大宁−吉县区块深部煤层气井排采制度研究与实践

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

  • 摘要: 与中浅层煤层气相比,深部煤层气在赋存方式、产气机理、举升工艺等方面存在较大不同。排采作为煤层气开发过程中的重要一环,直接影响投产后深部煤层气井的产气效果,探究与其相适应的精细化排采制度十分必要。基于大宁−吉县区块地质条件与储层物性分析,针对主力产层8号煤开展等温吸附试验,分析深部煤层气含气性和赋存方式;考虑气井生产规律和配套采气工艺,分析先导试验29口深部煤层气井生产动态数据,初步划分深部煤层气井生产阶段,明确了不同阶段主要任务和生产特征;结合“人造气藏”开发理论和气井全生命周期生产制度设计理念,整体将气井全生命周期划分为5个生产阶段,探索形成了1套标准模板;开展现场试验,按照设计的精细化排采制度进行生产,评价深部煤层气井排采制度适用性。研究结果表明:① 8号煤层整体发育稳定,煤体结构为原生结构煤,等温吸附试验显示兰氏体积普遍大于20 m3/t,兰氏压力大于2.8 MPa,深部煤层气储层含气饱和度较中浅层增加29.6%,以饱和−过饱和赋存状态为主,游离气占比可达20%~30%;② 利用煤岩兰氏曲线及曲率变化、弯曲情况,得到大宁−吉县区块8号煤启动压力、转折压力、敏感压力分别为8~10、5~7和2~3 MPa,进一步通过29口气井生产动态数据生产时间与水平段长归一化处理,初步认定深部煤层气井目前经历了4个生产阶段,第Ⅰ阶段为压后放喷返排,第Ⅱ阶段主要表现为投产初期快速上产,第Ⅲ阶段主要任务是摸索合理配产,第Ⅳ阶段为产气递减阶段,圈定气井各阶段压力、产气量、产液量、返排率等参数变化范围;③ 依托气井全生命周期生产的客观规律,以控压开采为核心,将深部煤层气井生产阶段划分为5个阶段,分别为返排阶段、上产阶段、稳产阶段、递减阶段和低产阶段,同时量化了各阶段的控制指标与技术界限,因尚未有气井进入低产阶段,低产阶段有待进一步验证;④ 4口试验井平均稳产时间已达360 d,远超先导试验井平均水平,首年平均单井产气量增加71.35%,精细化排采制度初步显示出较好的适应性。

     

    Abstract: Compared to Medium-shallow coalbed methane, deep coalbed methane exhibits significant differences in occurrence 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 analyze the gas content and occurrence state of deep coalbed methane. Considering the gas well production laws and supporting gas extraction processes, the research also analyzes dynamic production data from 29 deep coalbed methane wells, preliminarily divides the production stages of deep coalbed methane wells, and further demonstrates the main tasks and production 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 Langmuir 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 research figures out start-up pressure, inflection pressure, and sensitivity pressure as 8~10 MPa, 5~7 MPa, and 2~3 MPa respectively. 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 production, 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 production 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.

     

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