New Insights in Pressure-Transient Analysis for Stress-Sensitive Reservoirs

Abstract

Abstract Deep-seated low-permeability petroleum or geothermal reservoirs usually exhibit stress-sensitive permeability. For such reservoirs, pressure-transient analysis based on constant rock properties, especially permeability, can lead to significant errors in parameter estimation. Recently, an approximate analytical solution has been published to analyze pressure-transient tests in stress-sensitive reservoirs using the concept of "permeability modulus". However, because of the non-linear character of the partial differential equation for stress-sensitive reservoirs, application of superposition to model buildup tests is not straightforward. Using the concept of "permeability modulus", this study presents numerical solutions for stress-sensitive reservoirs. Pressure transient solutions with wellbore storage, skin, permeability modulus, and outer boundary effects have been studied in detail. Both drawdown and buildup solutions have been investigated. Solutions presented in this study also include multiple permeability modulus values for a given situation, where permeability modulus changes with respect to stress level in the reservoir in a stepwise mode. This study has pointed out inaccuracies involved in previous analytical solutions, especially for wellbore storage and skin effects, and buildup tests. Previously, it has been stated that stress-sensitive permeability effects can cause wellbore storage early-time unit slope line to be masked. This investigation shows that, irrespective of the severity of stress sensitivity, wellbore-storage-dominated unit-slope line always appears at early time. We also show that the use of multiple permeability modulus values to study pressure-transient analysis for stress-sensitive reservoirs is consistent with available laboratory measurements of permeability as a function of stress.