Measurement of Unloading Pore Volume Compressibility of Frio Sand Under Uniaxial Strain Stress Path and Implications on Reservoir Pressure Management

The injection of fluids into a compartmentalized formation induces pore pressure buildup and may result in reactivation of sealing faults. Among other variables, the pore volume compressibility (PVC) can affect the amount of pore pressure change during injection. PVC has been traditionally measured with isotropic loading compressibility tests. However, long and thin reservoirs subjected to depletion or injection typically follow a uniaxial strain stress path, rather than an isotropic stress path. Furthermore, injection unloads the reservoir rock by reducing effective stress, whereas depletion causes loading. This paper reports experimental measurements of the uniaxial strain unloading compressibility of Frio sand, a member of Tertiary strata in the Gulf of Mexico Basin. The uniaxial strain unloading compressibility increases nonlinearly from 0.29 to 1.45 GPa⁻¹ (2 to 10 µsip) as the mean effective stress is reduced from 26 to 5 MPa. The uniaxial strain unloading compressibility of Frio sand is about one third of the uniaxial strain loading compressibility at comparable levels of effective stress. The uniaxial strain compressibility of Frio sand is roughly one half of the isotropic compressibility. Reservoir simulation highlights that using incorrect pore compressibility values considerably underestimates the expected increase of pore pressure in a compartmentalized formation during injection. Uniaxial strain unloading compressibility of target reservoir rocks should be accurately estimated or measured to prevent excessive pressure build up in target storage formations during injection of CO₂ or any other fluid.