The Utility of CO2 as an Energizing Component for Fracturing Fluids

Abstract

Summary The use of CO2 as an energizer in hydraulic fracturing fluids has increased dramatically in the past few years. The history of CO2 usage for this application is discussed briefly, and CO2 is compared with the other commonly used energizer, N2. The design considerations for using CO2 in a fracturing treatment, both as a minor (energizer) and a major (foam) component, are presented. Special consideration is devoted to CO2 foam because CO2 is pumped as a compressible liquid that changes to a gas downhole. This paper presents the calculations required to estimate the surface liquid CO2 volumes and injection rates needed to obtain the desired downhole foam properties. Also, the instrumentation required to meter the liquid CO2 safely and accurately is discussed. Field results further evidence CO2's utility in hydraulic fracturing. This discussion includes a comparison of fracture stimulation with and without CO2 as an energizer as well as treatments that compare CO2-energized fluids with N2-energized fluids. Economic considerations of pumping CO2 vs. N2 vs. no energizer are discussed. Conclusions suggest pumping CO2 vs. N2 vs. no energizer are discussed. Conclusions suggest what further study is required to help improve design techniques and when CO2 should be considered as a component of a fracture stimulation treatment. Introduction The practical use of CO2 in stimulation fluids was developed in the early 1960's. In 1962, Bleakley reported the results of 50 successful fracture treatments that contained CO2 and described the operating procedures required to handle CO2 safely. These early treating fluids were foams and CO2 commingled with water. As carbonated fracturing fluids gained popularity, researchers began studying the properties of these fluids to improve treatment designs. An early investigation to characterize CO2 base fluids was conducted by Niel et al., who reported the following:foaming additives with CO2 gives better recoverability than with N2;no measurable corrosive effects were produced by fluids that contained CO2;CO2 buffers the pH of aqueous systems to about 3.5, which helps control swellable clays; andfluids containing CO2 exhibited greater leakoff. Several teams of investigators have studied the fluid properties of foams. An early work by David and properties of foams. An early work by David and Marsden proposed a practical rheological model for foam. The laboratory setup required for the evaluation of dynamic fluid properties for foams was discussed by Wendorff and Earl. Reidenbach et al., compared the actual properties of N2 - and CO2 - based foams and concluded that these fluids exhibit very similar laminar rheologies. Also, Harris demonstrated that N2- and CO2-based foams exhibit similar dynamic fluid-loss properties that can be enhanced by the addition of polymers to the aqueous phase. To predict surface treating pressures accurately, Blauer et al. studied the friction loss of foams in laminar, turbulent, and transitional flow regimes in tubing. Their investigation revealed that foam behaves as a single-phase pseudoplastic fluid whose effective viscosity must be used pseudoplastic fluid whose effective viscosity must be used to calculate pressure loss for flowing foam. The friction loss for any foam may be determined as for a single-phase fluid with conventional Reynolds numbers and a Moody diagram. The use of CO2 in fracture stimulation treatment yields several benefits. CO2 provides faster and increased load recovery; helps control clay swelling by buffering aqueous fracturing fluids to a pH of 3.5; replaces some of the water in the treatment, thus reducing the volume of water to be recovered; can be used to formulate highly efficient fracturing fluids that effectively transport proppants; and can be metered accurately. Although the in-situ properties of carbonated fracturing fluids are well documented, little work has been performed regarding the surface handling of CO2 so that the desired downhole concentrations are attained. The actual pumping and metering of CO2 is somewhat more complex than N2 because of its different phase behavior and chemical activity. A method for accurately metering the liquid CO2 at surface conditions and conversion to in-situ volume are discussed. Also, some specialized instrumentation that supports these processes is presented. First, however, a review of the physical presented. First, however, a review of the physical properties of CO2 is in order. properties of CO2 is in order. Physical Properties of CO2, Physical Properties of CO2, At ambient conditions, CO2 exists as a colorless, nontoxic, dense gas. when the gas is compressed and cooled, a stable liquid is formed. CO2 is delivered to the oil field as a liquid in 20-ton [18-Mg] loads at temperatures of 0 to -20F [-18 to -29C] and pressures of 200 to 300 psi [1.38 to 2.1 MPa]. Fig. 1 is a phase diagram that shows the pressure and temperature conditions at which CO2 exists as a solid, liquid, or gas. SPEPE P. 351