The effects of fluid pressure changes on fractured rock elastic moduli and surface deformation

Abstract

Reservoir models use the elastic moduli of rock, both bulk and shear, to compute deformation. These moduli may change with pressure and fracture density, but this effect is usually left out of models. This work shows effective elastic moduli of fluid-filled fractured rock through a self consistent method. The calculated effective elastic moduli for a penny-shaped crack are compared to literature values. Effective moduli values for rocks containing rough fractures with asperities are presented. The bulk and shear moduli increase with external stress. Increases in pore pressure cause an increase in bulk modulus but a decrease in shear modulus. The effect of using these determined effective moduli of fractured rock in modeling is investigated through a model of surface deformation over the In Salah gas reservoir in Algeria where carbon sequestration was performed. The In Salah CO₂ storage project is commonly studied due to the unexpected surface deformation observed. Surface deformation of less than a millimeter occurs from changing the material properties in this reservoir to that of saturated fractured rock containing 25 square rough fractures per cubic meter of 0.2 m side length and 0.22 m aperture, as determined in this study.