In recent years the use of microemulsions for remediating damaged wells has been a success story. Microemulsions are unique, thermodynamically stable, optically clear single phase blends of water, biodegradable water-immiscible solvent, co-solvent, and specially designed surfactant. Engineered mixtures of these ingredients can form stable microemulsions over a rather broad range of water to solvent to surfactant ratios. Are all microemulsions equally effective in their performance? The main focus of our study was to prepare various microemulsions and to examine a link between microemulsion composition and their performance.
Microemulsion treatments have proved to be effective for improving gas production rates by increasing formation permeability and for enhancing fluid recovery from sand-packed and shale-packed columns. For achieving maximum benefits, microemulsion formulations have to be properly designed. Both microemulsion composition and dose are important for their end performance. The effectiveness of a particular microemulsion may vary depending on the application, and may be sensitive to the type of formation.
In recent years a significant number of publications describing the use of microemulsions and surfactant solutions in stimulating oil and gas wells have appeared. These publications covered both studies performed in the lab, as well as the results from field case studies [1–5]. The benefits of surfactants and microemulsions are related to their surface activity, which is revealed as their ability to lower surface tension and contact angle on air/water interfaces. Surface activity is associated with both surfactants and environmentally friendly terpene solvents. Since the latter are insoluble in water, the synergistic effects between surfactants and solvents can be achieved by formulating these additives into microemulsions, which are thermodynamically stable optically transparent colloidal solutions. Many functions of microemulsions, essential for an effective remediation of damaged wells, are very well summarized in . Microemulsions have been shown to be effective in improving core permeability, reducing emulsion tendencies between reservoir oil and treatment fluids, enhancing fluid recovery from proppant packs, and minimizing leak-off into the formation [2–5]. Superior performance of microemulsions is believed to be related to microemulsion structure, described by the so-called Voronoi model , which provides a maximized surface area of contact between surfactants and formation. In a number of studies it has been emphasized that microemulsions play an important role in altering capillary pressure, and especially capillary end effects [4,8]. Capillary pressure is given by a well-known equation where ? is the surface tension, ? is contact angle, and r is the radius of pores. For a liquid of density ??, capillary rise, h, determining the depth of liquid penetration, can be calculated as