Cavings Field Guide

Cavings Field Guide AN AID TO MANAGING WELLBORE STABILITY IN REAL TIME April, 2008

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CAVINGS FIELD GUIDE CONTENTS

Contents Foreword ....................................................................................................................................4 1 Introduction – The Need for Cavings Monitoring .................................................................6 1.1 1.2 1.3 1.4 1.5 1.6 1.7

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Cavings Analysis ...................................................................................................................................................... 6 Cavings Identification................................................................................................................................................ 7 Cavings versus Cuttings ........................................................................................................................................... 8 Cuttings by Bit Type – Reamers vs. Drill Bit ............................................................................................................. 9 Cavings Overview................................................................................................................................................... 10 Identifying Cuttings from Cavings ........................................................................................................................... 11 Quantification of the Amount of Cavings................................................................................................................. 12

Common Drilling Problems................................................................................................14 Spectrum of Instability Mechanisms ..................................................................................18 3.1 3.2 3.3

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Wellbore Breakouts ................................................................................................................................................ 19 Roof Collapse ......................................................................................................................................................... 20 Rubble Zones ......................................................................................................................................................... 21

Cavings by Well Type and Drilling Scenario......................................................................22 4.1 4.2 4.3 4.4 4.5

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High Angle Wells .................................................................................................................................................... 23 Extended Reach Wells (ERD)................................................................................................................................. 24 Drilling Pressure Ramps ......................................................................................................................................... 25 Underbalanced Drilling ........................................................................................................................................... 26 Drilling Faults and Rubble Zones............................................................................................................................ 27

Cavings by Formation Type...............................................................................................30 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

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Salt ......................................................................................................................................................................... 31 Coal ........................................................................................................................................................................ 32 Sandstone............................................................................................................................................................... 33 Gumbo.................................................................................................................................................................... 34 Running Sands and Gravel..................................................................................................................................... 35 Basalt...................................................................................................................................................................... 36 Fissile Shale ........................................................................................................................................................... 37 Cement from the Previous Hole Section................................................................................................................. 38

Cavings Shape ..................................................................................................................40 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

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Overview................................................................................................................................................................. 41 Splintery Cavings.................................................................................................................................................... 43 Angular Cavings ..................................................................................................................................................... 45 Combination of Angular and Splintery Cavings....................................................................................................... 47 Tabular Cavings...................................................................................................................................................... 48 Cavings from Faults in Hard Rock .......................................................................................................................... 50 Reworked Cavings.................................................................................................................................................. 51 Remedial Action Summary ..................................................................................................................................... 52

Cavings Documentation ....................................................................................................54 7.1 7.2 7.3 7.4 7.5 7.6

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Cavings Mechanism Check Sheet .......................................................................................................................... 54 Cavings Daily Reports ............................................................................................................................................ 56 Cuttings Size........................................................................................................................................................... 58 Mud Type & Impact on Cavings Quality.................................................................................................................. 59 Collection and Preservation of Cavings .................................................................................................................. 59 Tips for Photographing Cavings.............................................................................................................................. 60

Contacts ............................................................................................................................66 References and Websites .................................................................................................68 9.1 9.2

References ............................................................................................................................................................. 68 Websites ................................................................................................................................................................. 68

Notes Exploration & Production Technology

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CAVINGS FIELD GUIDE CONTENTS

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CAVINGS FIELD GUIDE FOREWORD

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, mudloggers 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 formationrelated 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

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Tom Christopher, Drilling Engineering Excellence Network Director

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CAVINGS FIELD GUIDE 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 cleaning, increase mud weight, or change the mud chemistry formulation?

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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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CAVINGS FIELD GUIDE 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.

Samples of cuttings produced by a PDC bit

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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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CAVINGS FIELD GUIDE 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. NOTE: More than one cavings mechanism may occur in the open hole.

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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. BP Confidential

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CAVINGS FIELD GUIDE 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 (