A Rock Mechanical Experimental Study on the Effect of Hydrostatic Pressure and Directional Stress on Rock Sample Permeability

Abstract

ABSTRACT: The objective of this study is to compare the laboratory sensitivity of the permeability of cylindrical plugs to variations in hydrostatic and directional stress to determine how different stress evolution trajectories may affect fluid flow for different rock types. To achieve this, series of Stress-Dependent Permeability (SDk) tests were performed on suites of rock mechanical plugs, covering a wide range of lithology and permeability, employing different types of transients simulating varying geo-mechanical conditions during loading and/or unloading processes. Specific laboratory testing procedures were adapted depending on the permeability of the rock sample. Steady-state method was deployed for medium to high permeability samples, while complex transient (subjecting a sample to a pressure pulse and monitor the resulted pressure decay) methods were deployed to estimate the permeability of tighter rock samples. Test results suggest that plug samples with low to ultra-low permeability are more sensitive to changes in hydrostatic pressure, and less affected by directional stress variations. For more permeable samples, permeability responses to a dynamically evolving state of stress may exhibit a degree of sensitivity to directional stress variations difficult to predict without conducting a proper laboratory analysis. 1. INTRODUCTION In some cases, during the production cycle of a reservoir, the net effective reservoir stress is increasing due to reduction of the pore pressure; this continuous process induces an alteration of the reservoir flow capacity of the reservoir. Published studies (Fatt, 1952) showed that while the change in porosity with pressure is small, the effect of overburden stress is significant on permeability. Another study (Jones, 1979) showed that tight sand samples are profoundly affected by evolving net reservoir stress more than the permeable samples. Moreover, the effect of changing stress environment has been investigated on different formations, i.e. sandstone and carbonates (Dautriat et al, 2007) to accentuate a relationship between effective stress and permeability. In this lab study, we are attempting to investigate the stress-dependent permeability in tight and medium-to-high permeability rock samples, in response to the effect of both hydrostatic pressure and directional stress, aiming to program the relationships between hydrostatic pressure and directional stress in order to optimize permeability and maximize fluid flow.