Uniaxial Strain Unloading Compressibility of Frio Sand: Measurements and Implications on Reservoir Pressure Management for CO_2 Storage

摘要

The injection of fluids into a compartmentalized formation induces pore pressure buildup and decreases the local effective stress. Such changes of effective stress may result in re-activation of sealing faults. Variables that affect the amount of pore pressure change include the injected volume and the rock compressibility. This latter has been traditionally measured with isotropic loading compressibility tests. However, reservoir rocks under depletion or injection typically follow a uniaxial strain stress path rather than an isotropic stress path. Furthermore, injection unloads the reservoir rock rather than loading it as it occurs during depletion. This paper reports experimental measurements of the unloading uniaxial strain compressibility of Frio sand, a member of Tertiary strata in the Gulf of Mexico Basin. The uniaxial strain unloading compressibility of Frio sand is about one third of the uniaxial strain loading compressibility in the range of mean effective stress from 10 to 14 MPa. The uniaxial strain compressibility is expected to be roughly one half of the isotropic compressibility. Reservoir simulation of CO_2 injection in a compartmentalized formation indicate that using isotropic loading compressibility considerably underestimates the expected pore pressure increase.