中国深层煤层气勘探开发进展、关键评价参数与前景展望

经过30 a的不懈探索，我国煤层气开发井型逐渐由直井向水平井开发过渡，水平井开发煤层气的基础理论是制约当前煤层气开发的主要问题。和页岩相比，煤层页理和脆性矿物不发育，非均质性更强，不利于体积缝网的形成。但煤层顶底板封盖强，机械强度高，具备较好的水力压裂条件。部分煤层裂隙被方解石等填充，酸化有利于储层渗透率改善和复杂裂缝形成。因此，建立一套适合煤层气储层特征的水平井开发工艺尤为重要。针对煤层地质特征，从水平井长度优化、簇间距优化、压裂液选择、支撑剂选择、返排率、最终可采储量(EUR)特征6个方面，系统分析煤层气水平井开发技术经济政策，认为煤层气水平段长度一般不超过1 000 m，簇间距15~30 m储层改造效果较好；深部煤层气压裂液中加入滑溜水破胶剂及少量低温辅助破胶剂可提高破胶效率；相比中浅部煤层气储层，深部煤层压裂支撑剂应提高大粒径(0.425/0.850 mm)比例；深部煤层(如大吉区块8号煤)达到最高产气时的压裂液返排率与威远−长宁页岩气相近；煤层气EUR偏低，但用液强度、加砂强度大，应提高煤层气开发效率及经济效益等。煤层气开发效果受地质特征和开发工程技术协同控制，从地质工程一体化角度，提出下步煤层气水平井开发的对策，指出应进一步优化煤层气水平井开发工艺；完善水平井钻完井与配套技术，降低单井钻完井成本；依据煤粉迁移规律，实现煤层气水平井控粉有序返排，提高排采效果；依据游离与吸附气赋存比例，优化生产控制技术提升单井EUR。

Exploration and development progress,key evaluation parameters and prospect of deep CBM in China

Through 30 years of unremitting explorations, coalbed methane (CBM) wells in China have gradually transitioned from vertical to horizontal wells. However, the lack of basic theories on CBM production using horizontal wells is the main challenge to current CBM development. Compared to shales, coal seams exhibit heterogeneity and a minor presence of foliations and brittle minerals, hindering the formation of volumetric fracture networks. Nevertheless, the roofs and floors of coal seams have high sealing capacities and mechanical strength, creating excellent conditions for hydraulic fracturing. Some fractures in coals are filled with calcite, rendering acidification conducive to the improvement of reservoir permeability and the formation of complex fractures. Hence, it is particularly important to develop a technology for CBM production using horizontal wells that is tailored for CBM reservoir characteristics. Targeting the geological characteristics of coal seams, this study systematically analyzed the technical and economic policies for CBM production using horizontal wells from six aspects: the optimization of the horizontal well section length and cluster spacing, the selection of fracturing fluids and proppants, flowback rate, and estimated ultimate recovery (EUR). Key findings are as follows: (1) Encouraging reservoir stimulation performance can be achieved under a horizontal well section length of 1 000 m or below and a cluster spacing ranging between 15 m and 30 m; (2) Adding slippery water gel breakers and a small quantity of low-temperature auxiliary gel breakers into fracturing fluids can improve the gel-breaking efficiency of deep CBM reservoirs; (3) Compared to middle and shallow CBM reservoirs, the ratio of proppants with large grain sizes (0.425/0.850 mm) should be increased for deep CBM reservoir fracturing; (4) Under the highest gas yield, the fracturing fluids’ flowback rate for deep CBM reservoirs (e.g., the No. 8 coal seam in the Daji block) resembles that for shale gas reservoirs in the Weiyuan-Changning area; (5) Given the low EUR but high fracturing liquid and proppant intensities of CBM production, it is necessary to improve its efficiency and economic benefits. The CBM production performance is jointly controlled by geological characteristics and engineering technologies. From the perspective of the geo-engineering integration, this study put forward the countermeasures for future CBM production using horizontal wells and proposed suggestions. Specifically, it is necessary to further optimize the technology for CBM production using horizontal wells, improve the drilling and completion technologies of horizontal wells to reduce the cost of single-well drilling and completion, achieve the orderly flowback of pulverized coal in CBM horizontal wells as per the migration pattern of pulverized coals to improve the drainage and production efficiency, and optimize the production control technologies based on free and adsorbed gases’ proportions to increase single-well EUR.