Different deformation and failure processes of rock observed under critically and isotropically stressed conditions due to infiltration of compressible non-wetting fluid

The possibility of dynamic failure and degradation of sealing capacity of critically stressed cap rock due to infiltration of compressible non-wetting fluid was examined through laboratory experiments. A triaxial compression apparatus with a capacity to flow fluids through a specimen was used. A rock specimen was set up to mimic the condition of the cap rock just above the reservoir layer. Two kinds of external stress conditions were applied to the water saturated specimen, i.e., the conditions close to hydrostatic and those close to critical for normal faulting. Constant water pressure was applied to the upper end of the specimen. Air in a gas accumulator was infiltrated into the specimen from the bottom end. Both axial and circumferential strains at the center of the specimen and the air pressure at the inlet were monitored during the experiment. Axial strain (positive for contraction) of the specimen under hydrostatic condition increased at early stage, and then monotonically decreased. Circumferential strain of the specimen monotonically decreased. The deformation of the specimen was thought to be elastic. We tried to quantitatively evaluate the experimental results by using a simulator which solves coupled processes of two-phase fluid flow and poroelastic deformation of porous media. Experimental results showed that the specimen under critically stressed condition became more deformable than that under hydrostatic condition and resulted in failure. The increase in permeability was also recognized for critically stressed specimen. The following processes were thought to occur for the critically stressed specimen. At the bottom region where air was infiltrated, a sudden drop of effective stress caused by a sudden increase in pore pressure resulted in an occurrence of a small fault. Although volume of air increased by its migration along the fault, decrease in air pressure was quite small due to its high compressibility, and consequently, failure condition was kept at the region where air was infiltrated. Sequences of air migration and the formation of small faults from bottom to top of the specimen could explain the fact that the specimen became softer as time progressed. Finally, effective stress of the bulk specimen decreased and the failure occurred. These deformation and failure processes of critically stressed rock are important to understand sealing capacity of cap rock. This is because recent researches have argued that there are many sedimentary basins under critically stressed conditions in the world. Therefore, understanding of an in-situ stress state and consideration of the possible dynamic failure of cap rock are important to evaluate the sealing capacity of cap rock.