Laboratory hydraulic fracturing experiments on unconfined coal blocks were conducted to simulate fracture stimulation of shallow coal seams. Fractures were initiated by injecting a gelled water into an openhole section of a wellbore. Multiple fractures appeared at the sample periphery during injection period. This paper describes observations made during laboratory experiments, and discusses implications of the experimental results to field-scale treatments. The results have application in modifying fracture width calculations for shallow coal seams. When a shallow coal seam is hydraulically fractured, the created fracture width may be developed as a result of inflating some cleats and closure of surrounding cleats. Stress-strain curves for coal samples under uniaxial compression showed nonlinear, stress-dependent Young's modulus. The traditional approach for the fracture width calculation based on a constant Young's modulus is not valid in shallow coal seam fracturing. Therefore, an approach that accounts for nonlinear compaction using stress-dependent modulus in calculating fracture width is described.

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