Fracturing fluid leakoff in coalbed methane reservoirs can be highly damaging to the cleat permeability of coal. During the fracturing treatment, the cleats sub-parallel to the fracture are thought to open and accept fluid. Laboratory and field evidence suggests that severe damage can result deep within the formation. This work investigates mechanisms of fluid leakoff, damage and control in large scale cleated coal samples and in coal core samples from the San Juan and Black Warrior Basins. The leakoff properties, regained permeability and retained conductivity of the proppant pack have been studied with various cleat orientations and fluid loss additives, including 100 mesh sand. The damage of various fracturing fluids has been further measured vs penetration depth in fresh coal cores from the Black Warrior Basin in a multiport core flow apparatus.

Three damage mechanisms have been identified including 1) leakoff of whole gel during spurt or uncontrolled leakoff, 2) leakoff of water only during filter cake control prior to breaking and 3) leakoff of broken polymer while breaking. The application of 100 mesh sand can control leakoff to cleats: however, the cleats and pack only cleanup if appropriate breakers are applied. Leakoff of water only with filter cake control is the least damaging mechanism to cleat permeability. Finally, the leakoff of broken guar is as damaging as whole guar due to polymer adsorption onto coal surfaces; experimentally modified guars that show minimum adsorption are less damaging.

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