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Formation Pore Pressure Prediction Processes, Pitfalls and Challenges Formation Pore Pressure Prediction Processes, Pitfalls and Challenges Ernest C. Onyia

Some Geopressured Basins

Outline Introduction Pressure Analysis Process Pitfalls/Limitations and Examples Challenges Concluding remarks

Introduction The “Art and Science” of Pore Pressure Prediction Impacts – Exploration, Drilling, Development and Production Management

Business Impacts of Pressure in Exploration and Drilling Exploration - Applications of pore and fracture pressure in prospect quality. HC migration and seals. Drilling and completion - Cost Reduction by reducing unscheduled events. Safety and Environment.

Exploration Application Depth, ft. Density, ppg.

Drilling Impacts – Unscheduled Events Stuck Pipe Lost Circulation Well Control Wellbore Mechanical Instability

Photo Courtesy of Wild Well Control Inc. Cost, Safety and Environmental Impacts

The Process Seismic, Logging & Drilling Data LWD, Offset Data Calibrate Pressure Models - Choices & Decisions Interpretation Measured Pressure Refine

Pore Pressure Prediction Basic Steps Data Determine or Calculate Overburden Stress Select Shale Data (e.g. shale travel time) Select Appropriate Pore and Fracture Pressure Models Establish Shale Compaction Trend(s) Calculate Pore and Fracture Pressures

Determine Overburden Pressure Bulk DensityOverburden gradient, psi/ft

Select Shale Parameters From Log Data SP (mv) Sonic DtRtRt

Select Appropriate Pore and Fracture Pressure Models 0.3 n n 2.1 ) t t NHG)(-(OBG - OBG ) R R NHG)(-(OBG - OBG                       f f n n f f n n P P OR t t P P R R   ,  nn LLf  Resistivity, R t  Sonic Travel Time,

Establish Shale Compaction Trend Calculate Pore and Fracture Pressures P f P frac Overburden RT sh Compaction Trend

Process can be very subjective - Models and Methods Experience of the interpreter - Choices and Decisions Integration - Seismic, Wireline, LWD, Geology and Drilling Data The Process

Pitfalls and Limitations Geology Data - Seismic and Wireline Log Methods Pressure Prediction Models Examples

Causes of Overpressure Geological Pitfalls Compaction Disequilibrium Structure Lithology Diagenesis - Post Depositional Changes

Normal Compaction Process of Shales After Geoservice CLAY SLURRY IN WATER CLAY SLURRY IN WATER 1. At deposition: Shale Particle LEGEND Interstitial Water SHALE Self-supported Matrix 3. Final compaction: CLAYSTONE Overburden Load 2. Increasing overburden. Load-water escapes:

Undercompacted Shales CLAY SLURRY IN WATER CLAY SLURRY IN WATER 1. At deposition: After Geoservice UNDERCOMPACTED SHALE 2. Increasing overburden load, water cannot escape - porosity becomes greater than normal Excess trapped water Overburden Load

The Compaction Process After Dickinson (1953) 70-80%200  t e N o r m a l C o m p a c t i o n T r e n d 62% Shale PorosityShale Transit Time Depth AB (Arithmetic Scale) (Logarithmic Scale)

Compaction Trends - Pitfalls

Compaction Trend Lines Pitfalls Empirical trend lines fit to check shots: V o e bz (V o 2 + 4Az) 1/2 V o + z n V o + kz V i =

Pore Pressure Prediction Compaction Trends Shale D t, msec/ft P fmin Fracture Pressure P fmax

Rock Type Effects on Pressure Estimates

Effect of Geological Structure After Huffman Pore Pressure Distribution on a Seismic Section

Sonic Log Rock and Fluid Type Effects on Pressure Estimates  t, msec / ft.

Seismic Velocity Pitfalls Data Acquisition and Processing Velocity Step Size Lithology

Pre-Drill Seismic Velocities Example DEPTH FEET VEL_1F(ft/sec) VEL_2F(ft/sec) VEL_3F(ft/sec) VEL_4F(ft/sec) VEL_5F(ft/sec) VEL_6F(ft/sec) Velocity, ft/sec Depth (feet)

Extra pick removed Pick moved to boundary Velocity Analysis Example New pick may not be on bull's-eye Courtesy B. Lankston Time, Sec.

OriginalHorizon-Based Calculated Interval Velocities Courtesy B. Lankston Time Velocity, (ft/sec.)

Vertical Interpolation and Horizontal Smoothing

Wireline Log Data Pitfalls Sonic Compressional Travel Time Resistivity Porosity

Sonic Log Data Preferred - Less affected by borehole conditions. Near seabed high travel times. Gas effects.

Pore Pressure Estimates Resistivity & Sonic Logs Fracture Pressure, ppge Overburden, ppge Pore Pressure from sonic log data (PPS) Pore Pressure from resistivity (PPR) log data Shale travel time DTSH Shale Normal Compaction Trend, NCTL Fracture Pressure Overburden pressure

Resistivity Pitfalls - Environmental Effects Courtesy - Schlumberger

Resistivity – Anisotropy ?

Near Seabed Effects on Resistivity & Sonic Logs Pitfalls Sonic Resistivity SP (mv)

Idealized Salt Effect on Shale Resistivity and Pore Pressure

Pressure Prediction Models & Methods Crossplot Methods Effective Stress Methods –Intrinsic Model Limitations –Compaction Trends Pitfalls

What is a Crossplot Model? Reservoir Pressure, psi/ft Normal Pressured Rshn/Rsh Observed Equivalent Mud Weight, ppg

Concept of Effective Stress += Pore Pressure S V =P +  V S h =P +  h = P + K  V Total Stress Effective Stress S h S V Pore Pressure P P Effective Stress  h  V After Glenn Bowers

Commonly Used Effective Stress Models Eaton Bowers Equivalent Depth Equivalent Depth

Effective Stress Models Eaton Model  0.3 n n 2.1 ) t t NHG)(-(OBG - OBG t ) R R NHG)(-(OBG - OBG,                         f f n n f f n n nn P P OR t t P P R R LLf    Resistivity, R Sonic Travel Time,

Effective Stress Models Bowers Model (1/u) ^ Virgin Curve:         B A 5000 V  B A V  [  max   ] Unloading Curve:

Equivalent Depth Method Graphic Illustration Plot parameter of interest. Determine formation pressure by projecting pressure from Depth A upward until it intersects the normal trend at Depth B. Read the pressure value on the normal trend for Depth B. Equivalent Depth Method Graphic Illustration Shale Parameter Depth Normal Trend Depth A Depth B

Deepwater Pore Pressure Challenges Seismic Velocities Limitations Narrow Margins Between Pore and Fracture Pressure Salt Mini-basins

Pressure Data Deepwater Mudline SWF Zone

Suggested Work-Around Understand the causes(s) of overpressure in the area of interest. Decisions: Data, models and techniques. Integrated analysis.

Suggested Work-Around, cont. Country Specific Suggestions: –West Africa –Caribbean –South/Central America –Nile Delta –Caspian –UK North Sea –Norway –Middle East –Asia

Leak Off Tests (LOT) Choosing the Leak Off Pressure and Minimum Stress

Misconceptions The weakest point in an open hole section is at the shoe. Full LOT damages the wellbore permanently % of leak-off (based on offset data). Offset data mentality - “I know what happened in the offset!”

Limitations or Deficiencies Lack of a standard procedure in the industry. Tests usually performed in shales. Results may be unrepresentative. Mechanics and interpretation sometimes poorly understood. Environmental conditions - temperature, compressibility, permeability, etc. Directional wells - LOT may reflect reduced breakdown pressure while extension pressures still = minimum horizontal stress.

Leak Off Test Human Factor 11 7/8" LOT -Pumping l Unit Data :33:3610:40:4810:48:0010:55:1211:02:2411:09:36 Time psi LOP= 850 psi 880 psi

Salt Leak Off Test

Concluding Remarks on Pore Pressure

Concluding Remarks and Guidelines Pore Pressure Analysis is a process fraught with pitfalls. Integrated approach is recommended. Experience. Result is, at best, an estimate.

Pressure Management The overall objective is to prevent the risk and expense of kicks, lost circulation and stuck pipe. Maintain a keen awareness of safety and environmental implications of your decisions and interpretations.

Pressure Management Good communication is essential - with rig and office based personnel. Filter - keen knowledge of how to filter or identify the onset of abnormal pressure in the presence of several other parameters while drilling. Selectivity - Not all parameters will work. When indicators warrant, take DECISIVE action. It may not be popular.

Concluding Remarks on Leak Off Tests

Concluding Remarks and Guidelines Casing depths - Consider rat-hole requirement to ensure test in shale and other contingencies. Conditioning - At least obtain consistent mud densities in and out - especially if hole cleaning problems are indicated.

Concluding Remarks and Guidelines on LOT In salt, there is no merit in taking LOT beyond the overburden pressure equivalent. Oil Based Mud - Pressure While Drilling (PWD) - a standard for deepwater.

Salt Leak Off Test

Concluding Remarks and Guidelines on LOT Oil Base & Water Base Mud LOTs - –Test using the mud planned for use in drilling the next hole section. If it is not possible, and LOT was performed before switching mud, perform an FIT or re-test with new mud.

Concluding Remarks and Guidelines Single versus Extended LOTs –Exploration –Development

Thank You

Distribution of Oil & Gas Fields’ Relationship to Pressure Belonin & Slavin - AAPG Mem 70.

Samaan Field Trinidad Heppard - AAPG Mem 70

Pressure Estimates - Samaan Field Trinidad Heppard - AAPG Mem 70

Estimated Pore Pressure Offshore, Trinidad Heppard - AAPG Mem 70

Nile Delta Wells Egypt AAPG Mem 70

Generalized Stratigraphic Section Nile Delta, Egypt AAPG Mem 70

Wakar #1 Well Nile Delta, Egypt Nashaat, et al. AAPG Mem 70

Pore Pressure Example Offshore, Nigeria

BASINS ASSOCIATED WITH ABNORMAL PRESSURES