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Abstract

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 HCl/HF solutions were prolonged flushing with water and HCl/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 polymers can cause significant damage to coal permeability. These results are alarming since extensive 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 productivities suggest that, in general, small low cost propped water fractures are as effective, if not more, as 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.

Background

The productivity of a coalbed methane well can be improved by hydraulic fracturing. Over time, crosslinked guar or HPG (Hydroxy Propyl Guar, a derivative of guar gum) gelled fluids have evolved as the preferred fracturing fluids for transporting and distributing large mesh proppant at very high concentrations within a fracture during the hydraulic fracturing operation. On occasions when only water is used in fracture treatments, low concentrations of HEC (Hydroxy Ethyl Cellulose) polymer is often used as a friction reducing agent. The HEC polymer is widely acknowledged to be the lowest residue polymer used as a friction reducing agent by the oil industry. Our field studies have shown that 20-30% of the injected frac gel volume is not recovered, and is suspected to be trapped in the coal. No such accounting has ever been made of the friction reducing agent used during water fracture treatments.

It is recognized that a side-effect of all fracturing operations with gelled fluids is the potential for formation damage from filtrate invasion. For conventional reservoirs, the damage to permeability caused by the "plugging" of pores by gelled fluids (either broken or unbroken) is a very nominal price to pay relative to the large production increase as a direct consequence of the increase in effective wellbore radius achieved during the fracture treatment. Unfortunately, the magnitude of damage to coal permeability caused by gelled fracture fluids, friction reducing agents, and other chemicals used during a hydraulic fracture treatment has never been published. The research presented in this paper was published. The research presented in this paper was initiated to understand and measure the magnitude of change in coal permeability when it is brought in contact with fracturing fluids.

Permeability Damage Due to Cleat Plugging Permeability Damage Due to Cleat Plugging Coal is a dual porosity reservoir rock containing a microporous matrix and a network of natural fractures known as cleats. While cleats have very low porosity (of the order of 1-2%), they are solely responsible for the permeability of a coal seam. permeability of a coal seam. P. 109