Impact of Effective Pressure Variations on Reservoir Rock Porosity and Compressibility

Pressure transient analysis programs typically use a linear diffusion equation. However, according to extensive petrophysical investigations by various authors, the relationship between porosity and pressure is nonlinear. This factor, like the reservoir compressibility coefficient, can significantly impact the accuracy of various transient flow problems. This applies particularly to determining the distance to reservoir boundaries and estimating reserves using pressure buildup tests for exploratory wells. Therefore, the aim of this study was to investigate the predominant type of porosity pressure function for subsequent refinement of the diffusion equation used for PTA. To solve the set task, petrophysical studies of core samples from terrigenous and carbonate formations were conducted in a pressure range of 1.44–64.65 MPa (200–9000 psi) using a compression rig. The results were compared with studies by other authors. These studies are not unique, but the results obtained differ from generally accepted ones. It is shown that the dependence of porosity on pressure follows a logarithmic law, rather than an exponential one, as is commonly believed. PTA calculations have demonstrated a significant impact of compressibility coefficient variability on the accuracy of determining the distance to reservoir boundaries. A detailed study revealed that core unloading results in an abrupt change in the compressibility coefficient, caused by the formation of microcracks. Therefore, using linear and exponential porosity-pressure relationships with a constant compressibility coefficient is highly inappropriate for solving PTA problems.