Stability analysis method of CBM reservoir during depletion and its application:A case study of Zhengzhuang Block

The original in-situ stress varies during the depletion of coalbed methane (CBM) reservoir.which may lead to the failure of coal rocks and significantly reduce the CBM recover efficiency. In order to analyze the var-iation law of in-situ stress during the CBM reservoir depletion.a theoretical stress-depletion response model was derived based on a uniaxial strain model. Besides.a stability analysis model during depletion was built and an evaluation index (H) was proposed to evaluate the CBM reservoir stability under different in-situ stress circum-stances. This research indicates that:① In the drainage stage.with the drop of the pore pressure.the horizontal principal stress decreases linearly.However.in the gas production stage.under the strong matrix shrinkage effect.the decrease rate is enhanced.② The failure laws of CBM reservoirs under different in-situ stress regimes are different. For the normal faulting regime.in the drainage stage or gas production stage.the radius of Mohr circle rises and the center moves right. In this process.the Mohr circle approaches the Mohr failure envelopes and the reservoir may be damaged. With the increase of desorption effect.the rise rate of Mohr circle radius enhance and the coal reservoir is prone to destruction. For the striking-slip faulting regime.in the drainage stage and initial gas production stage.the Mohr circle radius remains constant and its center moves right.away from the Mohr failure envelopes. The reservoir remain stable. While the reservoir failure may occur in the stable gas production stage and the gas rate decay stage due to the strong matrix shrinkage effect which cause the Mohr circle moves left and approaches the Mohr failure envelopes. For the thrust faulting regime.the Mohr circle shrinks during the whole depletion process. The reservoir will not be damaged.③ According to the data of in-situ stress testing wells in the Zhengzhuang Block.the reservoir stability was evaluated. Before CBM depletion.the H value mainly is between 32-40 MPa and the reservoir remains moderately stable. The reservoir stability reduce from the middle to the northwest and south region of Zhengzhuang block. In the drainage stage.the H value will rise linearly with the decrease of pore pressure and the reservoir remains highly or moderately stable. In the desorption stage.the H value declines rapidly and the reservoir stability is reduced. It is notable that as the pore pressure drops at 0.2 MPa.the H value declines gradually from the middle region to the western region of Zhengzhuang Block. ④ The gas production curve of some CBM wells shows that gas production rates were unstable and the duration of gas production peaks were short. When the reservoir pressure decreased to a relative low value.the gas production rate declined rapidly and coal fines can be monitored. It inferred that the CBM reservoirs may be damaged in the process of depletion.