Abstract: Deep coalbed methane (CBM), typically located at depths greater than 2 000 meters, exhibits distinct geological and production characteristics. These are primarily reflected in the unique characteristics of coal seams as well as the occurrence conditions, specific flow properties and production patterns of CBM under high temperature, high pressure, high stress, and high transformation conditions. The results show as follows. (1) The reservoir space in deep coal seams is dominated by micropores with a diameter less than 2 nm, indicating that these reservoirs are typical nanoscale systems. As the coal rank increases, the volume fraction of micropores in coal seams gradually rises. These micropores constitute the primary occurrence space for adsorbed gas, and their volume proportion to some extent reflects the relative content of adsorbed gas in the reservoir. Deep coal seams feature a typical dual pore-fracture structure; however, their porosity and permeability significantly decrease with the increasing burial depth under the high-temperature and high-pressure conditions. (2) The distribution changes among different occurrence states of deep CBM result from the combined coupling effects of temperature, pressure and occurrence space. As burial depth increases, the proportion of adsorbed gas in coal seams decreases, while that of free gas correspondingly increases. A clear critical transition depth exists, marking the shift in dominance between adsorbed gas and total gas content. The enrichment and accumulation of deep CBM attributed to the coupled interplay of coal reservoir composition, burial depth and preservation conditions, forming a "triple-control" mechanism on gas accumulation. Among these factors, depth effect represents a distinctive characteristic of deep CBM, while preservation conditions are critical for the enrichment of free gas. (3) Under the influence of deep high-temperature conditions, the desorption amount, desorption rate, desorption ratio, and diffusion coefficient of adsorbed gas in coal seams increase significantly. This leads to a pronounced methane desorption compensation effect, allowing coal rocks to release more methane despite a decrease in adsorption capacity. Meanwhile, the peak period of adsorbed gas desorption occurs earlier. Under high-temperature conditions, low gas-water viscosity ratios and diminished interfacial tension enhance the seepage flow of gas-water two-phase fluids within coal seams. (4) The production behavior of deep CBM fundamentally represents the integrated flow dynamics of multi-phase and multi-state fluids under elevated temperature and pressure conditions combined with extensive hydraulic fracturing. The CBM production process also reflects the gradual dissipation of ultra-static pore pressure, typically exhibiting a declining production pattern characterized by an initial high peak, a mid-term stable output, and a late-stage steady decrease. During the evolution of deep CBM, free gas dominates production output in the early production phase, while adsorbed gas plays a dominant role in the middle to late stages. Throughout the entire production lifecycle of a CBM well, the contribution of free gas to the total output generally approximates its original proportion in the virgin coal reservoir. The carbon stable isotope values of methane (δ¹³C₁) in deep CBM wells exhibit a "L-shaped" variation trend over the drainage period, reflecting certain geological significance. In the early production stage that dominated by free gas, δ¹³C₁ decreases rapidly and then stabilizes with minor fluctuations; in the middle to late stages, when adsorbed gas becomes dominant, δ¹³C₁ values approximate the corresponding predicted values of original reservoirs and remain consistently stable over time. These findings are of great significance in enriching and deepening the geological theories related to deep CBM.

Key words: deep coalbed methane, reservoir, flow, production pattern, particularity

摘要： 深部(埋深＞2 000 m)煤 层气的地质特殊性和产出特殊性主要体现在高温、高压、高应力和高改造条件下煤层特征以及煤层气赋存条件、流动性质和产出规律的特殊性上。研究结果表明:(1)深部煤层的储集空间以小于2 nm的微孔为主,储层为典型的纳米储层。随着煤阶的增大,煤层中微孔体积占比逐渐增大。微孔是吸附气赋存的主要空间,微孔体积占比在一定程度上代表了煤层中吸附气含量的占比。深部煤层具有典型的双重孔-缝结构,但其孔隙度和渗透率在深部高温、高压条件下会发生衰减。(2)深部煤层气不同赋存状态间的分配变化是温度、压力和赋存空间共同耦合作用的结果,随埋深增加,煤层中的吸附气含量占比降低,游离气含量占比将增高,吸附气含量和总含气量存在明显的临界转换深度。深部煤层气的富集成藏是煤层物质组分、深度效应和保存条件"三元控气"共同作用的结果。其中,深度效应是深部煤层气特有的效应,保存条件对于游离气的富集至关重要。(3)在深部高温效应下,煤层中吸附气的解吸量、解吸速率、解吸率和扩散系数都将明显提高,由此可以产生明显的甲烷解吸补偿效应,这使得煤岩在吸附气量减少的情况下也可以释放出更多的甲烷,同时,吸附气的解吸高峰期会更快地发生。高温条件下,较低的气水黏度比及界面张力更有利于煤层中气水两相的渗流。(4)深部煤层气的产出行为实质上是在高温、高压和大规模水力压裂强化后,特殊的多相、多赋存态流体运动的综合表现。煤层气的产出过程也是超静孔隙压力逐渐消散的过程,进而普遍表现为初期高峰高产、中期稳定中产、后期稳定低产的衰竭式产出特征。深部煤层气在产出初期以游离气为主,在产出的中—后期以吸附气为主,在整个煤层气井的生产周期,游离气的产量占比接近原始煤层中游离气的含量占比。在深部煤层气井的产出气中,甲烷的碳稳定同位素值(δ¹³C₁)随排采时间呈"L"型变化趋势,具有地质响应意义,即在排采初期以游离气产出为主的阶段,δ¹³C₁快速降低且呈小幅稳定,而在排采中—后期以吸附气产出为主的阶段,δ¹³C₁接近原始气藏的预测值且保持长期稳定。上述研究成果对于丰富和深化深部煤层气的地质理论具有重要意义。

关键词: 深部煤层气, 储层, 流动, 产出规律, 特殊性