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 closure of cleats. Stress-strain curves for coal samples under uniaxial compression showed nonlinear, load-dependent tangent moduli values. The traditional approach for the fracture width calculation based on a constant Youngs modulus value may not be valid in shallow coal seam fracturing. An approach for use of non-linear compaction combined with load-dependent modulus for calculating fracture width is described.
Because it provides safer mining operations and a high-quality energy resource, methane recovery from coal seams has generated great activities in the coal mining and petroleum industries. The tax credit given to any investment in coalbed methane recovery has significantly augmented exploration and development of coal seams. Hydraulic fracturing is commonly performed to commercially deplete a coalbed methane reservoir. The purpose of hydraulic fracturing of coal is to interconnect the cleat system to the wellbore; therefore, if the cleat system is not well developed, creating a hydraulic fracture may not yield expected results. The cleat system is usually water saturated and depletion of pressure below the desorption pressure of the coal is essential to initiate the gas desorption process. A single, long, planar hydraulic fracture is the key for effective stimulation treatment in coal seams. Unconventional behavior has been observed when fracturing shallow coal seams. Typical unconventional observations are (1) unusually high treatment pressure; (2) fracturing with no proppant on many occasions yields better production results than fracturing with proppant; (3) relatively short and wide fractures result from a treatment; and (4) creation of multiple, parallel, T-shaped and I-shaped fractures. Despite a lack of understanding of fracturing behavior in coal seams and a flurry of activity, there have been few laboratory studies reported in the literature. Field experience has been the source of knowledge to improve hydraulic fracturing of coal seams. In shallow coal seams where lack of confinement is evident, cycling stimulation treatments have been applied in the field. This method entails a "huff and puff" fracturing process to change fracture direction, hence optimizing well productivity. It is the purpose of this paper to provide experimental results that would explain the unconventional field observations encountered during coal fracturing.
Explanation of unconventional results experienced in fracturing shallow coal seams requires understanding reservoir and mechanical properties of coal which affect a hydraulic fracturing process.
The overall medium of coal seam is characterized by three interacted systems: coal grains, micropores, and cleats.
