Coalbed methane wells are routinely fracture stimulated using gelled fluids and proppants. Surprisingly, there is no published report on the compatibility of the coals with fracturing fluids containing polymers, surfactants, biocides, friction reducers, and other chemicals. The objective of the research presented here was to understand and measure the change in coal permeability after it was brought in contact with hydraulic fracturing fluids.
The permeabilities of whole core samples of coals from San Juan and Warrior Basins were severely damaged (5-10 fold reduction) after they were exposed to gelled fracturing fluids that had been broken and filtered repeatedly. Efforts to restore coal permeability by prolonged flushing with water and HC1/HF solutions were unsuccessful. While damage to coal permeability appears to have been caused by sorption-induced swelling of the coal matrix, some plugging of the cleat system by residual gels cannot be ruled out.
Based on limited lab data, it appears that even water containing low concentrations of friction reducing polymers can cause significant damage to coal permeability. These results are alarming since extensive damage to coal permeability by gelled fluids and friction reducing polymers could negate most of the benefits of a large effective wellbore radius created during an expensive fracture stimulation. Field comparison of well productivities suggest that, in general, small low cost propped water fractures are as effective, if not more, as large high cost propped gel fractures. It is anticipated that coal permeability damage would be exacerbated in thin, low cleat porosity, high permeability coal seams.
It is recommended that every effort be made to avoid contacting coals with gelled fluids, polymers, or liquid chemicals. Furthermore, remedial workover treatment should be considered for coal wells that could have been damaged in the past by hydraulic fracture stimulation.