Proces for filtration of oil and gas well treatment fluids.

A filtering process is disclosed for obtaining a clear effluent filtrate from a turbid oil or gas well treatment fluid contaminated with solid particulates comprised of drilling and formation debris having particle sizes in the range of from about 0.1 to about 30 micrometers comprising passing the treatment fluid through a surface filter having an absolute pore rating of about 40 micrometers or less at a flow density of from about 0.05 to about 0.5 gallons per minute per square foot of filter surface such that (1) initially at least a portion of the particulates is collected on the upstream surface of the surface filter and begins forming a filter cake with enhanced capability for removing particulates smaller than the absolute pore rating of said surface filter and (2) thereafter the treatment fluid is filtered through a filter composite of the surface filter and the filter cake whereby a clear effluent substantially free of solid particulates having particle sizes in the range of from about 0.1 to about 30 micrometers or higher is obtained and the useful life of the surface filter is extended. The process has particular application to the treatment of completion and stimulation fluids used in oil and gas wells.