Injection Water Quality-A Key Factor to Successful Waterflooding

Abstract

Abstract Poor injection water quality is a prime factor in the reduction in injectivity in many water injection and disposal wells. These reductions in injectivity often result in costly workovers, stimulation jobs and recompletions, or, in many cases, the uncontrolled fracturing of wells by high bottomhole pressures resulting in poor water injection conformance and reduced overall sweep efficiency and recovery. This paper discusses many commonly occurring water quality issues and how they impact injectivity, including damage due to injection of suspended solids, fines migration, clay swelling and deflocculation, formation dissolution, chemical adsorption and wettability alterations, relative permeability effects associated with the injection of skim oil or grease and the injection of entrained free gas, biologically and bacterially induced damage, formation of insoluble scales and precipitates, emulsification, wax and asphaltene deposition. Screening criteria are presented to allow for a rigorous evaluation of a particular injection water source to investigate potential areas of sensitivity and to attempt to minimize problems associated with impaired injectivity. Introduction Water injection processes are utilized throughout the world to dispose of produced aqueous fluids and as a means of increasing the recovery efficiency in many oil reservoirs. A key factor in the success of these operations is contingent on being able to inject a sufficient quantity of the water of interest into the target zone. Injectivity can be restricted by:Poor inherent reservoir quality;Insufficient pay or contact of the pay zone of interest by the injection well;Formation damage effects associated with the actual water injection process. The subject matter of this paper will concentrate on the topic of injection water quality and how this factor relates to impaired injectivity. Impaired injectivity causes problems in that it restricts the volume of water which can be injected in a given well (causing potential problems with voidage replacement for a waterflood, or the buildup on surface of a large volume of produced water in a disposal operation). Often downhole injection pressure may exceed fracture pressure causing the initiation and propagation of uncontrolled induced fractures. These fractures may reduce overall efficiency of the waterflood process by lowering areal sweep efficiency and possibly directing injected fluids out of the zones of interest. However, in some cases, fractures may provide connections to zones of interest. Almost all problems associated with impaired injectivity can ultimately be related back to problems associated with water quality. Potential damage mechanisms which can be associated with water injection processes include:Mechanically induced damage, including: a) Injection of solids, b) Velocity induced damage (fines migration) and settling, where fines are presentInjection water/formation rock interactions, including: a) Clay swelling, b) Clay deflocculation, c) Formation dissolution, d) Chemical adsorption/wettability alterations.Relative permeability effects, including: a) Skim oil entrainment, b) Free gas entrainment.Biologically induced impairment, including: a) Bacterial entrainment and growth.Injection water/in situ fluid interactions, including: a) Formation of insoluble scales, b) Emulsification and emulsion blocks, c) Precipitation, d) Wax/asphaltene deposition.