Reservoir Stress-Sensitivity BGD Smart JM Somerville M Jin

Reservoir Stress-Sensitivity Reservoir properties and therefore behaviour influenced by changes in stress Caused by either changes in pore pressure or temperature, or combination Properties = permeability, dimensions, integrity

Stress-Sensitivity Scales Near wellbore permeability – (stress skin cf skin caused by invasion) failure Increasingly distant from the wellbore permeability Whole reservoir permeability, directional floods Field compaction, subsidence, seal alteration

Stress-Sensitivity Scales Near wellbore – Influenced by UBD permeability – (stress skin, no skin caused by invasion) failure Increasingly distant from the wellbore permeability

Reservoir Stress-Sensitivity: a multi-disciplinary challenge More Realistic Reservoir Model Better Decisions

Reservoir Stress-Sensitivity: a multi-disciplinary challenge Better Decisions More Realistic Reservoir Model Stress Sensitivity

Better Decisions Re:- All impacting recovery factor and costs Reserves Well design PI Well locations Production strategy Reservoir management (inc 4D seismic) Seal integrity Compartmentalisation Facilities Efficacy of UBD technology and methodology All impacting recovery factor and costs

HWYH-399 Key: Breakout from CBIL(A) Drilling-induced tension from STAR The figure above shows a selection from a RECALL image analysis display. The data are from well HWYH-399 between 6324 and 6327 feet measured depth (MD). From left to right, the tracks are: natural Gamma Ray (GR) for the whole logged interval of the STAR+CBIL tool run interpreted breakouts from the CBIL image and dips from the STAR resistivity image the CBIL(A) acoustic image in amplitude mode the STAR resistivity image. Both displayed images have dynamic gain correction selected. Note: the prominent high angle tension crack on both the CBIL(A) and STAR images between 6324 and 6325.25 feet MD. The crack has the same azimuth in both images. Breakout is common in well HWYH-399, always on a near north-south axis. The azimuth control of the two image logs seems to be in agreement. According to the GeoScience Phase-1 report, the strike of tension gash, or induced axial failures, parallel to the maximum principle stress of the regional Zagros trend should be around 150 - i.e. almost perpendicular to the trend shown. However, Kes reports that “max principal stress Shmax over Ghawar has a mean direction of N55E, but there are examples of rotation, even to NW-SE ... [he] … would not be surprised by an Shmax azimuth of N90E in HWYH-399.” So the orientation processing for the HWYH-399 image logs may not be suspect. At least the two logs agree with each other.

HWYH-394 Key: Drilling-induced tension cracks Bed boundary Fracture All from STAR Bed boundary Fracture Unclassified, possible stylolite The figure above shows a selection from a RECALL image analysis display. The data are from well HWYH-394 between 6201 and 6204 feet measured depth (MD). From left to right, the tracks are: natural Gamma Ray (GR) for the whole logged interval of the STAR+CBIL tool run caliper, GR and interpreted dips from the STAR resistivity image the CBIL(T) acoustic image in travel time mode for breakout analysis – though the crack is much more obvious in amplitude mode, where the mismatch between the two image logs stands out. the STAR resistivity image. Both displayed images have dynamic gain correction selected. Note the prominent sub-vertical tension crack on both the CBIL(T) and STAR images between 6202 and 6202.75 feet MD. The crack’s apparent azimuth differs between the two images: CBIL(T): around 180, or midway between 0 and 360 STAR: at about 60 and 240 It seems that the azimuth control of at least one of the two data sets may be incorrect. The STAR tool image has been extensively picked for dips and the strike of tension gash, or induced axial failures. These are consistently around N080E, similar to that for HWY-399. So, perhaps the CBIL image log remains incorrectly oriented after processing? It would be as well to check on the raw data files provided to HWU.

The Conceptual Model The reservoir consists of blocks or layers of intact rock bounded by discontinuities The reservoir is stressed in an anisotropic manner The whole system exhibits hysteresis

Thinly-bedded interval in the Annot Sandstone Thinly-bedded interval in the Annot Sandstone.This interval is underlain and overlain by more ‘massive’ sandstones.

The Reservoir “Intact” Rock Discontinuities

Boundary and Local Stresses within the Reservoir Boundary or Regional Stresses sh sH sv Local Stresses

Intact Rock Properties (stress-sensitive values where appropriate) Ambient porosity and permeability Elastic constants E and v Biot’s coefficient Failure (Fracture) Criteria Vp and Vs velocities Vp anisotropy at ambient conditions Permeability at reservoir stress conditions Palaeomagnetic trial

Tests with Specimen in Triaxial Cell P and S waves Fluid flowing at pressure Stress-Sensitive Values of:- Elastic Moduli Biot’s Coefficient Permeability Vp,Vs Failure Criterion Tests with Specimen in Triaxial Cell

Single State Triaxial Testing Failure e1 s2 = constant s2 s2 e1 s1 Single State Triaxial Testing

Failure Criterion - Triaxial Factor s2’’’’ s1 s1 x x x s2’’’ x x Tan = Triaxial Factor x s2’’ x x s2’ e1 s2 Failure Criterion - Triaxial Factor

Multi-Failure vs Single State

Multi-Failure State Triaxial Testing Tan = Triaxial Factor x x s2’’ x x s2’ e1 s2 Multi-Failure State Triaxial Testing

Multi-Failure vs Single State

Failure Criterion s1 s1 s2 s2 s2

Vp at 27MPa vs Porosity

Young’s Modulus at 27 MPa vs Porosity

Angle of Internal Friction vs Porosity

Sampling Rationale - Intact Rock Wireline Log Correlation Rock Mechanics Property Sample Core, then Test Petrophysical Property

Populating Model - Intact Rock Convert Reservoir Characterisation Model into a Geomechanical Model Synthetic Rock Mechanics Log Correlation

Discontinuity Properties 1. Mechanical Pre-failure properties Failure criterion Post-failure properties 2. Petrophysical/Permeability Sensitivity to Stress and Strain

Types of Fracture Behaviour

Direct shear-test F=const

Fracture Permeability Sensitivity to Stress and Strain K/K0 Discontinuity dilating only Dilation 1 1000 TYPE 1 Shearing under moderate normal stress Shear Displacement 100 TYPE 2 TYPE 3 0.1 Shearing under high normal stress Fracture Permeability Sensitivity to Stress and Strain

The Process Populate the Conceptual Model with properties and data So create a Geomechanical Model of the reservoir (plus surrounding rock) Impose process-induced changes on the Geomechanical Model using analytical or numerical solutions Numerical offers more realism than analytical – hence coupled modelling

Coupled Modelling

Fluid Flow Simulator Stress-Analysis Simulator More realistic simulation results Fluid Flow Simulator Change in Pore Pressure, Temperature, Saturations Change in Permeability Change in Effective Stresses Rock Movements, Change in Stress and Strain Stress-Analysis Simulator Reservoir and o/b stresses, strains and displacements

Differentiating Filter (Synthetic) Enhanced 4D Seismic Interpretation/Reservoir Management Fluid Flow Simulator Differentiating Filter (Synthetic) Change in Pore Pressure, Temperature, Saturations Saturation-Related changes in Impedance Stress-Related changes in Impedance Change in Permeability Change in Effective Stresses Rock Movements, Change in Stress and Strain Changes in Velocity and Density Stress-Analysis Simulator

Differentiating Filter (Synthetic) Fluid Flow Simulator Stress-Analysis Simulator Change in Pore Pressure, Temperature, Saturations Change in Effective Stresses Rock Movements, Change in Stress and Strain Change in Permeability Enhanced 4D Seismic Interpretation/Reservoir Management Saturation-Related changes in Impedance Stress-Related changes in Impedance Changes in Velocity and Density More realistic simulation results Reservoir and o/b stresses, strains and displacements

UKNS, Perm Stress Sensitivity (ECLIPSE coupled with VISAGE) Example 1 UKNS, Perm Stress Sensitivity (ECLIPSE coupled with VISAGE)

Production Prediction: permeability reduction The diagram shows the absolute reduction (k1-k18). The maximum reduction in permeability is in the central part of the field Perm sensitivity modelled with hysteresis (ECLIPSE Output)

Stress Sensitive Permeability with hysteresis Depressurisation in Miller Injection in Miller induced unloading Injection in South Brae induced unloading in Miller Field

Comparison of GOPR Predictions Oil Production Rate is sharply reduced because the permeability reduction in the area causes a reduction in BHP and leads to a increase in gas production (ECLIPSE Output)

Horizontal Ground Displacements - 1

Horizontal Ground Displacements - 2

Horizontal Ground Displacements - 3

Stress Ratio vs. time Between wells Far from well Close to well

Stress Status in p-q terms (anisotropy) close to wells far from wells

Stress Path Distribution

Permeability Stress Path Sensitivity MATLAB Excel

Compaction and subsidence Compaction in 1987 1 2 Compaction IN 1995 in which the result of injection is shown

UKNS, Seismic Stress-Sensitivity (ECLIPSE, VISAGE, H-WU software) Example 2 UKNS, Seismic Stress-Sensitivity (ECLIPSE, VISAGE, H-WU software)

Features of a 2D flow model grid embedded for coupled geomechanical simulation Well Overburden Faults Caprock Sideburden Model mesh of a gas-water reservoir Flank to crest of a long structure transected by faults mostly parallel to the longitudinal direction - allows 2D modelling Moderate to low porosity stiff frame sandstone reservoir with an evaporite sequence caprock - not a particularly good candidate 4D seismic (or any seismic in practice!) Flow model grid embedded and conditioned for coupled geomechanical simulation Gas , Water in the flow model grid

Displaced shape of the geomechanical model Surface subsidence Differential compaction across faults in reservoir Typical location of shear strain on faults Features of the displaced shape of the geomechanical model due to pore pressure changes differential compaction across faults surface subsidence - changes in the overburden localized shear strain on faults (VISAGE Output)

Mean effective stress distribution at the end of the simulation Localized effects at faults Perturbed stress field above and below reservoir Unperturbed stress field (constant gradient) Apparent deepening of reservoir due to decreasing pore pressure Mean effective stress effects unperturbed stress field away from reservoir apparent deepening of resevoir due to decreasing pore pressure perturbed stress field directly above and below reservoir localized effects at faults (VISAGE Output)

Time-lapsed compressional acoustic impedance Changes in overburden/caprock due to stress redistribution Changes in reservoir due to pore pressure decline Top of caprock Time-lapsed acoustic impedance from the acoustic model database with fluid change and stress change effects showing changes in reservoir due to fluid movement at GWC changes in reservoir due to pore pressure decline (increasing stress) changes in overburden/caprock due to stress redistribution Initial gas-water contact Changes in reservoir due to fluid movement (VISAGE Output)

Initial Modelling: Before Production Begins

Time Lapse Model: Saturation Changes Only

Time Lapse Model: Saturation + Stress

Time-lapsed seismic trace model Reflector at top of caprock Perturbations at reflector event due to fluid change effects Reservoir top Time-lapsed seismic trace model generated from the acoustic model database showing strong reflector event at the top of the reservoir caprock slight perturbations at the GWC due to fluid change effects pull-up in reflector event at the reservoir base due to stress change effects These effects are better displayed dynamically Reservoir base Pull-up in reflector event due to stress change effects

Where are we now? Extreme examples of reservoir stress-sensitivity accepted: Ekofisk, HP/HT, Gulf of Mexico, Angola? The processes required exist in usable form Non-uniform levels of commitment What about the more subtle reservoirs?

Technical Challenges Discontinuity distributions Discontinuity properties Rel perm stress-sensitivity In situ stress state Coping with anisotropy Seamless software

Organisational Challenges Realising the full value of the data we already have Cost vs value of the process Coping with multi-disciplinarity

Is this too much to ask for? Better performance Fully owned decisions Shared belief Shared analysis

Decision Making Straight from the geomechanical model, aided possibly by some calcs, e.g. fracture density = well locations for max PI subsidence = yes or no With the aid of coupled modeling, e.g improvement of appraisal impact of perm sensitivity = recovery, GOR etc Ground movements and subsidence = threat to wells and facilities 4D seismic enhancement = better management

Thank You

What do we want to achieve today? Overview of the main tasks of the project Select candidate reservoirs for study Set up communications Agree next meeting date 17th August?

Hysteresis K Increasing Stress

Hysteresis K X Increasing Stress

Hysteresis K Increasing Stress

UBD site history very important Effective Stress around the wellbore Failure Level Time Drilling Completion Production

Building the Geomechanical Model *Structure and anisotropy analysis from Seismic *Geomechanical Core Analysis Multi-Disciplinary Tasks assembling data for Model *Published and proprietary studies *Log analysis Basin process simulations *Geomechanics of fracture genesis *Genetic Units expertise Analogue studies Characterise Structural Setting of the Reservoir Creation of the Geomechanical Model Characterise Reservoir Rocks Characterise Reservoir Faults & Fractures Reservoir Geomechanical Model feedback to improve characterisation feedback to improve characterisation Stress-Sensitive Coupled Modelling Stress-Sensitive Reservoir Modelling and Coupled Simulations (Ground movements, Fluid Flow and 4D seismic) Deliverables Better Decisions Reservoir Management