Drainage system research and application of deep coalbed methane gas reservoirs in Daning-Jixian block

CHEN Ming; WANG Dameng; YU Lizhu; SUN Junyi; FENG Xin; ZHANG Kang; GU Xianfeng; WANG Liwei; YIN Zesong; ZHANG Huan

doi:10.13225/j.cnki.jccs.2024.0318

Abstract：Compared to Medium-shallow coalbed methane,deep coalbed methane exhibits significant differences in oc-currence state,production mechanism,lifting technology,etc.As an important part of coalbed methane development,drainage directly impact the gas production of deep coalbed methane wells.Therefore,it is essential to explore a refined drainage system that is suitable for deep coalbed methane.Based on the geological conditions and reservoir properties of the Daning-Jixian block,isothermal adsorption tests were conducted on the main producing layer-coal seam No.8 to ana-lyze the gas content and occurrence state of deep coalbed methane.Considering the gas well production laws and support-ing gas extraction processes,the research also analyzes dynamic production data from 29 deep coalbed methane wells,pre-liminarily divides the production stages of deep coalbed methane wells,and further demonstrates the main tasks and pro-duction characteristics of different stages.Combining the development theory of"artificial gas reservoirs"and the design concept of a production system for the entire life cycle of gas wells,the research divides the entire life cycle of gas wells into 5 production stages and forms a standard template.Moreover,field experiments were conducted to produce according to the designed refined drainage system and evaluate the applicability of drainage system for deep coalbed methane wells.Research results indicate:① No.8 coal seam is generally stable.Isothermal adsorption experiments show that the Lang-muir volume is generally greater than 20 m3/t,Langmuir pressure is greater than 2.8 MPa,and gas saturation in deep coal seams increases by 29.6％compared to medium-shallow layers,mainly existing in a saturated to supersaturated state,with free gas content reaching up to 20％-30％;② Utilizing the coal-rock Langmuir curve and changes in curvature,the re-search figures out start-up pressure,inflection pressure,and sensitivity pressure as 8-10 MPa,5-7 MPa,and 2-3 MPa re-spectively.Furthermore,through the normalization of production time and horizontal segment length from 29 gas wells'dynamic production data,the research preliminarily identifies that deep coalbed methane have gone through 4 production stages,with Stage Ⅰ being post-fracture flowback,Stage Ⅱ characterized by rapid initial production,Stage Ⅲ focusing on exploring rational production allocation,and Stage Ⅳ indicating a decrease in gas production.The pressure,gas produc-tion,liquid production,and flowback rate parameters at each stage of the gas wells were also delineated;③ Based on the objective laws of gas well production throughout its life cycle,with pressure control as the core,the research also divides the production stages of deep coalbed methane wells into 5 stages:flowback stage,initial production stage,stable produc-tion stage,decreasing production stage,and low production stage,with quantifying the control indicators and technical limits for each stage.The low production stage is awaiting further verification;④ The average stable production time for 4 test wells has reached 360 days,far exceeding the average level of the pilot wells,with an average annual production of 33.279 million cubic meters from the day of self-gas production,accumulating a high increase of 71.35％.That proves good adaptability of the refined drainage system in the Daning-Jixian block.