ABSTRACT:

Permeability is commonly assumed to be a function of effective stress, which considers only the difference between stress and fluid pressure. This approach allows permeability to be measured with very low values of fluid pressure in the laboratory. However, it is likely that permeability at high stress and high fluid pressure will be different than permeability at low stress and low fluid pressure, even for the same effective stress (stress minus fluid pressure). This is because pore pressure may influence permeability differently than confining stress. To test this, we developed a system to measure permeability along the axis of a rock sample, for which axial stress, confining stress, and fluid pressure can all be varied independently, and with fluid pressures approaching 138 MPa (20,000 psi). The highest stress is orthogonal to flow, to duplicate conditions that are important for reservoir production. We present results from several consolidated reservoir sandstones, showing that the effective stress coefficient for permeability is less than or equal to one over a large range of stress and pressure conditions, but that it can exceed one when pore pressure is high and effective stress is low.

1. INTRODUCTION

Stresses in petroleum reservoirs are not equal in all directions. Permeability also varies with direction, and permeability is affected by both the inherent nature of the formation as well as the stresses. For most reservoirs, the most important permeability to measure is the horizontal permeability. This permeability can be strongly affected by the stresses acting orthogonal to the flow direction, rather than the stress acting parallel to the flow direction. Furthermore, as the reservoir pressure declines due to production, the ''effective'' vertical stress (vertical stress minus fluid pressure) increases, and this reduces the formation permeability in the horizontal direction.

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