petroleum geomechanics

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Petroleum GeomechanicsPetroleum Geomechanics

Examples where Geomechanics Knowledge is Vital

Maurice Dusseault

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Petroleum Geomechanics AreasPetroleum Geomechanics Areas

Borehole stability (+ chemistry, diffusion, wellbore hydraulics)

Hydraulic fracturing (+ fracture and porous media flow of fluids & slurries, filtration)

Reservoir compaction (+ reservoir mechanics, geological history, mineralogy)

Sand production (+ capillarity, hydrodynamic forces)

Thermally-induced casing shear (+ thermo-dynamics, fluid flow)

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Coupled ProblemsCoupled Problems

All these are coupled problems! Solve a fluid flow

problem to get a general pressure distribution, {p}

Pressures change stress so use {p} in a stress-strain (-) problem

generates V, so we have to put this back into

the pressure solution

ReservoirSimulator for p & T

Geomechanics Simulator for '- solution

Diffusion mechanics

Continuum mechanics

Measurements for verification and model calibration

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Three Chosen Case HistoriesThree Chosen Case Histories

Borehole Stability at Cusiana (Colombia)$35,000,000 per hole before changes$12-15,000,000 per hole after changes

Compaction at Ekofisk-Valhall (North Sea)Massive surface subsidencePlatform rebuilding as necessary, casing shear

Casing Shear in Cyclic Steam (Canada)Massive shear stresses from thermal stressesHorizontal wells help solve the problem

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Colombia TectonicsColombia Tectonics

COLOMBIA

Venezuela

Panama

Ecuador

Peru

Brasil

http://geology.about.com/library/bl/maps/blcolombiamap.htm http://geology.com/world/colombia-satellite-image.shtml

Cusiana

Massive Borehole Stability

Problems

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The General Stability ProblemThe General Stability Problem

Cusiana Field, 4270 m, discovered in 1989 In foothills just east of last major Andes range Lithology: alternating sand-shale sequence Tectonics: active faulting, overthrust belt, very

high in situ shear stress at depth (near failure) 10 months drilling time, lost 20-30% of wells

that were spudded, >10% of time spent on cleaning holes, expensive mud, stuck pipe, sidetracks, distorted casings, and so on

$35,000,000 per well in 1990-1992 period

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The Geomechanics ProblemThe Geomechanics Problem

Very high stresses in situ Relatively weak rocks (mechanical weakness) Fissile shales dipping at up to 30-40 Inclined joints and fissures that can slip if the

MW is too high, creating hole constrictions These issues led to the problems observed:

Massive breakouts, sloughing, hole enlargementOil-based muds not effective (ie: natural fractures)Stuck pipe from joint displacementsHole cleaning problems, tripping problems

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StressesStresses

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Borehole StabilityBorehole Stability

Borehole management involves not only stresses, rock strength, MW, mud type!

It also depends on hydraulics & chemistry:Pumping strategy and cleaning capabilitiesGel strength, viscosity, mud density, filtrate chem.BHA design, ECD, even tripping policy

Rock

mecha

nics Hydraulics

Chemistry

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HOW DO WE REDUCE HOW DO WE REDUCE ECONOMIC RISKS IN ECONOMIC RISKS IN

DRILLING AT CUSIANA?DRILLING AT CUSIANA?

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Massive Subsidence Massive Subsidence -- EkofiskEkofisk

Huge reservoir, over 300 m thick in center High porosity (high-) Chalk (see next slide) Oil production causes:

Drawdown of reservoir pressure p Increase in effective stress Collapse of high- Chalk to a lower porosityMassive reservoir compaction (> 9 m at present)Seafloor subsidence (>8 m at present)And - good reservoir drive energy!

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CoccolithicCoccolithic Structure of ChalkStructure of Chalk

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Hollow Hollow CoccolithsCoccoliths, Little Cement, Little Cement

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EkofiskEkofisk

10 km

Reservoir structure

Subsidence bowl

saltPorosity, 50% in this

zone

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ValhallValhall DispositionDisposition

From: Zoback MD, Zinke JC. Production-induced Normal Faulting in the Valhall and Ekofisk Oil Fields. Pure Appl.

Geophys. 159 (2002) 403-420

Ekofisk and Valhall are similar Chalk reservoirs, formed because of deep-seated salt doming creating an anticline.EKOFISK

Geography

VALHALL

A

A

Deep salt ridge

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Massive Reservoir Compaction Massive Reservoir Compaction

Seafloor subsidence Leads to wave risks

for platforms Casing shearing

(some wells redrilledseveral times)

Wonderful drive sustaining process!

1973

1984

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Casing Shear at Casing Shear at EkofiskEkofisk

Source: EPOKE, Phillips Petroleum, No 7, 1999

6262000

6263000

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510000 511000 512000 513000 514000 515000 516000E-W Coordinates

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Overburden

Reservoir

Distribution of overburden shear damage and producing interval compaction damage

at Ekofisk(SPE 71695)

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Rock Mechanics ImplicationsRock Mechanics Implications

During development (1969-1975), engineers failed to predict massive subsidence

When noted in 1981-83, maximum subsidence was greatly underestimated (~6-7 m)

Also, engineers failed for many years to understand the water-weakening phenomenon

Some consequences:Platform jacking by 6.3 m in 1988-89Complete platform redevelopment in 1999-2002Far greater OOIP and production rate

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ProductionProduction--Injection HistoryInjection History

Start water injection

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ButBut

Ekofisk taught us a great deal The need for very careful rock testing under in

situ conditions without altering the material Links between rock behavior, capillarity,

chemistry and fluid flow (p) are important. Coupled modeling is needed for realistic

predictions in reservoir geomechanics Measurements are always important Risks can be reduced by careful geomechanics There will always be some surprises!

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HOW DO WE MANAGE HOW DO WE MANAGE ECONOMIC RISKS BECAUSE ECONOMIC RISKS BECAUSE

OF SUBSIDENCE AT EKOFISK?OF SUBSIDENCE AT EKOFISK?

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Imperial Oil Cold LakeImperial Oil Cold Lake

50 km

Imperial Oil Cold Lake Property

Edmonton

Calgary

Saskatoon

Regina

Alberta Saskatchewan

Lloydminster

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Casing Shear in Thermal ProjectsCasing Shear in Thermal Projects

High temperature (To ~ 25C, Ti ~ 225-325C) Large degree of thermal expansion of the sand

reservoir that is convectively (flow) heated This also leads to thermal dilation of the sand,

giving a large volume increase (1-3%) Overlying shale does not increase in volume

(conductive heating is slow, no dilation) This means a great deal of shear stress is

concentrated at the sand-shale interface Cased wellbores cannot withstand this (shear)

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mod. Stancliffe & van der Kooij, AAPG 2001

+285 mm

+200

-210

+260

+130 mm

-165km

+100 Vertical displacements

(mm)over 86 days

subsidenceheave

Imperial Oil Imperial Oil Cold LakeCold Lake

Mega-RowCSS

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ThermallyThermally--Induced ShearInduced Shear

thermal front,V - expansion

displacementpattern

Shale: impermeable, stiff

Sandstone, permeable, softer

ovalization and collapsetendencies near interfaces

(large stress concentrations)

dogleg over an interval

Note: casing experiences substantial distortional loading far in advance of the arrival of the thermal front. Largest shear stresses probably coincide with the thermal front.

Shale: impermeable, stiff

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Thermal Compaction as WellThermal Compaction as Well

Lost Hills, California z Interferogram

Subsidence Induced Fissure

Courtesy: Michael S. Bruno

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HOW DO WE MITIGATE HOW DO WE MITIGATE ECONOMIC IMPACTS OF ECONOMIC IMPACTS OF

CASING SHEAR?CASING SHEAR?

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ExerciseExercise Three separate groupsThree separate groups

Each group develop a list of strategies for managing the risk associated withGroup 1: Borehole instability, high stress areaGroup 2: Massive subsidence, offshore fieldGroup 3: Casing shear, thermal project

Write down your choices, we will present them and discuss them briefly

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CusianaCusiana and Borehole Stabilityand Borehole Stability

Choice of inclined well trajectory to penetrate shales at 90 (15), in the optimum direction

Additives to plug induced shear fractures and delay hole sloughing behavior

Careful mud control Change casing points Better hole cleaning & hydraulics to cope with

the large sloughing Etc.

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The Risk Minimization LoopThe Risk Minimization Loop

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Mechanica

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Earth Mod

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Now at Now at EkofiskEkofisk

Water injection is sustaining the reservoir pressure. This reduces but does not eliminate the reservoir compaction.

Wells are drilled and equipped with more compliant casing so that more shear distortion can occur without shearing or breaching

Measurements: seafloor gauges, logging, etc. Better modeling and therefore predictions However, Ekofisk made us humble! We did

not predict its behavior in advance!

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Handling Casing ShearHandling Casing Shear

Mitigation strategies include:Adjusting well trajectories (avoidance)Adjusting injection/production strategiesModifying hole size and cementing practiceChanging casing grade (D/t ratio)Changing inner completion designAccept damage risk and adjust economicsAccept risk, design for easier re-drills, repairs

Geomechanics analysis helps quantify risk

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e Well PerformanceComparison Tool

CommonDesign AnalysisDatabase

AnalyticalAnalysisTool

Simple DecisionAnalysis Tool

ProprietaryReservoir Analysis

ProprietaryWell Damage Analysis

ProprietaryDecisionAnalysis

Well PerformanceComparison Tool

Well PerformanceComparison

Tool



AnalyticalAnalysis

Tool

AnalyticalAnalysis

Tool

Simple DecisionAnalysis Tool

Simple Decision

Analysis Tool



Estimate

Reservoir Deformation

Well Deformation

Limits

ProprietaryWell Damage Analysis

ProprietaryWell Damage

Analysis



Optimum Well Design

Risk Mitigation in Casing ShearRisk Mitigation in Casing Shear

Courtesy: Michael S. Bruno

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Shell Oil Canada Shell Oil Canada Peace RiverPeace River

ref. Nickles New Technology Magazine, Jan-Feb 2005

Surfaceuplift / tilt

data

reservoir inversion gridwith 50x50m grid cells

Multi-lateralCSS

Less well shear because of well

trajectories!

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Lessons LearnedLessons Learned

In many instances, geomechanics knowledge is vital to oil and gas development

Borehole stability (Cusiana example) Subsidence (Ekofisk example) Casing shear (Oil sands Cold Lake example) And many others These are coupled problems (combined

stress-flow problems p, T, ) Implementing geomechanics into planning

reduces risk, avoids problems, lowers costs

petroleum geomechanics

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