Numerical and Experiments Investigation of Wellbore Cavitation for Openhole Cavity Completion in CBM Reservoirs: A Case Study of Linxian Area in Ordos Basin, China

The remarkable coalbed methane (CBM) production achieved by cavity wells in San Juan Basin, USA, provide a strong impetus for testing the cavity completion method in China. Currently the problems, such as cavity can not be created or the buildup pressure by gas injecting can not be determined, often obstruct the operation of China’s CBM openhole cavity completion pilot wells. In view of the questions mentioned above, in this paper, the CBM reservoir in the Linxian area of Ordos Basin is taken as the target, a coupled gas-water flow and coal deformation numerical model and a series of laboratory cavitation experiments have been carried out to analyse the mechanism and the relevant influencing factors about CBM wellbore cavitation. The study shows that: a threshold value of coal cohesion affecting the successful application of CBM openhole cavity completion exists, when the coal cohesion is more than the threshold value, cavity cannot be created after the first pressure circle; the larger the permeability of reservoir, the lower the threshold value of coal cohesion, and with the increase of the initial permeability, the tensile failure zone decreases while the plastic failure zone increases. Based on the results of numerical study, an engineering model for judging the feasibility of cavity completion in Ordos Basin, China, is built based on the concept of threshold cohesion, also this model can be used to predict the minimum buildup pressure needed for the cavity creation, laboratory experiments were completed on large scale raw coal and synthetic block to simulate the process of cavity completion, experiments data are used to validate the engineering model. This study can guide the application of CBM openhole cavity completion in China.