Economic gas production from coal seams (in which cavitation is not applicable) requires hydraulic fracture stimulation due to the low reservoir permeability of most target coal seams. Effective hydraulic fracture stimulation of coal seams, however, presents a formidable challenge due to a number of factors including: the mechanical complexity of coal; prevalent natural fracturing; extreme sensitivity of coal seams to fracturing fluid damage; stress sensitive permeability of coal; and the (often) complex geometry of the induced hydraulic fractures.1 

The authors began by performing an in-depth analysis of previous hydraulic fracturing treatments performed in coal seam reservoirs in China. The results of this post-treatment analysis, together with the collected reservoir data, led to the development of a modified stimulation strategy. As part of a pilot investigation by the North China Bureau of Petroleum Geology (NCBPG) of a (potentially) large coalbed methane field in China, eight hydraulic fracturing treatments were executed in the summer of 1994 using the modified stimulation strategy -- including on-site real-data three-dimensional hydraulic fracture modeling and treatment re-design.

This paper describes the analysis theory and methodology, the novel stimulation techniques employed, the on-site implementation (including difficulties), and the results of the fracturing treatments. While the paper is a case study of fracturing at a particular field, the paper will also contain a discussion of some of the complexities of coal seam fracturing. The issues to be discussed are: simultaneous propagation of multiple hydraulic fractures; (often) excessive near-wellbore fracture tortuosity; rapid downward proppant convection; and the high net fracturing pressures which may result in exceeding more than one of the reservoir's principle stresses.

Analysis of the collected data shows that 2-D models and "conventional" 3-D models of the hydraulic fracturing process apply very poorly to hydraulic fracturing in coal seams. Engineering decisions based on these more "conventional" fracture modeling techniques can lead to inappropriate fracture treatment design, as well as significant problems in predicting the post-frac production performance. The paper summarizes a few enhancements to coal seam fracturing technology, as well as the present limitations and the necessary advancements required for superior coal seam fracture performance in the future.

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