Introduction
Geomechanical models simulate changes in rock properties with time and pressure. Geomechanical models are useful for (and may be necessary additions to) analyses of overpressured reservoirs, tight/low permeability reservoirs, and more.
Stress Dependence
Stress (or pressure) dependence of permeability is a commonly observed and generally accepted
dynamic behaviour that can affect production performance forecasting. This phenomena has been observed in both CSG applications through to shale and tight gas.
In CSG applications, it is sometimes ignored. According to Burgoyne and Shrivastava (2015) Reasons for this omission typically include (a) the difficulties in reliably characterizing stress dependent effects from a limited number of pressure build-up (PBU) tests, and (b) large uncertainties in our understanding of both the porosity and compressibility of coal. Similar behaviour has been observed in tight gas reservoirs as well.
Stress changes the shape of coal cleats (coal is mechanically weak). And of course, alteration of cleat shape will affect cleat permeability. For any given coal seam, stresses increase over the course of depletion leading to the expectation permeability decrease.
Matrix Shrinkage
Regardless of age, rank, or purity, coal matrix is a reactive, organic rock composed of complex hydrocarbons. The matrix of coal swells as gas adsorbs on the surface and shrinks when the gas desorbs. Matrix shrinkage increases cleat width and therefore increases permeability
The most widely used analytical models for coal permeability changes are: Palmer and Mansoor (1988), Shi and Durucan (2005), and finally Cui and Bustin (2005, 2007).
Additional Reading
-
Burgoyne & Shrivastava
References:
-
Rodriguez, Cesar Alexander, Stress-Dependent Permeability on Tight Gas Reservoirs, Texas A&M University, 2004
-
Barbara, Mariela Franquet, Effect of Pressure-Dependent Permeability on Tight Gas Wells, 2004 Texas A&M University.
-
Burgoyne, M.. , and R.. Shrivastava. "A Practical Workflow for Characterizing Stress-Dependent Behaviour of Coal from Changes in Well Productivity." Paper presented at the SPE Asia Pacific Unconventional Resources Conference and Exhibition, Brisbane, Australia, November 2015.