Abstract
Wells drilled with oil-phase muds are subject to severe damage due to loss of mud filtrate or whole mud to the reservoir matrix or hairline fractures. It is essential that this mud be removed prior to well production in order to gain maximum productivity. A system of aqueous-based remedial treatments has been employed to remove oil-phase drilling muds from damaged reservoirs. Mud solids are suspended and viscosity is permanently lowered by the aqueous-based system so that the mud can be flowed to the surface. Special surfactants restore the affected reservoir to a water-wet state and clay stabilizers prevent formation damage. The new system represents an improvement over conventional hydrocarbon/mutual solvent treatments. The conventional treatments do not effectively suspend mud solids and only temporarily reduce mud viscosity, while leaving the treated reservoir in an oil-wet condition. The aqueous-based system also lowers surface injection pressure below that required by hydrocarbon/mutual solvent systems.
Laboratory data are presented showing the relative effects of the aqueous-based system and a hydrocarbon/mutual solvent treatment on the degree and permanence of oil-phase mud viscosity reduction. The aqueous-based system more effectively and irreversibly lowers mud viscosity as compared to the hydrocarbon/mutual solvent treatment. Cores damaged by oil-phase mud filtrate or whole mud were treated with either hydrocarbon/mutual solvent or the aqueous-based system. Regained core permeabilities following the treatments indicate that the aqueous-based system yields significantly higher recoveries than the hydrocarbon/mutual solvent treatment.
The aqueous-based system has been widely used in wells drilled with oil-phase muds to effect significant production increases. The aqueous-based remedial treatments have more effectively removed damaging oil-phase mud filtrate and solids than conventional hydrocarbon/mutual solvent treatments. In the process, problems associated with the use of hydrocarbon/mutual solvent treatments have been alleviated. These problems include high pumping pressures, "regelling" of the mud, oil-wetting of the formation, and incomplete removal of mud solids.
