Abstract

Long-term prediction of reservoir performance can be strongly influenced by stress sensitivity. As the effective stress in a reservoir increases due to production, the porosity and permeability decreases. Inelastic rock deformation mechanisms play a significant role in bringing about these play a significant role in bringing about these changes in porosity and permeability. This paper shows that inelastic pore collapse can be the dominant mechanism in reducing pore space and permeability even when reservoir conditions are permeability even when reservoir conditions are hydrostatic. The presence of a shear component in the in-situ stress can increase these effects. A numerical simulated reservoir study has shown that well work-over and stimulation projects could be warranted earlier than currently practiced.

Introduction

Long-range predictions of reservoir performance should take account of stress sensitivity, performance should take account of stress sensitivity, as porosity and permeability can change significantly as the effective stress is increased by pore pressure reduction during production. Inelastic pressure reduction during production. Inelastic (irreversible) rock deformation mechanisms play a significant role in bringing about these changes in porosity and permeability.

Inelastic effects in homogeneous media are usually associated with shearing deformation. In heterogeneous media, however, the application of hydrostatic pressure (without a shearing component) gives rise to local shearing due to stress concentrations at inhomogeneities or inclusions. This is particularly true of porous rocks, which may be quite homogeneous on the scale of a laboratory sample but which are heterogeneous on the scale of the grains, pores and cracks. In this case, the hydrostatic response may be quite inelastic, as evidenced by the considerable hysteresis observed on unloading in some porous specimens. This inelastic pore collapse can be the dominant mechanism of reduction of pore space and permeability, even when reservoir conditions are hydrostatic. A second cause of inelastic deformation, of course, is the actual presence of a macroscopic shear component in the in-situ stress.

In this paper we treat both types of inelastic effects, with emphasis on the former, and we examine their implications with respect to reservoir performance. In many real situations the performance. In many real situations the reservoir performance can be predicted, with acceptable accuracy, without consideration of inelastic effects. In other cases, consideration of these effects is necessary; this may be true both for shallow reservoirs with highly porous soft rock and for deeper reservoirs with less porous hard rock which, however, experiences a porous hard rock which, however, experiences a much larger increase in effective pressure during production. One description of the deformation production. One description of the deformation effects is based on a model for the response of fluid-saturated porous media developed in earlier papers. papers.

HYDROSTATIC RESPONSE

Our treatment of the hydrostatic response of a fluid-saturated rock is based on an identification and separation of two distinct deformation mechanisms - matrix or solid compaction and pore collapse - as described by the following set of equations:

(1)

(2)

(3)

(4)

(5)

(6)

Here V, Vs and Vp denote total volume, solid volume and pore volume, respectively, and the porosity is defined by equation.

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