A high percentage of oil and gas wells drilled throughout the world have been completed using perforated casing. In older cased-hole wells, the production pressure may be depleted and/or hydrocarbon flow may be impeded by various damaging mechanisms such as in-situ emulsions, water blockage, solids migration, asphaltenes and paraffin deposition. In newer wells, drilled using oil- or synthetic-based drilling fluids, emulsion blockage may cause near-wellbore damage when the filtrate or whole mud interacts with formation fluids or completion brine.
Properties of microemulsions, such as high oil solubilization rates and ultra-low interfacial tension make this type of fluid ideal for addressing all of the described problems in old and new cased-hole wells.
This paper presents development procedures and case history results from numerous field applications that demonstrate the efficiency to reduce skin damage and improve productivity by using a single-phase microemulsion fluid specially designed for cased-hole wells.
In cased-hole wells, the production pressure may be depleted and/or hydrocarbon flow may be impeded by formation damage. In the majority of oil and gas wells completed by perforating casing, formation damage may include wettability changes, in-situ emulsions formation, and water blockage.1–3
Operators have been using various remedial treatment methods for removing near-wellbore damage in cased-hole wells, including the injection of acids and solvents such as xylene, oils, or alcohols, in combination with surfactants.4, 5 Although they may partially or completely restore the permeability of the porous media, many of these treatments are not completely effective, because they do not restore the wettability properties of the rock. Consequently the production of the oil and gas will not reach the expected production level.
The type of wettability of the formation rock is important, because it controls flow and distribution of fluids in a reservoir.6 The original water-wet characteristics of most reservoirs are changed by the adsorption or deposition of organic molecules present in the crude oil. These organic molecules contain polar groups, generally consisting of oxygen, nitrogen, sulfur, in addition to hydrocarbon tails. These components are also known as natural surfactants.6 The polar groups adsorb onto the rock, exposing the hydrocarbon tail and changing the surface to oil-wet. The wettability of formation rock can be also altered by surface-active additives used in drilling and completion fluids.2
Another significant damage mechanism is the formation of in-situ viscous emulsions produced by incompatibility of fluids when the formation fluids, including crude oil or formation water, contact the drilling, completion or treatment fluids.7
Phase trapping is one of the most severe damage mechanisms in low permeability gas reservoirs. This effect can occur in gas reservoirs with the presence of water- or hydrocarbon-based fluids. The porous media of a reservoir is initially at a low liquid saturation which provides the maximum cross sectional area for flow through the rock matrix, and therefore has the highest level of permeability. If a fluid, such as completion brine, is introduced into the porous system a higher water saturation in the flushed zone could be generated and would result in trapped gas. If the capillary drawdown gradient is insufficient to overcome the capillary pressure when the well starts production, a high pressure trapped liquid saturation results.8
Waterblock is one of the foremost formation damage mechanisms that requires attention. Waterblock may result from the invasion of water-base fluid or flitrate into an oil-bearing formation which increases the near-wellbore water saturation. This effect is elevated with the retention and adsorption phenomena of polymeric additives which pass through the drilling fluid filter cake and invade deeply into the reservoir.9, 10