The stimulation of gas flow from coal seams by hydraulic fracturing has been pursued in the U.S. for more than ten years. However, there currently is no concensus among the industry as to the best approach for coalbed stimulation design; there are many different fracturing techniques used by operators.
Under sponsorship of the Gas Research Institute, data on more than 130 separate hydraulic fracture treatments in coal were acquired from the industry. An empirical study of fracture treatments in coalbeds of the Black Warrior Basin in Alabama reveals a trend for the maximum treatment pressures observed during hydraulic fracture treatments in coal to approach twice the values of the minimum horizontal stresses. This trend far exceeds the observed maximum treatment pressures in conventional sandstones at equivalent depths.
To enable simulation of such high treatment pressures, several physical mechanisms were investigated, including pressure buildup due to fluid leakoff, multiple fracture propagation, high viscosity fluid due to coal fines generation, and fracture-tip plugging. A 3-D hydraulic fracture computer code was modified to account for an incomplete fracture-tip screenout due to coal fines plugging within the fluid pad. The ensuing fracture treatment predictions more closely reproduce field results and provide better insight to the proper design of fracture treatments in coal deposits.