Coupled Poroelastic Solutions for the Reservoir and Caprock Layers with the Overburden Confinement Effects

Abstract:

Induced seismicity, caprock failure and possible fluid leakage are among the commonly reported mechanisms which would hinder the long-term storage of the fluid in the reservoir (compartment). Consequently, estimation of the injection rate and volume for the case of fluid injection or the time scale of dissipation into deforming rocks in the case of naturally pressurized compartments have been subjects of research interest in assessing the sealing efficiency of caprocks.

A three layers uniaxial model comprising the reservoir, caprock and burden rock is considered. Coupled theory of poroelasticity is used to develop an analytical solution for the time-dependent solid stress and pore pressure of the subsurface rocks. The stress-displacement formulation at the interface of the caprock and overburden is calibrated against the published three-dimensional analytical solutions for reservoir compaction to account for the confinement effect of the burden rock.

The obtained solution is applied into two problems: (1) Caprock integrity upon fluid injection into the reservoir. (2) Seal rock efficiency of an abnormally pressured reservoir compartment. Results of solution to the former problem indicate that the elastic moduli of the reservoir and caprock have substantial effect on the rate and magnitude of overpressure dissipation from the reservoir. For a given volume of injected fluid the peak values of shear or tensile failure tendencies of the caprock strongly depend on the injection time. Results of solution to the second problem indicate that the sealing efficiency of seal rocks has substantial dependence on the ratio of the reservoir to caprock thicknesses, as well as Young’s moduli, and to a less degree on the stiffness of the burden rock. However, for sufficiently small permeability of the seal rock, the efficiency of the seal rock in containing fluid transport from the high-pressure compartment rock would not be as sensitive to the permeability ratio of the rock layers.

In the two applied problems, the proposed generic coupled poroelastic solution can be used to optimize the injection time, and estimate the time scale of pressure dissipation from the abnormally pressured compartment.