Fracturing Fluid for low Permebility Gas Reservors: Emulsion of Carbon Dioxide With Aqueous Methonol Base Fluid: Chemistry and Applications

Abstract

Abstract High-quality emulsion of carbon dioxide (CO2) in aqueous alcohol-based gel (CO2 emulsion) was introduced into the Western Canadian Sedimentary Basin (WCSB) as a fracturing fluid in 1981. Since that time, the use of the fluid has been very successful, particularly in low-pressure, tight gas applications. The fluid has all the advantages of conventional high-quality CO2 foams/emulsions, with the added advantage of minimizing the amount of water introduced into the well. The present paper will discuss a fluid that is an emulsion of liquid carbon dioxide in a base fluid of aqueous methanol. The discussion will include chemistry, rheological evaluations, and successful field utilization of these fluids in the Western Canadian Sedimentary Basin over the last decade. Introduction With ever-increasing need for resources world wide, the industry continues its trend to exploit gas reservoirs with ever-lower permeabilities. Very low-permeability reservoirs are typically in a state of capillary under-saturation, where the initial water and sometimes hydrocarbon saturation is less than would be expected from conventional capillary mechanics for the pore system under consideration (1). These formations are also called desiccated or dehydrated formations and have been known to exist all over the world. Introducing an additional immiscible phase, or increasing the existing phase saturation within porous media, can substantially damage permeability and relative permeability to hydrocarbons. This phenomenon is commonly described as aqueous or hydrocarbon phase trapping, depending on the situation underconsideration. The most common technique for preventing these problems involves eliminating the use of water-based fluids. Even fluids with very low fluid loss (to minimize invasion depth into the formation) may be susceptible to countercurrent spontaneous imbibition effects in desiccated reservoirs. The most successful means of mitigating these effects has been to use interfacial tension-reducing agents (e.g., mutual solvents or surfactants), or miscible gases such as carbon dioxide or LPG. When translated to fracturing fluids, this involves developming fluids that utilize the above mentioned agents in a non-damaging way. The use of CO2 as an energizing medium for fracturing fluids is an old concept (2). The concept of high-quality foam as fracturing fluid has also been previously reviewed (3). Bennion has discussed the use of methanol and CO2 to minimize and remove damage in low-permeability gas reservoir (4). Thus, a natural extension of these concepts is to combine all the benefits of these fluids, i.e., develop an emulsion fluid that uses high-quality CO2 (80 Mitchell quality or higher) with 40% methanol in place of water in the external gel phase. Properties of Methanol and use in Fracturing Fluid Formulations Methanol has some attractive properties that make its use in fracturing fluid formulations attractive (5). Using 40-% methanol in water lowers the surface tension of water from 72 dynes/cm to around 40 dynes/cm, the freezing point from 0 oC to -40 oC, the specific gravity from 1 to 0.95. At the same time, the vapor pressure of water is increased from 17.5 to 46.5 mm of Hg at 20 oC and from 150 to 300 mm of Hg at 60 oC, which helps in recovering the fluid. Likewise, the fluid viscosity goes up by 60% when methanol is added to water at a concentration of 40%. A 40% methanol-containing aqueous system can be gelled with several polymers including conventional hydroxypropyl guar (HPG), carboxymethyl hydroxypropyl guar (CMHPG) and polymers used to viscosify pure methanol (5,6). Figure 1 displays the viscosities of the various polymers in 40% methanol. These gels are compatible with both liquid and gaseous CO2 without precipitation.