The Effect of Stress on Permeability of Sandstone Cores

Abstract

Introduction The effects of mechanical stress on reservoir rock parameters are now generally accepted as beingimportantinreservoirengineeringcalculations. In several oil fields the unique behavior of the reservoir has been attributed to the influence of overburden pressure or internal fluid pressure on rock permeability or fluid content.Although the gross effects of overburden pressure on permeability, porosity, waterflood efficiency, electrical resistance and sonic velocity have now been established, there are some very interesting, and perhaps important, details of rock behavior under stress that have not heretofore been studied.In this paper we will show that permeability anistropy of several sandstones is a function of overburden pressure. We will also show that permeability reduction of cylindrical samples of two sandstones, when subjected to mechanical stress, is a function of the ratio of radial to axial stress.Most studies of overburden pressure effects made in the past have attempted to duplicate reservoir conditions by applying hydrostatic external pressure on all surfaces of the sample. Assuming perfect elasticity, that is, no viscoelastic flow during geological time, reservoir rock in situ cannot be under hydrostatic stress as is apparent from the value of 0.25 for Poisson's ratio for most rock. In the absence of tectonic forces the horizontal stress component (1) (2) If there are directional permeability effects, as has been reported from field and laboratory studies, 10, 11 then one would expect hydrostatic stress as used in the laboratory to give a different permeability reduction from that occurring under reservoir stress conditions.The influence of stress on directional permeability was measured by a modification of the method first described by Maasland and Kirkham for use on soil samples. Their method permits calculation of the permeabilities along each of the three mutually perpendicular axes of a cubic sample. The modification reported in this paper extends the method to observing these permeabilities when the sample is under hydrostatic stress.The Hassler sleeve type of core holder is satisfactory for studying overburden pressure effects on permeability if a known force is applied to the end plugs. Wyble used a sleeve core holder but did not load the end plugs. In this paper we combine sleeve pressure and axial force to give permeability changes which we believe are close to those occurring in reservoirs when reservoir pressure changes. The combination of sleeve pressure and axial force assumes, however, that the sleeve pressure simulates both vertical stress and horizontal stress in one conditions. METHOD PERMEABILITY ANISOTROPY The Maasland and Kirkham method of measuring permeabiHtyanisocopyusesacubeofporousmaterial which is open on three faces and has discs with a center hole for an air inlet cemented to the other three mutually perpendicular faces (Fig. 1). SPEJ P. 95^