Impact of Stress Sensitive Permeability on Production Data Analysis

Abstract

Abstract The dependence of reservoir permeability and porosity on pressure can have a significant impact on well performance. This paper addresses this issue theoretically, via reservoir simulation, and via production data analysis type curves. This paper presents the single phase fluid flow equation for a slightly compressible fluid in a stress sensitive reservoir in a form which can be solved in Laplace space to generate profiles of well flow rate versus time (for constant bottomhole pressure production). This is achieved by writing the flow equation in terms of the derivatives of a function involving the sum of pressure and pressure-squared terms. Reservoir simulation results concur with the theoretical prediction that depletion in a stress sensitive reservoir will be slower than in a non-stress sensitive reservoir, however ultimate recovery in the simulation cases was not affected by stress sensitivity. p/z plots for stress sensitive reservoirs show volumetrics are unaffected by stress sensitivity. Analysis of simulated production data from stress sensitive reservoirs shows that drainage area is likely to be underestimated by this approach. Permeabilities predicted from analysis using standard type curves are lower than the permeability at initial reservoir pressure. The magnitude of the reduction in the interpreted permeability naturally depends on the magnitude of the stress sensitivity. Introduction Stress sensitivity is a particularly important phenomenon in tight gas reservoirs. Rushing and Newsham (2001) noted this and recommended measuring rock properties at a range of stress conditions. McKinney et al. (2002) attribute the hyperbolic decline often observed in tight gas reservoirs to stress sensitivity. Cox et al. (2007) considered the impact of stress sensitivity on a couple of simulated cases in a wider study but did not comprehensively address the impact of stress sensitivity of production data analysis. The topic is of considerable practical interest to reservoir engineers e.g. Amar et al. (1995), Lei et al. (2007) and Hedong et al. (2007). Much of theoretical analysis of flow in stress sensitive reservoirs has focused on pressure transient analysis (e.g. Pedrosa, 1986, Samaniego and Cinco-Ley, 1989). This study considers rate transient analysis which has received less attention. The approach presented in this paper avoids the use of a pseudopressure to handle stress sensitivity, however the approach presented in this paper is limited to a linear variation of permeability and porosity with pressure change. Theory Fluid Flow Equations To assess the impact of stress sensitive permeability on well performance the nature of the differential equation governing single phase fluid flow was considered. Note that this analysis uses a form of the fluid flow equation which is applicable to a slightly compressible (i.e. liquid flow) case. This section aims to provide some theoretical insight to compliment observations based on reservoir simulation results presented in later sections.