Coalbed methane (CBM) is one of the important unconventional gas resources in the worldwide. However, we are still facing the challenge of low gas production rate for a single well. Here, we proposed a new completion method: horizontal cavity completion achieved by abrasive water jet, which is expected to attain stress relief in large area and enhance the coal permeability in coal seam. In this work, numerical simulations were carried out with fluid-solid coupled method to find out the suitable nozzle type for horizontal cavity completion. First, we compared the fluid field of a rotate nozzle and a conical nozzle. Then, the coal-breakage mechanism was analyzed based on the stress distribution. The rock failure can be attribute to the two coupled mechanisms: tensile failure combined with shear failure. Results reveal that the abrasive rotating jet has a larger damaging area than the conical jet, while conical jet has greater tensile stress near the stagnation point. The key findings are expected to provide theoretical basis and design reference for the horizontal cavity completion in coalbed methane wells.

1. Introduction

The low gas production rate for a single CBM well is still a great challenge in China (Lou, 2013). Depressurization and desorption are the theoretical basis for the current in-suit CBM surface wells production (Liu et al., 2011). The core concept is to reduce the reservoir pressure of coal bed methane through well drainage. When the reservoir pressure is lower than the critical desorption pressure, a large amount of methane will be desorbed, and free gas migrates to the wellbore (Tang et al., 2004). However, due to relatively low permeability (Chen et al., 2017), the flow path is not well established and the hydraulic fracturing performance is also poor in China. Moreover, multistage horizontal fracturing, CO2-ECBM (Sampath et al.,2017), N2-ECBM and other EOR technologies have not been widely used due to the high costs. Hence, it is necessary to explore a low cost and high efficiency exploitation method.

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