Previous Talks

Wednesday, December 9th, 2009

Pore-Pressure Depletion and Effective Stress Issues in the Gulf of Mexico's Lower Tertiary Play

G. N. Boitnott, J. L. Shafer and T. W. Miller

Abstract


We present the results of a study of pore pressure depletion in the Gulf of Mexico’s Lower Tertiary Play. Permeability and velocity measurements collected during multiple cycle stress paths are presented. We demonstrate how the data are used for evaluating effective stress coefficient and the calculation of net effective stress for compaction, permeability, and elastic wave velocities.

Rock properties models for the deep water Lower Tertiary sandstones need better calibration for conditions of high initial pore pressures and large pore pressure depletions. Available data indicates that inelastic pore compaction at high drawdowns is possible and significant permeability reduction may occur due to both elastic and inelastic deformations. Velocity changes in response to pore pressure need better calibration in order to improve both characterization and eventual reservoir monitoring activities.

Core measurements used to calibrate logs, interpret seismic data, and constrain reservoir simulations are commonly made at low pore pressures. To mimic reservoir conditions, the tests are performed at reduced external stresses (sigma), so as to compensate for the low pore pressures (Pp) used in rock testing. It is common practice to choose a net effective stress (sigma-alpha*Pp) that is thought to result in identical rock properties. We show that due to a variety of pore pressure phenomena, the net effective stress for each property is different, and is a direct result of the fact that pore pressure and external stresses act in different ways, causing different stress distributions and deformations within the grains.

Our results lead to the conclusion that the “effective stress” should not be thought of as an equivalent state of stress. Failure to properly apply effective stress concepts can lead to potentially significant errors. For example, laboratory measurements of brine permeability indicate that for these sandstones, the routine core permeability measurements at low pore pressure may underestimate the in-situ rock permeability by 20% to 50% and underestimate permeability reduction due to depletion. Other examples include pore pressure effects that influence interpretation of stress and pore pressure from sonic and seismic data, where the effective stress coefficient (alpha) has the added complexity of including both rock and fluid components.

Speaker Biographies

Greg N. Boitnott is Vice President of New England Research Inc. He specializes in core measurements and analysis with an emphasis on the development of physical models. Areas of current interest include petrophysical upscaling and core to log integration. Greg received a B.S. in Geology from Beloit College, and a Ph.D. in Rock Mechanics from Columbia University in New York.

John L. Shafer has been a consultant to the oil and gas industry for the past ten years since retiring from Exxon after 19 nineteen years. For the past six years he has been a consultant to Devon Energy’s Gulf Division. Quantification of reservoir quality with low field NMR, core image analysis, and petrology has been the focus of his research. He is a past President of the Society of Core Analysts (SCA), a chapter of SPWLA. John obtained a B.S. in Chemistry from Allegheny College in 1963, his Ph.D. in chemistry from University of California, Berkeley in 1970, and a M.S. degree in petroleum engineering from the University of Houston in 1992..

Terry Miller has a PhD in ME from Rice, and was for 32 years a research scientist for Exxon primarily in the area of geomechanics (soil, ice, rocks), several years with Knowledge Systems as Scientific Consultant and VP for Technology for a shorter time, and now independent consultant specializing in rock mechanics testing issues and in situ stress analyses.