Effect of chemical environment and rock composition on fracture mechanics properties of reservoir lithologies in context of CO2 sequestration

Abstract:

The coupled chemical and mechanical response of reservoir and seal rocks to injection of CO₂ have major implications on the short and long term security of sequestered carbon. Many current numerical models evaluating behavior of reservoirs and seals during and after CO₂ injection in the subsurface consider chemistry and mechanics separately and use only simple mechanical stability criteria while ignoring time-dependent failure parameters. CO₂ injection irreversibly alters the subsurface chemical environment which can then affect geomechanical properties on a range of time scales by altering rock mineralogy and cements through dissolution, remobilization, and precipitation. It has also been documented that geomechanical parameters such as fracture toughness (K_IC) and subcritical index (SCI) are sensitive to chemical environment.

Double torsion fracture mechanics testing of reservoir lithologies under controlled environmental conditions relevant to CO₂ sequestration show that chemical environment can measurably affect K_IC and SCI. This coupled chemical-mechanical behavior is also influenced by rock composition, grains, amount and types of cement, and fabric. Fracture mechanics testing of the Aztec Sandstone, a largely silica-cemented, subarkose sandstone demonstrate it is less sensitive to chemical environment than Entrada Sandstone, a silty, clay-rich sandstone. The presence of de-ionized water lowers K_IC by approximately 20% and SCI 30% in the Aztec Sandstone relative to tests performed in air, whereas the Entrada Sandstone shows reductions on the order of 70% and 90%, respectively. These results indicate that rock composition influences the chemical-mechanical response to deformation, and that the relative chemical reactivity of target reservoirs should be recognized in context of CO₂ sequestration. In general, inert grains and cements such as quartz will be less sensitive to the changing subsurface environment than carbonates and clays.