ABSTRACT
A medium-to-high porosity St. Peter sandstone was subjected to hydrostatic confining pressure Pc(=S) under fully-drained conditions, simulating the stressed rock in situ during depletion or injection. We recorded variations of static strain and dynamic velocities with confining pressure and pore pressure to estimate the corresponding effective stress coefficient. For the static deformation data, α is clearly less than unity, ranging between 0.3 and 0.7 for any tested stress conditions. The effective stress coefficient is dependent on Pp during depletion but not so during injection. Given the same stress condition, the effective stress coefficient during injection is consistently higher than during depletion. The dynamic effective stress coefficient for Vp is generally close to unity when σ is less than 20 MPa. During depletion, α for Vp first decreases with σ when the latter is up to 30 MPa. However α jumps to unity for σ ≥ 30 MPa. The effective stress coefficient for VS increases significantly with σ, regardless of the loading path. It is important to note that the effective stress coefficient with respect to velocities is different from that of static deformation, in both the magnitude and the dependency on Pc and Pp.
The production of hydrocarbons typically causes the reservoir to contract. Significant deformation can cause wellbore failure, surface subsidence and production loss. On the other hand, hydraulic fracturing stimulation and enhanced oil recovery inject large volumes of fluid into the reservoirs, which can potentially trigger faulting or earthquakes. What accompanies such deformation is the stress changes around and inside the reservoir, which may enhance or inhibit the rate of deformation. 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. The effective stress coefficient α, as a key parameter in poroelasticity, is used to evaluate the relative contribution of total stress S and pore pressure Pp on rock properties (Biot, 1962). While there is a continuing interest in the effective stress coefficient, the experimental studies on α has been scarce. Motivated by the effective stress alteration due to depletion and subsequent fluid injection in conventional reservoirs, we characterized a St. Peter sandstone in order to obtain its dependencies of α on S and Pp with respect to volumetric deformation and velocities. We present our preliminary test results here.
