Carbon Dioxide/Aqueous Mixtures As Fracturing Fluids In Western Canada
Authors: Leinan A.B. , Harris P.C. 
Published: March 2010

Carbon dioxide has been used for many years as an energy source to aid in fluid recovery of well stimulation fluids. CO2aqueous fluid ratios of up to 50% have provided a means of quick fluid recovery when used with conventional fracturing fluids.

Several advantages are realized when using these fluids. CO2 reduces the viscosity of formation crude, and also lowers interfacial tension.

There is potential benefit in increasing the percentage of CO2 in the stimulation fluid. Liquid CO2 has a lower viscosity than water, so a better means of transporting proppant must be devised. Gelling agents for the CO2 may be used, but their utility above the critical temperature of CO2 must be considered. The use of a two-phase structured fluid as an emulsion and/or foam can provide a reasonable way of using high percentages of CO2.

Recently, fracturing fluid systems have been developed which incorporate much greater volumes of CO2, systems with a CO2 aqueous fluid ratio as high as 90%. Viscosities of the two-phase system are considered both above and below the critical temperature of the CO2, Consideration is given to the sand transportation capabilities of the system.

This paper examines the development of the technique from its conception as a research project through its implementation as a field technique. Many jobs of this type have been done in western Canada. The techniques, problems and results of some of these jobs are examined.

Introduction

CO2 has been recognized since the 1950s as an aid to improving oil recovery. Its use as a lift gas in fracturing fluids began about 1960 and its popularity for this purpose has steadily risen since then. Recently it has been used as part of a two-phase structured fluid.

Several benefits are realized using CO2 in fracturing operations. It is soluble in both oil and water at reservoir conditions. Then, as pressure is released at the wellbore during flow back, it begins to come out of solution and acts as a solution gas drive.

Fluids saturated with CO2 have low interfacial tension(6, 7). This reduces the capillary pressure in the formation and therefore improves the recovery of fracturing fluids from the formation matrix.

In a water solution, CO2 forms carbonic acid, which has a pH of about 3.5. This is the optimum range for the protection of clay-bearing formations. At the same time, the pH is not so low as to cause a problem with the dissolution of iron from iron-bearing minerals and subsequent re-deposition reducing well productivity.

While CO2 offers all these benefits as a fracturing fluid, there are problems associated with its use. Liquid CO2 has very low viscosity and consequently very little proppant carrying capacity. Above Tc, the critical temperature of CO2, it is gaseous and has even less carrying capacity.

Rheological Properties

One way of providing a much higher viscosity is the use of a two-phase fluid, with a high ratio of CO2 as the internal phase, surrounded by a thin film of viscosified aqueous fluid. This type of fluid provides the advantages of CO2 without many of the disadvantages.