Stress Evolution and Permeability Enhancement Mechanism of Multistage Cavity Completion in Coalbed Methane Horizontal Wells

Summary Coalbed methane (CBM) is an important clean energy resource. However, low gas production rate, especially in areas where hydraulic fracturing is notoriously inefficient, is the major obstacle that restricts the commercial development of CBM. Multistage horizontal well cavity completion has been observed to be successful in improving gas production rates in the Zhengzhuang block, Qinshui Basin, China. It has resulted in rates that are 1.5 times higher than the average production level achieved through horizontal well hydraulic fracturing. However, the stimulation mechanisms and major factors determining completion efficiency are still poorly understood. In this paper, we established a numerical model using the finite discrete element method (FDEM) to compute the stress evolution and fracture-network patterns. The accuracy of the model has been confirmed by analytical and numerical solutions. Subsequently, a series of parametric studies were performed to quantitatively analyze the mechanisms of multistage cavities influencing the stress evolution and fracture geometries in CBM reservoirs. Finally, we investigated a field case in an actual horizontal well located at the Qinshui Basin, where 17 cavity stages were completed. This case study further shed light upon the well completion strategies and optimization decisions. Implications and suggestions were also provided for field treatments to enhance the completion efficiency. The results demonstrate that FDEM can provide new insights into cavity completion mechanisms by explicitly accounting for fracture and fragmentation process at the field scale. The complex-fracture networks originated from multistage cavities consist of cavity-induced shear fractures, tensile fractures, mixed-mode fractures, and activated multiscale natural fractures, which is the primary reason for enhanced permeability and the essential difference from hydraulic fracturing. Compared with a single cavity, the interactions among multiple cavities can further promote the fracture-network connectivity and thus enlarge the stress-relief area and fracture area substantially. The selections of cavity geometrical parameters, including spacing, length, diameter, and number, have significant impacts on stress evolution (both magnitude and stress-relief area) and fracture patterns (such as fracture-network geometry, interconnectivity, propagation direction, and area). Stress evolution and fracture patterns reproduced from a field case in the Qinshui Basin can provide critical learnings for the industry in designing horizontal well cavity completion schemes. The key findings of this study are expected to deliver fundamental and practical guidelines for the horizontal well cavity completion in CBM or other unconventional oil and gas exploitation.