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Society of Petroleum Engineers Distinguished Lecturer Programwww.spe.org/dl

How to Stabilize and Strengthen the Wellbore during Drilling OperationsWellbore during Drilling Operations

Dr Fred GrowcockDr. Fred Growcock

Society of Petroleum EngineersSociety of Petroleum Engineers Distinguished Lecturer Programwww.spe.org/dl

Presentation Outline

The Lost Circulation Problem

Strategy for Managing Lost Circulation

Wellbore StrengtheningWellbore Strengthening

Wellbore Stress Enhancement

Application Strategies on the Rig

Case HistoryCase History

Summary

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The Lost Circulation Problem: High-Risk Operations

eigh

t

Stable Wellbore

Fractured Wellbore 

Pressure Gradient / Mud Density

Wei

ght

Mud

We Stable Wellbore

Collapsed Wellbore

Weak zone (depleted)

Frac Gradient

Mud

Mud

W

Well Inclination0 30 60 90

Deviated Wellbores

( p )Weight

Pore Pressure

Depleted ZonesDepleted Zones

Deep Water

The Lost Circulation Problem

• Estimated industry cost in the Gulf of Mexico:$1 billion/yr. Worldwide: ~ $ 2 to 4 billion/yr. y y

• On average, 10-20% of the total cost of drilling anHTHP well is expended on mud losses (U.S.Department of Energy)Department of Energy).

• No consistent approach to manage lost circulation.• Nearly 200 products offered by 50 drilling fluidNearly 200 products offered by 50 drilling fluid

companies to control lost circulation.

A comprehensive Lost Circulation mitigation and prevention program is required. g

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Managing Lost Circulation: A 4-Tiered Strategy

6

Tier I. Best Current or New

Accurate, precise Geomechanics Model, esp.Pore Pressure / Fracture Gradient

Minimum and invariant Equivalent Circulating Density (ECD)• Accurate hydraulics profile for pre-drilling & at well site

• Good hole-cleaning practicesg p

• Optimized solids control equipment

Managed Pressure DrillingManaged Pressure Drilling

Casing While Drilling

Expandable Casing/Liner7

Managing Lost Circulation: A 4-Tiered Strategy

8

Tier II. which Minimize Losses:

WBM:Full pressure shielded from tip by

solids filter cake buildup.

OBM/SBM:Full pressure at fracture tip.

based on Morita et al, SPE 20409

Tier II. which Minimize Losses:

High Low-Shear-Rate Viscosity (LSRV)Reservoir FluidsReservoir FluidsUnderbalanced FluidsAphron and Mixed Metal Fluids

Minimum and Invariant ECDSoluble or Micronized Weighting MaterialTemperature Insensitive RheologyTemperature-Insensitive Rheology

Wellbore IsolationSilicate, Gilsonite Fluids

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Managing Lost Circulation: A 4-Tiered Strategy

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Tier IV. in Pills or Whole MudContingency: losses in high perm zones/fractures

Blends of materials of

Contingency: losses in high-perm zones/fractures

Optimum Bridging Agent Blend for FluidBlends of materials ofdifferent sizes, shapesand properties;

D10 TD50 TD90 T

F0.8

0.9

1.0

Particle size distributionmatches distribution ofopenings in rock

BrandMudA=PuB=PuC=SnD=Om0.6

0.7

ize

Dis

trib

utio

n

openings in rock;

Used in squeeze orsweep treatments; 0.3

0.4

0.5C

umul

ativ

e Pa

rtic

le-S

sweep treatments;

May be used one timeor on regular basis Calc

Avg0

0.1

0.2

C

gwhile drilling ahead.

12

Avg Max 1x10 -2 1x10 -1 1x10 0 1x10 1 1x10 2 1x10 3 1x10 40

Particle Size (microns)

Managing Lost Circulation: A 4-Tiered Strategy

13

vs

Fl kFlakes

Most Fibers Plates

Insoluble Salts

Plates

L i t

MarbleSynthetic Graphite

LaminatesHard,

Granular Fibers

Swellable or Reactive Materials

Soft Granules14

Strategies for Strengthening the Wellbore

• Physical– Wellbore Isolation: Swellable or Reactive Materials

F t Ti I l ti (F t P ti R i t )– Fracture Tip Isolation (Fracture Propagation Resistance)• Chemical

Decrease clay swelling pressure (inhibitors such as K+– Decrease clay swelling pressure (inhibitors such as K+ and invert emulsions)

• Thermale a– Increase temp of drilling fluid above bottom-hole static

temp: stiff formations, high thermal expansion• Mechanical

– Fracture Closure Stress– Hoop Stress Enhancement (Stress Cage)

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Fracture Propagation Pressure (FPP)Original Formation Breakdown Pressure (FBP)

Leak ‐OffPressure(LOP)

FBP after Wellbore Strengthening

e

Fracture Propagation Pressure (FPP)(LOP)

Fracture Gradient  (FG)

Pressure Fracture Closure Pressure (FCP) 

(= Minimum  Horizontal Stress, Shmin)

P

Ti V l P d ( )Time or Volume Pumped (constant pump rate)

Fracture Closure Stress

• High-fluid-loss treatment for existing fractures

A li d hi h fl id l ill h l d• Applied as high-fluid-loss pills or whole mudtreatment (Drill-n-Stress)

• Pill may be water-based in a non-aqueoussystem

• May follow with cross-linked polymer plugs orcement

Fracture Closure Stress

As existing fracture is widened, particles are forced deep within the fracture.

Liquid leaks from the slurrythrough the fracture walls or tip.

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Fracture Closure Stress

As the slurry deliquifies, it consolidates.

The residual solid plug supports the fracture and isolates the tipisolates the tip.

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Hoop Stress Enhancement

• Wellbore is pre-fractured and sealed to preventsubsequent fracturing

• Mathematical models are used to design the : calculate the required fracture width and particle size q pdistribution of

• is added at moderate concentration whileis added at moderate concentration whiledrilling (involves solids control management):Continuous addition is much more effective than pillsp

• The formation is pressured every stand to build hoopstress or ECD is raised to desired levelstress, or ECD is raised to desired level

Hoop Stress Enhancement

As fracture is widened, large particles are forced inand wedged in the fracture mouth to create bridge

Smaller particles come in behind the bridgeand seal the openingg

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Hoop Stress Enhancement

Trapped fluid filtersthrough permeable walls of fracture

The pressure drops and compressive p p pforces are transferred to WSM

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Hoop Stress Enhancement Application Cycle

Suitability of Treatment Data CollectionModeling of FractureCreation and Fillingg

Output:-Fracture dimensions-Formulation of LossPrevention MaterialLessons Learned

Fluids Program-Type of treatment:

*Pills*ContinuousData Collected

-Handling WSM-Fluid property management-Monitoring tools

• Slot Tester• Sieves

-Mud Pump restrictionsD h l t l t i ti

ProgramImplementation

Data Collected&Analyzed

-Downhole tools restriction-Lost circulation contingency

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Design of Hoop Stress Enhancement

• Fracture Width required to achieve desired wellbore pressure – elastic model:

Desired Equiv. Circ. Density, or Wellbore Pressureq y

Rock Properties – Poisson’s Ratio, Young’s Modulus, Minimum Horizontal Stress (or Fracture Gradient)

Well Geometry – Depth, Inclination, Hole Size

• Acceptable types of WSMp yp• WSM particle size distribution (PSD) and

concentration

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AcceptableLarge, Tough and GranularLarge, Tough and Granular

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Unacceptable

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Continuous Addition of to whole mud is best

• For short intervals, bypass the shale shakers

• For long intervals screen out the WSM with the shakersFor long intervals, screen out the WSM with the shakers

• The most effective solution: an WSM Recovery System

Screens out drilled cuttingsScreens out drilled cuttings

Recovers and l WSMrecycles WSM

Screens out fines

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Screens out fines

Monitoring Hoop Stress Enhancement

• Techniques to be used at the rigsite:– Permeability Plugging Testsy gg g– Wet Sieve Analysis– Sand kit determination

• Techniques available at shore bases:Techniques available at shore bases:– Laser Light-Scattering Particle Size

Distribution– Fracture Sealing and Wellbore

Strengthening tests

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eepwater Gulf of Mexico, Above and Below Salteepwater Gulf of Mexico, Above and Below SaltCase History – Deepwater Gulf of Mexico

Major challenges• Mud losses…> 15,000 bbl/well

on previous wells especially in

SALTSALT

on previous wells, especially inintervals below the salt!

• Wellbore stability

S l tiSALTSALT Solution• Strengthen five intervals- One above the Salt

O i th S ltDrilling ECD exceeds min Frac Gradient

- One in the Salt- Three below the Salt• Shakers dressed with

14/20 mesh screens14/20 mesh screens• [WSM] ~ 15 to 30 lb/bbl• WSM Design

Si ed Marble- Sized Marble- Granular Synthetic Carbons

Case History – Deepwater Gulf of Mexico

No losses in 2900 ft sand section above salt No losses in 7500 ft salt section, screened upNo losses in 9200 ft sand section below salt (where majority of losses occurred in offset wells)(where majority of losses occurred in offset wells)

Savings• Total cost for the WSM ~ $ 0 7 MM Previous mud• Total cost for the WSM ~ $ 0.7 MM. Previous mud

losses ~ $ 2.7 MM. Net savings ~ $ 2.0 MM on materials alone.

• On subsequent 4 wells, similar net savings of ~ $ 2.0 MM on materials alone.

The 4-tier strategy for managing lost circulationSummary

must be an integral part of well planning

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