New Insights of Stress Sensitivity Experiment in the Kuqa Deep Gas Reservoir

In order to reveal the stress sensitivity characteristics of the Kuqa deep gas reservoir, this study investigates the problem from two complementary aspects. First, conventional variable confining pressure experiments and well-test interpretation are employed to clarify the basic stress sensitivity characteristics of the Kela 2 gas field under conditions of monotonically increasing effective stress. Second, considering that field operations such as gas injection and temperature rise may cause periodic pore-pressure fluctuations under nearly constant overburden pressure, this paper establishes a novel physical simulation method for multi-round charging and depletion recovery to investigate the additional reservoir responses under cyclic effective-stress evolution. The results show that (1) when the confining pressure increases from 5 MPa~40 MPa, the permeability of the core generally decreases, with a decrease of 5~85%. In contrast, the porosity decreased by only 2% to 12%. The number of cores with conventional air permeability greater than or equal to 1 mD in the Kela 2 gas field reservoir accounts for 63.4%. The stress sensitivity causes the permeability to decrease by less than or equal to 40%, and the overall stress sensitivity is not strong. (2) Post-test observations showed fracture development in some cores after the experiment, indicating that during the gas reservoir mining process, the stress cycle changes will cause some closed cracks in the core to reopen or produce new cracks, which will play a role in increasing permeability. After the crack is opened, the comprehensive recovery degree at the end of the stable production period increases by 21.7 percentage points, and 9.9 percentage points increase the comprehensive recovery degree at the end of the abandoned production. (3) The new understanding of this experiment has changed the traditional understanding that stress sensitivity can only lead to reservoir damage, and also pointed out a new technical direction for the field to improve reservoir physical properties and enhance oil recovery by changing stress effects such as heating-condensation, intermittent gas injection, and directional blasting.