Preliminarily exploring the theories and technologies for coalbed methane production using horizontal wells: Comparison of conditions for coalbed methane and shale gas exploitation

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.