Abstract:

The effective stress coefficient (a) is important to the analysis of poroelastic stress changes and deformations in the reservoirs. A Middle Bakken core was tested to study its effective stress in scenarios of depletion and injection. We designed an innovative experiment configuration to expedite the pore pressure equilibration within the tight specimen, and an exhaustive loading path to characterize the dependencies of a (with respect to static deformation) on confining pressure and pore pressure. Our test results reveal that the specimen's a is less than unity, and varies between 0.3 and 0.8. It suggests the effective stress is simply not equivalent to pressure difference between confining pressure and pore pressure. The effective stress coefficient a generally increases with pore pressure for constant confining pressure but decreases with confining pressure for constant pore pressure. The variation of a during depletion is more significant than during injection. These observations are also indicative of the microstructures of the specimen in terms of its responses to pore pressure and confining pressure variation.

Introduction

The production of hydrocarbons typically causes the reservoir to dwindle. Significant deformation can cause well failure, surface subsidence and production loss. What accompanies the deformation is the stress changes around and inside the reservoir (Segall and Fitzgerald, 1998; Zoback and Zinke, 2002), which may enhance or inhibit the rate of deformation. On the other hand, activities of hydraulic fracturing stimulation and enhanced oil recovery in unconventional plays inject large volume of fluid into the tight reservoirs. Like depletion, injection also induces significant deformation and stress changes, and potentially, triggers faulting and earthquakes (Zoback, 2012; Ellsworth, 2013). Thus, it is imperative to fundamentally understand the change in stress and strain as the reservoir depletes or undergoes injection.

Commonly, the stress changes associated with depletion and injection are considered as poroelastic and treated as such. How total stress (S) and pore pressure (Pp) interact and affect rock deformation is dealt by poroelasticity. The concept of effective stress (s) is important to evaluate the net effects of S and Pp on the deformation of rocks (Terzaghi, 1923; Biot, 1962). The Biot coefficient, or the effective stress coefficient, a, is used to couple S and Pp. For reservoir stress change and accompanying deformation, the effective stress is dictated by the particular a with respect to rock stiffness.

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