Dynamic gas supply mechanisms and theoretical production modes in deep free gas-rich coal reservoirs
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摘要: 【目的和方法】 吸附气、游离气产出过程及其产出效率的动态配分机制是深部煤层气勘探开发亟待解决的关键科学问题。基于探井取心测试化验数据，系统分析了深部煤层气资源的理论可动性，结合数学/数值模型构建、甲烷碳同位素监测和排采曲线解剖，揭示了排采诱导的储层压降扩展、吸附气解吸、游离气渗流的时空演化过程及其产出效应，提出了深部富游离气煤储层多态甲烷协同供气机制和理论生产模式。【结果和结论】 (1)深部煤层气井生产过程中，游离气和吸附气具备“连续-协同”的供气特点和“竞争产出”的配分关系，任意时刻产出气均为二者的混合气，不同赋存态甲烷的动态配分比例取决于不同生产阶段压力传播域内以“降压诱导解吸、压差驱动渗流”为核心的游离气传质效率和吸附气解吸补充效率的叠合。(2)深部煤储层将经历解吸全过程，降至关键解吸节点所需压力降幅较大，初期高储层压力-低解吸效率与后期高解吸效率-低压降空间的矛盾难以调和，压降漏斗内吸附气平均解吸率偏低且供气单元主要集中在高渗改造区，单井排采且压降不触及气藏边界时游离气供气半径可持续拓展并始终占据较大产量比重，井组联采的井间干扰行为会导致排采中后期游离气占比下降、吸附气产能主导。(3)游离气和吸附气分别具有“骤增-缓降或骤增-骤降-缓降”和“缓增-趋稳-缓降”的生产特征，总体产能存在快速上产、相对稳产和缓慢递减3个主要阶段，排采曲线形态受控于游离气量、原位渗透率、储层改造效果、排采降压制度等因素，部分井在相对稳产阶段存在先骤降后趋稳两个次级阶段。(4)增大改造体积、提高水平段长、寻找富游离气-高孔渗区段是增产核心，探索提高吸附气解吸效率和压降整体下沉幅度的工艺技术是增加深部煤层气下探深度的关键。(5)以地质-工程一体化原理为指导，确定深部不同地质单元煤层气合理产能目标及其所需井控面积，协同优化完井方式、井网密度、压裂参数、配产速率和生产周期，是深部煤层气效益建产的重要攻关方向。Abstract: [Objective and Methods] The dynamic partitioning mechanism of the production efficiency of adsorbed gas and free gas is a critical scientific issue that urgently needs to be addressed in the exploration and development of deep coalbed methane (CBM). Based on core testing and laboratory data from exploration wells, this study systematically analyzed the theoretical mobility of deep CBM resources. By integrating the construction of mathematical/numerical models, monitoring methane carbon isotopes, and analysis of drainage curves, the research revealed the co-evolutionary process of pressure drop expansion, adsorbed gas desorption, and free gas seepage induced by drainage. Furthermore, a collaborative gas supply mechanism and theoretical production model for multi-state methane in deep coal reservoirs was proposed. [Results and Conclusions] (1) During the production process of deep CBM wells, free gas and adsorbed gas exhibit a "continuous-synergistic" gas supply characteristic and a "competitive production" allocation relationship. At any given moment, the produced gas is a mixture of both, where the dynamic partitioning ratio of methane in different occurrence states depends on the superposition of free gas mass transfer efficiency and adsorbed gas desorption efficiency within the pressure propagation domain at different production stages. (2) Deep coal reservoirs undergo the entire desorption process, requiring substantial pressure drawdown to reach critical desorption nodes. The inherent conflict between early-stage high reservoir pressure with low desorption efficiency and late-stage high desorption efficiency with limited pressure drop space remains challenging to reconcile. Within the pressure drawdown funnel, the average desorption rate of adsorbed gas remains low, with gas supply units predominantly concentrated in high-permeability stimulation zones. Under single-well drainage conditions where pressure drawdown does not reach reservoir boundaries, the free gas supply radius continues to expand and consistently maintains a dominant production share. However, inter-well interference in clustered well groups may lead to a decline in free gas contribution during mid-to-late production stages, shifting productivity dominance to adsorbed gas. (3) Free gas and adsorbed gas demonstrate distinct production characteristics: "sharp increase followed by gradual decline" or "sudden increase-sharp decline-gradual decline" for free gas, and "gradual increase-stabilization-gentle decline" for adsorbed gas. Overall production capacity progresses through three primary phases: rapid buildup, relatively stable output, and slow decline. Production curve morphology is governed by free gas volume, in-situ permeability, reservoir stimulation effectiveness, and pressure drawdown management strategy. Some wells exhibit two substages: an initial sharp decline followed by stabilization during the relatively stable production phase. (4) Enhancing stimulation volume, extending horizontal well sections, and targeting free gas-rich zones with high porosity-permeability constitute core strategies for production enhancement. The development of technologies to improve adsorbed gas desorption efficiency and achieve comprehensive pressure drawdown magnitude emerges as the key to advancing exploration depths. (5) Guided by the principle of geology-engineering integration, determining reasonable productivity targets and required well control areas in different deep geological units, while synergistically optimizing completion methods, well spacing density, fracturing parameters, production allocation rates, and operational cycles, constitutes the paramount priority for cost-effective development of deep CBM.
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Keywords:
- deep CBM /
- adsorbed gas /
- free gas /
- production mechanism /
- production modes
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