Abstract
Hydraulic fracturing is used widely to stimulate coalbed methane production in coal mines. However, some factors associated with conventional hydraulic fracturing, such as the simple morphology of the fractures it generates and inhomogeneous stress relief, limit its scope of application in coal mines. These problems mean that gas extraction efficiency is low. Conventional fracturing may leave hidden pockets of gas, which will be safety hazards for subsequent coal mining operations. Based on a new drilling technique applicable to drilling boreholes in coal seams, this paper proposes a tree-type fracturing technique for stimulating reservoir volumes. Tree-type fracturing simulation experiments using a large-scale triaxial testing apparatus were conducted in the laboratory. In contrast to the single hole drilled for conventional hydraulic fracturing, the tree-type sub-boreholes induce radial and tangential fractures that form complex fracture networks. These fracture networks can eliminate the “blank area” that may host dangerous gas pockets. Gas seepage in tree-type fractures was analyzed, and gas seepage tests after tree-type fracturing showed that permeability was greatly enhanced. The equipment developed for tree-type fracturing was tested in the Fengchun underground coal mine in China. After implementing tree-type fracturing, the gas extraction rate was around 2.3 times greater than that for traditional fracturing, and the extraction rate remained high for a long time during a 30-day test. This shortened the gas drainage time and improved gas extraction efficiency.
Highlights
Coalbed methane (CBM), a source of clean energy, can be exploited either through boreholes drilled from the surface or from the workings of underground coal mines
Chinese CBM recovery mainly depends on underground coal mining
At CBM production sites in most coal mines in China, the vertical stress. Experiments to simulate both conventional Hydraulic fracturing (HF) and tree-type fracturing under different conditions on the coal seam is in most cases larger than the horizontal stresses [27]
Summary
Coalbed methane (CBM), a source of clean energy, can be exploited either through boreholes drilled from the surface or from the workings of underground coal mines. Chinese CBM recovery mainly depends on underground coal mining. In 2015, underground CBM production was 13.6 billion cubic meters, accounting for 76% of the total CBM recovered in China [1]. The geology of CBM reservoirs in China in particular means that more than 70% of China’s coal fields are not suitable for development of large-scale ground CBM production [3]. These geological conditions make it necessary for underground mines in China to exploit coal and CBM simultaneously
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