Hydraulic fracturing treatments in unconventional (coalbedand carboniferous shale) natural gas wells have beensuccessful in stimulating gas production. However, the wellsoften do not perform up to potential following the treatmentsfor various reasons. Some of the factors that can contribute tothe relatively poor performance include low coal or shalepermeability, complex cleating and natural fracture networks,gas content and adsorption characteristics, water content, andfracturing fluid interaction with formation surfaces.Fracturing fluid damage in coal formations has been shown tocause significant reductions in permeability, and whole gelleakoff into the cleat networks can further impair production.The same damage mechanisms can impair the production ofcarboniferous shale formations.To avoid the damage associated with polymer-based fluids,a coal/carboniferous shale-compatible solids-free (CCSF) fluidwas developed for unconventional natural gas wellapplications. The CCSF fluid is a nitrogen-foamed fluid thatwas designed to be environmentally friendly to minimize riskto ground water that could be contacted during fracturingtreatments.This study investigates the effects of various hydraulicfracturing fluids including conventional polymer-based fluids,water containing friction reducers, and a coal/carboniferousshale-compatible solids-free fluid on coal and shale packcleanup. Laboratory coal pack cleanup tests demonstrate asignificant reduction in retained permeability with polymerbasedfluids. In addition, a high cleanup factor (a parameterintroduced to provide a measure of the pressure and timerequired for cleanup) is required to establish only marginalflow conditions with brine injected in the production direction.Even water containing low concentrations of friction reducermay result in less than 50% retained permeability. The CCSFfluid provides 70 to 100% retained permeability and a lowcleanup factor for effective coal pack cleanup with coalsamples from seven coal basins in North America. Similarimprovements were observed with carboniferous shale packcleanup tests.Field case histories in coal and carboniferous shaleformations are consistent with laboratory observations anddemonstrate a dependence of fracture cleanup on retainedpermeability and the cleanup factor measured in thelaboratory. Production results demonstrate a 30 to 60%increase in production with the CCSF fluid compared toconventional fracturing fluids in coal and carboniferous shalebasins in the United States.
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