Failure mechanism of cavitation-induced shear of the plugging layer in high-temperature high-pressure fractured gas reservoirs in the Tazhong block, NW China

Abstract

To profoundly understand the plugging layer failure behavior of high-temperature high-pressure fractured gas reservoirs in the Tazhong block, the failure characteristics of high-temperature high-pressure fractured gas reservoirs were studied and analyzed based on the combination of reservoir characteristics, fluid properties and the microscaled physical structure of the plugging layer. The concept of cavitation-induced shear failure is proposed, and the physical model of cavitation-induced shear failure is established. The cavitation-induced shear failure process of the plugging layer is systematically studied with the aid of granular material mechanics and liquid bridge theory. In addition, laboratory experiments of reversed pressure-bearing were carried out based on the principle of reversed cavitation. Results show that the cavitation-induced shear failure of the fractured sealing layer is a special kind of failure modes of the plugging layerin gas reservoirs. This process can be divided into three steps: viscosity reduction because of gas diffusion, cavitation-induced shear stripping and fluids displacement mismatch shearing destructions. In addition, the experimental results show that the plugging layer with a positive pressure of 6 MPa has different shear failure resistances for different fluids. With the displacement medium changing from liquid to gas, the reversed pressure-bearing value of the plugging layer decreases from 2.0 MPa (22 min) and 2.5 MPa (30 min) to 1.5 MPa (10 min) and 1.0 MPa (12 min), respectively. The comprehensive shear resistance is reduced by approximately 50%, indicating that gas has different destructive ability and destructive mechanism compared to liquid.

Keywords

• high-temperature high-pressure
• fractured gas reservoir
• plugging layer failure
• cavitation-induced shear failure
• reversed pressure-bearing