Investigation of hydraulic jet multistage cavity completion in coalbed methane horizontal wells

China is rich in coalbed methane resource. However,a low production rate is the long-time bottle-neck problem. In this paper,an innovative proposal of creating multistage cavities by hydraulic jet(abrasive jetting) in coalbed methane horizontal wells is presented. First, the advantages of horizontal cavity completion by hydraulic jet were discussed. Flexibility in achieving any cavity shape and size,pin-point jetting,precise segmentation,multi-stage pressure-relief,creating complex networks and meeting the demand of in-situ fluidized mining of underground coal resources are the major benefits. Subsequently,the FDEM method was implemented to calculate the stress-relief area and describe the fracture generation characteristics around the cavities,and then illuminate the production enhancement mechanisms. Then,the laboratory experiments of abrasive round jetting and rotating jetting on various types of coals under submerged conditions were conducted. Shapes of jetted holes,rock-breaking modes and impacts of natural fractures were investigated. The results indicated that the stratification cracking along the weak planes was the most important coal-breaking mechanism. Next,the critical downhole tools,technological processes and coal particles flow-back related to different completion methods such as open-hole,slotted liner,casing-perforated and roof indirect stimulated wells were introduced and discussed. Finally,the application prospects and geological adaptability of the proposed method were illustrated. The proposed technique can be applied in tectonic coal,deep coal,coal measured-strata,soft coal beds where hydraulic fracturing is ineffective or multi-lateral boreholes are difficult to re-entry. © 2022 China Coal Society. All rights reserved.