National Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum (Beijing)
Coalbed Methane Research Center, China University of Petroleum (Beijing)
Chinese Petroleum Society
PetroChina Coalbed Methane Co., Ltd.
China United Coalbed Methane National Engineering Research Center Co. Ltd., Beijing
State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources, China University of Mining and Technology

近几年全国深部煤层气基于精细地质研究和水平井多段加砂压裂取得重大突破，部分井日产气量高达十万方，给煤层气产业重新树立了信心。但是，由于深部煤储层处于高地应力、高地温、高孔隙压力、低渗透率的复杂地质环境，不同深度煤储层典型参数和煤层气赋存方式的分布特征以及对储量和产量的影响亟需揭示。基于Langmuir等温吸附式、亨利定律及物质平衡原理，考虑吸附层和溶解气的影响，建立了深部煤层气游离气饱和度计算模型；以国内鄂尔多斯盆地大宁−吉县区块深部煤层气藏为例，分析不同深度深部煤层气赋存方式及分布特征，并评价游离气饱和度对深部煤层气储量、产量与合理配产的影响。研究认为：当煤层埋深大于溶解饱和对应的深度，游离气才会出现，且随着埋深的增加，游离气饱和度先快速增加后缓慢增加，目标区块埋深1 875 m处才出现游离气，在埋深2 800 m处游离气饱和度高达90%，游离气的占比高达17.3%。游离气饱和度对深部煤层气储量计算、产气特征和合理配产影响很大，随着游离气饱和度的增大，煤层气储量线性增大，累产气量持续上升但后期上升幅度逐渐变缓，深部煤层气井最优配产增加，井底流压下降速度加快，压裂改造区的内外压差降低，未改造区动用程度增加。目标区块主力开发煤层埋深位于2 100~2 300 m，游离气饱和度介于48%~68%，游离气占比介于10%~13%，建议气井合理配产介于(4~10)×104 m3/d。研究结果可为深部煤层气进一步开发提供理论依据和方法支撑。

In recent years, breakthroughs have been achieved in the exploration of deep coalbed methane (CBM) in China based on fine-scale geological research and techniques such as multistage hydraulic fracturing using proppants in a horizontal well. As a result, some wells yielded daily gas production of up to 100000 m3, inspiring confidence again in the CBM industry. However, since deep coal reservoirs occur in a complex geological environment characterized by high in-situ stress, high geotemperature, high pore pressure, and low permeability, there is an urgent need to reveal the typical parameters of coal reservoirs at different depths and the distribution of CBM in varying occurrence forms in the reservoirs, as well as their effects on CBM reserves and production. Based on Langmuir equation of isothermal adsorption, Henry's law, and the material balance principle, this study established a calculation model of free gas saturation of deep CBM reservoirs by considering the effects of adsorption layers and dissolved gas. With the deep CBM reservoirs in the Daning-Jixian block in the Ordos Basin in China as a case study, this study investigated the occurrence forms and distribution of deep CBM at varying depths and assessed the effects of free gas saturation on the reserves, production, and rational production allocation of deep CBM. Key findings include: (1) Free gas appears only when the coal seams’ burial depth exceeds the depth corresponding to the dissolution saturation of CBM reservoirs, with the free gas saturation increasing rapidly initially and then slowly as the burial depth increases. In the Daning-Jixian block, free gas emerges at a burial depth of 1875 m, and the free gas saturation reaches 90% at a burial depth of 2800 m, where the free gas accounts for up to 17.3%. (2) The free gas saturation has significant effects on the reserves calculation, gas production characteristics, and reasonable production allocation of deep CBM. With an increase in the free gas saturation, the CBM reserves linearly increase, and the cumulative gas production keeps rising, with the increased amplitude gradually decreasing in the late stage. Furthermore, an increase in the free gas saturation is accompanied by an increase in the optimal production allocation of a deep CBM well, an increase in the drop rate of bottomhole pressure, a decrease in the pressure difference between the stimulated reservoir volume (SRV) and the unstimulated reservoir volume (USRV), and an increase in the producing degree of the USRV. The dominant coal seams to be exploited in the target block are located at depths between 2100 and 2300 m, with free gas saturation ranging from 48% to 68% and a proportion of free gas varying between 10% and 13%. It is recommended that the rational production allocation of gas wells in the target block should be (4−10)×104 m3/d. The results of this study will provide a theoretical basis and methods for the further exploitation of deep CBM.

国家自然科学基金重点项目(42230814)；山西省科技重大专项项目(20201101002)；中石油股份公司前瞻性基础性攻关项目(2021DJ2301)；中国石油大学(北京)优秀青年学者基金项目(2462020QNXZ003)

石军太，曹敬添，徐凤银，等. 深部煤层气游离气饱和度计算模型及其应用[J]. 煤田地质与勘探，2024，52(2)：134−146.

SHI Juntai，CAO Jingtian，XU Fengyin，et al. A calculation model of free gas saturation in deep coalbed methane reservoirs and its application[J]. Coal Geology & Exploration，2024，52(2)：134−146.