Dynamic modeling of the crack deformation in dry/gas-saturated rocks with increasing normal stress

Abstract

The crack widely exists in reservoir rocks and the change of the stress state may lead to its deformation or even closure, which has a great influence on the oil and gas accumulation and percolation in reservoir rocks. In order to investigate the crack deformation at different stress conditions, a two-dimensional dynamic modeling method for crack deformation under progressively increasing compressional stress normal to the crack surface is proposed to simulate the natural process of crack closing. The crack deformation is extracted to update the geometric model of cracked rocks at each loading step. Such dynamic modeling method is verified by the velocity measurement of synthetic samples with known crack structures at different effective stresses. Dynamic modelings of different crack structures are designed to numerically test the effects of initial crack porosity, crack aspect ratio and crack density on the stress-dependent crack deformation. The modeling results indicate that the crack porosity and the crack aspect ratio decrease negative-exponentially with the increase of normal stress, while the crack density decreases positive-exponentially as the normal stress increases. Rocks containing cracks with lower crack aspect ratio, higher crack density and (or) higher crack porosity have stronger stress sensitivity. For different crack structures, the stress sensitivity coefficient of crack density shows much more wide variation, which indicates that the stress sensitivity coefficient of crack density should be a potential effective attribute for quantitatively identifying and predicting cracks within the underground rocks.