavings and cuttings recovered from an extended reach well

Extended reach wells can be considered a special case of a high-angled well. Due to longer well lengths, the time taken for cavings to reach the shakers is correspondingly longer too. There may be fewer large cavings seen at surface, as these may become ground up when transported up the borehole from depth. For these wells, it may be too late to wait until significant amounts of cavings are seen at surface. Early detection and intervention is very important. Monitor the shakers for isolated large cavings, and look for changes in the total cavings volume that might tell you that a problem is developing. Downhole data will likely show the first indications of a wellbore instability problem. Monitor drilling trends, such as downhole annulus pressure, pick-up weights, torque and drag, and

ensure that good hole cleaning practices are being followed.

Remediation: Angular or tabular cavings are most commonly produced when

drilling ERD wells. If drilling nearly parallel to faults, rubble zones may be

problematic. At the onset of any indicators of instability (e.g. the hole beginning to pull tight), take time to ensure the borehole is cleaned of cuttings and cavings before drilling ahead. Be prepared to increase mud weight by small amounts if problems persist.

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4.3 Drilling Pressure Ramps

Splintery and angular cavings produced when drilling into a pressure ramp in the Mad Dog Field

When drilling into a pressure ramp, it is often the case that the weight-up schedule adopted is in response to the drilling indicators of becoming close to balance, or even underbalanced. In low- to medium-strength rocks, a mixture of both splintery and angular cavings will be produced as the mud overbalance is

reduced. Watch for this. Splintery cavings only come from very strong rocks.5 ft

Remediation: Increase mud weight by small amounts – 0.3 ppg / 0.04 SG – in

response to cavings seen at surface and keep track with the increasing pore pressure. Avoid going underbalanced where permeable sands are expected.

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4.4 Underbalanced Drilling

Underbalanced drilling is often performed in relatively strong rocks with few wellbore instability problems. This is because the rock is usually strong enough in compression to withstand the stress buildup around the borehole as it is being drilled. The rock is also strong enough in tension to withstand the effects of the higher formation pore pressure that wants to spall (chip or flake) fragments from the borehole wall.

Splintery cavings from underbalanced drilling

When it does happen, instability when underbalanced is usually first seen by the production of splintery cavings. These can be long slivers, several inches or centimetres in length, or very tiny splintery fragments. The onset of splintery cavings, when knowingly underbalanced, is an indicator that weaker rock is being drilled and that there is a risk of pack-off occurring. This is a real possibility if air-drilling, where cavings transport may be less efficient than when using a

lightweight fluid. Here, the larger angular cavings may not be cleaned from the well.

Remediation: Increase downhole pressure or mud weight by small amounts if

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4.5 Drilling Faults and Rubble Zones

A mechanically weakened zone is sometimes encountered when drilling close to a fault in rocks of medium- to high-strength. This is often referred to as a rubble zone. Similarly, broken up rocks can be found when drilling around or

immediately below salt bodies.

Rubble zone cavings are often chaotic in size and shape. In certain geologic conditions, cavings from fault zones can have a shiny, polished surface, worn

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smooth by the relative movement of the rocks across the fault. Rubble zone material can be relatively fragile once a borehole is drilled through it. Continued removal of cavings makes a rubble zone prone to catastrophic borehole collapse that is likely to lead to a stuck pipe incident. Rubble zones can be destabilized easily by the effects of swab and surge pressure fluctuations.

Remediation: An increase in mud weight and good hole cleaning will help, but

there is no long-term fix for rubble zone instability. Adding gilsonite, SteelSeal or other fine asphaltene additives may help reduce fluid loss and plug the fine fractures. A small increase in mud weight (0.2 ppg / 0.02 SG) may help temporarily, but any benefit is likely to be short-lived. Minimizing the risk by designing the well path to minimise the length and time that the fault or rubble zone section is left exposed in the open borehole is the best preventative measure. Use a flat rheology drilling fluid to reduce downhole pressure

fluctuations. Avoid using a mud salinity higher than that of the formation pore fluid; a balanced salinity fluid is preferred. This will minimise dehydration of the rock fragments comprising the rubble zone which can lead to instability. Treat the drilling fluid with suitable fluid loss additives prior to drilling the rubble zone.

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5 CAVINGS BY FORMATION TYPE

5 CAVINGS BY FORMATION TYPE

Shale is the dominant formation within the overburden, and it can come in many different forms, depending upon its strength, composition and layering. BP often drills other rock types. Sands and sandstones are encountered frequently within the overburden; these can range from being weak, uncemented and highly

porous, to very strong, hard rocks with low porosity. Large thicknesses of salt are drilled in some deepwater wells. Coal seams may be present, and in some cases these might be the hydrocarbon reservoirs themselves. Oil reserves are found in chalk and other carbonate reservoirs. Each of these rock types has its own recognizable cavings shape and description. This section discusses some of these formation types.

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BP Confidential Page 31 Exploration & Production

Technology

5.1 Salt

Because of the equal stresses that exist in salt, cavings do not occur when drilling salt. The salt cuttings will dissolve in under-saturated water-based drilling fluids. When using synthetic oil-based drilling fluids, or water-based fluids that are salt-saturated, cuttings only a few tenths of an inch, or a few millimetres in size, will be produced.

Cuttings from halite, the most common salt, are typically a light grey or cloudy cream colour. Any variance from this colour

indicates the presence of

different salt minerals. Polyhalite is known to have a red-staining colour. Tachyhydrite and

carnalite salts (found offshore Angola) also have a brown or red staining.

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5.2 Coal

Coal cavings from Nilam Field, Indonesia

Coal cavings, some as heavy as 5 kg, were recovered from the 12-1/4 in hole section at a depth of 8,500 ft (2,590 m) MD from the Nilam field, Indonesia. The mud weight had to be raised from 9.6 ppg to 10.6 ppg to stop the flow of coal.

Core from CBM Well, San Juan Basin

Core from a coal bed methane (CBM) well, San Juan Basin, USA. Sugar cube-sized cavings may be generated from coal as the cleated fabric of the coal disaggregates due to the presence of the borehole.

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5.3 Sandstone

Sandstone outcrop and disaggregated sand grains recovered at the shakers

Sandstones comprise the majority of BP’s reservoirs. Cuttings and any cavings from unconsolidated and weakly-cemented sands will disperse in the mud

system and be produced as individual grains at surface. This is typical when drilling sandstones.

In certain geologic settings caliper logs have shown that breakouts do occur in some sandstones.

If breakouts do occur, cavings are normally angular in shape. These breakouts most often occur in strong, stiff rocks in tectonically-stressed areas such as the U.S. and Canadian Rockies; in Algeria; parts of Libya; and in Colombia. There are few recorded instances of cavings problems in these sandstones. Reducing fluid loss into the formation and raising mud weight usually cures the occurrence of sandstone cavings.

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5.4 Gumbo

PDC bit suffering from bit-balling in gumbo shales

Pellets of water-softened gumbo shale due to adverse shale-influenced Interaction

Gumbo shales typically

comprise a high proportion of smectite-rich clays. This clay mineral tends to swell and become soft and sticky when exposed to water, particularly water in the drilling fluid that has a lower salt concentration than that occurring in-situ. In some formations, exposure to a different type of brine (e.g. potassium brines, rather than the more common sodium brines found in pore waters) can cause a similar adverse reaction.

Smectite-rich clay is usually found at depths shallower than 10,000 ft (3,048 m) below surface (or seabed) and where temperatures are less than 190°F (88°C). At depths and temperatures greater than this, smectite converts to the less

reactive clay, illite. Fin

Swelling and dispersion of clays in the drilling fluid can occur from other processes, though salinity imbalance is the major cause of gumbo attacks. Altering the mud type (to synthetic oil-based drilling fluids) or increasing the mud salinity usually remedies the problem of gumbo shales.

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5.5 Running Sands and Gravel

Sands and gravel

Sands and gravels as unconsolidated as the beach deposit and as extensive as that pictured above occur in some offshore locations, and are very difficult to drill through.

These running sands can be very unstable when drilled at relatively shallow depths. As these large hole sizes may be drilled riserless, the main challenge is to adequately clean the borehole of the material washed into the borehole. Very large washouts can be formed through these formations. The recommended remedial action is to isolate these formations by setting pipe.

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5.6 Basalt

Basalt is a dark-coloured, very fine-grained igneous rock derived from volcanic eruptions. Basalt flows cover about 70% of the earth's surface. Slowly cooled, basalt may crack into hexagonal columns. When drilled, tabular cavings may be produced as the columnar structure breaks apart. Increases in mud weight and reducing drilling fluid loss may help mitigate drilling problems.

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5.7 Fissile Shale

Fissile shale seen in outcrop

Borehole collapse experiment in a fissile shale

Fissile shales are quite common, and

frequently cause wellbore instability problems. Many are source rocks, and their often over-pressured nature compounds the instability problem. Examples include the Kimmeridge, Sola and Draupne Shales of the North Sea, the

Frasnian Formation of Algeria, and the HRZ and Kingak shales of the Alaska North Slope. Wellbore instability can be very difficult to

manage in these

formations, particularly in high-angle wells. In fissile shales, the thin layers de-bond after drilling and result in the classic roof collapse mode of failure.

The best solution is to optimize the well

trajectory through these shales and to design the well to minimise the exposure time of these formations before the well is cased.

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5.8 Cement from the Previous Hole Section

Cement fragments recovered along with borehole cavings

Occasionally, if the previous casing string is set off bottom, a length of cement will cover the borehole wall of the hole section currently being drilled. Cement cavings are sometimes produced when underreaming the hole, when

backreaming to the shoe, or from the mechanical impact of the drill string against the sides of the hole. Cement cavings can potentially be large in size. The red-brown cavings in the picture above are cement. These are clearly

distinguishable from the dark grey shale cavings. Cements can also be grey in colour and might not be so distinguishable from shales. Cement can be

identified by using phenolphthalein, a fluid that turns pink on contact with the alkaline cement.

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6 CAVINGS SHAPES

6 CAVINGS SHAPE

Frequently, the first

indications of a developing wellbore instability

problem will be a small number of cavings coming over the shakers along with the regular drill cuttings. Proactive cavings monitoring can prompt appropriate and timely remedial actions to prevent a minor hole enlargement from

becoming a major stuck pipe incident.

This section describes cavings shapes, the

modes of borehole failure that cause them, and suggests remedial measures that have proven effective in

stopping the problem from becoming worse.

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6.1 Overview

Typical angular cavings shape

Angular cavings, with freshly sheared surfaces, indicate borehole failure due to insufficient mud weight.

Small tabular cavings separated from a sample of recovered drill cuttings

Tabular, parallel-sided cavings indicate delamination of weak bedding planes. High-angles wells are prone to this mode of wellbore instability.

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Exploration & Production Page 42 BP Confidential A mix of splintery and angular cavings, typical of drilling into a pressure ramp

Mixture of splintery and angular cavings indicates insufficient mud weight due to

high pore pressure.

Chaotically sized and shaped cavings from a rubble zone

Large volume of cavings with multiple bedding planes indicates a rubble zone destabilized by mud invasion into a fracture network.

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6.2 Splintery Cavings

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6.2.1 Caving Mechanism

Splintery cavings are produced from the entire circumference of the wellbore as a result of drilling underbalanced through over-pressured shales or drilling too fast through low permeability rocks.

6.2.2 Diagnostic Characteristics

The cavings are splintery and elongated. Cavings may be small splinters of rock in brittle, hard formations. In high-angle wells, and in fissile formations with a pronounced bedding, splintery cavings may be larger, having split along weak bedding planes. The cavings are often twisted or propeller (toroidal) shaped.

6.2.3 Remedial Procedures

Splintery cavings are an indication of borehole instability due to underbalanced drilling. Increasing the mud weight is the usual solution. Reducing ROP may reduce the volume of splintery cavings if the cavings are suspected of being produced by drilling with a high ROP.

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Angular cavings are produced from rock shear failure. When the sheared surfaces are fresh, the cavings are from borehole breakouts (drilling induced near-wellbore shear failure). When the sheared surfaces are old, the cavings most likely come from a rubble zone.

The cavings are arrow head or triangular in shape. There may be fresh fractures on the caving surface. In cross section, the cavings are often tapered or wedge shaped. One side of the caving may be curved, corresponding to part of the cut borehole circumference.

Remediation: Raise mud weight if the pore/frac window allows. This is the most

effective remediation measure to overcome the shear-failure mode of instability that produces angular cavings. If the mud weight is already close to the fracture pressure, then angular cavings need to be managed by implementation of careful drilling procedures. Good hole cleaning is essential; follow appropriate hole

cleaning guidelines. Reduce ROP to avoid loading the annulus with cuttings and cavings. Consider changing the well trajectory in future wells.

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6.4 Combination of Angular and Splintery Cavings

Mohr’s circle schematic of borehole failure due to insufficient mud weight

Mohr’s circle schematic shows why weak shales fail in shear first when

attempting to drill underbalanced, whereas stronger shales fail in tension. Both images of cavings are notionally at the same scale. When drilling close to balanced in weak rock the cavings produced generally fall into two categories, with both distinctive splintery and angular cavings. (In stronger, more brittle rocks, only splintery cavings will be produced.) Increasing mud weight is the appropriate remedial action under these circumstances.

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6.5 Tabular Cavings

Tabular cavings are usually produced as a result of bedding collapse on the high side of the hole when the borehole angle is within 20° of the bedding-parallel (down-dip) direction.

Tabular cavings may occur due to cleavages in pre-existing fractures or joint sets.

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The cavings are tabular in shape – with a length that is greater than its thickness. The caving surfaces are bounded by pre-existing planes of weakness. The

cavings are characterized by one or more flat and smooth surfaces; parallel surfaces are common.

Sometimes tabular cavings are described as being blocky. Blocky is a confusing term, as this may imply a more cubic shape to some (describing cavings from coal beds, for example). It is recommended that cavings produced from a

bedding-collapse mode of instability be described as tabular. The picture below is a good example of a tabular shaped caving.

Remediation: A small increase in mud weight (0.2 ppg / 0.02 SG) may help

temporarily to relieve the problems, but any benefit is likely to be short-lived. Minimise fluid loss. Avoid backreaming through the unstable zone. Maintain good hole cleaning. Avoid damaging the borehole wall through excessive drill pipe rotation (>150 rpm) and severe drill string vibrations. Minimise swab and surge fluctuations which might further destabilize the formation.

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6.6 Cavings from Faults in Hard Rock

Cavings from a fault zone showing slickenside marks

The cavings shown are from a well in Colombia. These cavings are unusual because of their slickensided characteristics. Slickensides are parallel striations (scratches) on the rock surfaces produced by the relative motion across opposite sides of fault planes. These cavings are mostly seen in hard rock fault zones. An increase in mud weight and good hole cleaning will help minimise the flow of cavings, but there is no reliable long-term fix for fault zone instability.

Avoid using a mud salinity higher than that of the formation pore fluid. A balanced salinity fluid is preferred. This will minimise dehydration of the rock fragments comprising the fault rubble zone which can lead to instability. It is recommended that the drilling fluid is treated with suitable fluid loss additives prior to drilling the fault zone.

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6.7 Reworked Cavings

Rounded, Worn Cavings

Rounded, worn cavings are cavings that have been reworked by the drill string while transported up the borehole. These cavings represent an intermediate stage between fresh cavings and coffee ground size/shape cavings.

Identification of the original cavings shape from these reworked cavings is difficult as the characteristic shapes may have become rounded. Originally angular cavings can become rounded pebbles by the time they reach surface.

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6.8 Remedial Action Summary

6.8.1 Angular cavings from fresh wellbore failure:

• Raise mud weight if pore/frac drilling window allows.

• Manage hole cleaning in order to reduce equivalent circulation density (ECD), especially if little extra mud weight window is available.

• Optimize well trajectory in future wells.

6.8.2 Tabular cavings from weak bedding planes:

• Raise mud weight slightly – the cavings rate should decrease.

• Ensure good hole cleaning.

• Minimise backreaming.

• Reduce hole angle in future wells.

• Consider changing the mud chemistry formulation in future wells.

6.8.3 Cavings from multiple fracture network and rubble zones:

• Raise mud weight slightly – this should result in a short-term decrease in cavings rate.

• Minimise fluid penetration into the cracks – minimise the fluid loss coefficient of the drilling mud.

• Use deformable crack blocking additives (e.g. asphaltenes – SteelSeal, BaraSeal …).

• Apply gentle drilling practices - minimise surge and swab pressure fluctuations, drill string vibration and excessive rpm.

• Minimise backreaming.

• Reduce hole angle in future wells.

• Avoid using a drilling fluid salinity higher than that of the formation pore fluid. A balanced salinity mud formulation is recommended.

6.8.4 Splintery cavings:

• Increase mud weight.

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7 CAVINGS DOCUMENTATION

7 CAVINGS DOCUMENTATION

7.1 Cavings Mechanism Check Sheet

The cavings mechanism check sheet may be used to identify the source of the cavings. It requires information to be provided on the following:

• the regional geologic setting

• a basic description of the cavings shape

• a basic description of the well

• a description of any other drilling parameters indicative of wellbore instability

• information of bedding dips and well/bedding intersection angles

• whether gas indicators are increasing

• information on the mud type

• the timing of the onset of the wellbore instability problem

Based on the above, a scorecard system will indicate the most likely cavings mechanism at play in the well.

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7.2 Cavings Daily Reports

Cavings Daily Report

A cavings / cuttings daily report kit is available to document the cuttings and cavings generated from the well on a periodic basis. The figure above shows

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cuttings and cavings records, with photographs, on a 2-hour (approximate) basis. It provides a snapshot record of how a wellbore instability problem might evolve over time. The activity box to the right of the photograph records the

circumstances that apply at the time the photograph was taken – e.g. cuttings produced when drilling ahead, or cavings produced after pumping a sweep. The cavings daily report is normally completed by the mud engineer, mud loggers or other qualified and designated rig site personnel as part of their normal daily duties. A photographic record should be taken of cuttings and cavings volumes before and after pumping sweeps; prior to, during, and after drilling known problematic formations. Photographs of cuttings and any cavings during regular drilling operations either on a depth basis, or on a time basis (e.g. hourly), should also be taken. Not all photographs have to be included in the report, and the formal reporting should be kept to an acceptable level. Since hole conditions are subject to rapid change, taking frequent cavings photographs is recommended, especially in critical wells.

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7.3 Cuttings Size

Regular cuttings from 1,300 m depth

Regular cuttings from 2,180 m depth

In addition to describing cavings shape, drill cuttings size or changing trends in the cuttings size should be monitored. Typically, as the hole becomes deeper there is a corresponding decrease in the drill cuttings size. This change is influenced by the increasing overbalance with depth and increasing ECD with progressive reduction in hole sizes. If a pressure ramp is drilled without

increasing the mud weight, then the hydrostatic mud overbalance is reduced. Shale cuttings in particular will typically increase in size in response to this reduced overbalance.

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7.4 Mud Type & Impact on Cavings Quality

Many of the cavings pictures reproduced in this Guide are from wells drilled with oil-based drilling fluids. When using oil-based fluids, adverse chemical reactions between the drilling fluid and the rock are less likely, and so the cavings will arrive at surface in better shape than when water-based drilling fluids are used. Cavings description and analysis is easier when using very inhibitive drilling fluids. When using water-based drilling fluids, the cavings shapes will tend to become more rounded, as some dispersion of the cuttings and cavings will occur as they are transported up from the wellbore. Care should be taken when

identifying cavings shape when using water-based drilling fluids.

Cavings recovered from a Kapok, Trinidad well, drilled with water-based drilling fluid

7.5 Collection and Preservation of Cavings

Carefully collect and preserve all cavings for further description and analysis. For shale cavings, avoid washing or contact with water. It is recommended that the surplus mud is wiped off the cavings and rinsed in base oil or diesel. The cavings lithology and shape can then be recorded and the cavings

photographed. After this, the cavings should be placed on paper to dry. Once thoroughly dried, store the cavings in a suitable container, either for further studies or as archived reference material.

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7.6 Tips for Photographing Cavings

Caving photographic records are an indispensable part of the record keeping process. Good quality photographs should be included in the daily cutting description sheets. The photos can help describe and document wellbore instability problems which may occur. This section contains tips on

photographing cavings.

NOTE – Be sure to verify rig HSSE requirements and restrictions for photographic equipment (camera and flash) prior to taking any

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Get the cavings in focus for a proper description. Include a scale so cavings size can be assessed.

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A good photo of different cavings types (angular and splintery), but the image quality gets poorer when the image is enlarged to get a detailed close-up of the cavings. Take the photo as close to the cavings as your camera permits, while still keeping the cavings in focus.

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A sharp, clear photograph, though no scale was included to assess the cavings size.

A clear photo, but it lacks detail. A good representative sample of cavings is seen among the cuttings.

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Taking top view and side view photographs is a good practice. This helps identify angular from tabular cavings, and brings out the three-dimensional size and shape of the cavings.

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8 CONTACTS

8 CONTACTS

The contacts listed below form part of BP’s Exploration & Production Technology Group (EPT). They are experts in wellbore stability or drilling fluids. In case of any questions on cavings identification, please contact one of the following. They would also be happy to receive photographs of interesting or unusual cavings that you would like to submit to a future update of this guide.

Stephen Willson,

Drilling Rock Mechanics Advisor, EPT Houston, USA.

+ 1 281.366.5842. [email protected] Stephen Edwards,

Geomechanics Engineer, EPT Houston, USA.

+ 1 281.366.4679. [email protected] Nigel Last,

Senior Advisor, EPT Sunbury, UK.

+44 (0)19 3273 9515. [email protected] Juan-Carlos Rojas,

Drilling Fluids Specialist, EPT Houston, USA.

+1 281.366.4605; [email protected] Jianguo Zhang,

Rock Mechanics/Fluids Specialist, EPT Houston, USA.

+1 281.366.6724; [email protected] Bryan Chambers,

Senior Advisor, EPT Aberdeen, UK

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9 REFERENCES

9 REFERENCES AND WEBSITES

9.1 References

Aldred et al. (1999), Managing drilling risk, Oilfield Review, Summer 1999, 2-19. Ewy, R.T. and Cook, N.G.W. (1990), Deformation and fracture around cylindrical openings in rock – II. Initiation, growth and interaction of fractures, Int. J. Mech. Min. Sci. & Geomech. Abstr., Vol. 27, No. 5, pp. 409-427

Økland, D. and Cook, J. (1998), Bedding-related instability in high-angle wells, paper SPE/ISRM 47285, presented at the SPE/ISRM Eurock’98 Conference, Trondheim, Norway, 8-10 July, 1998.

9.2 Websites

The following BP Intranet websites provide useful additional information on wellbore stability, well planning and execution, and the reduction of formation-related NPT.

Wellbore Stability Website: http://wbs1.bpweb.bp.com/

Pore Pressure/Fracture Gradient Community of Practice:

http://ppfg.bpweb.bp.com/

Geoscience Well Planning & Execution Community of Practice:

http://wpen.bpweb.bp.com/

Drilling & Completions Knowledge Exchange – Wellbore Stability Global Recommended Practice:

https://epti.bpglobal.com/C5/C6/Wellbore%20Stability

Drilling & Completions Knowledge Exchange – Rig Site Tools:

https://epti.bpglobal.com/C0/C5/Rig%20Site%20Tools

Subsurface and Wells Networks and Communities of Practice:

http://ssw.bpweb.bp.com/Default.aspx?tabid=430

EPT Drilling & Completions Knowledge Exchange:

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Cavings Field Guide