Liquid CO2 system has been pumped as fracturing fluids in North America since the early 1980s and the liquid CO2/N2system from 1994. The fluids have been used in over 1000 wells in a variety offormations and in wells of permeabilities from 0.1mD to 10 Darcies, depths in excess of 3000 meters and BHT from 10 to 100 degrees Celsius. The chemistry and physics of these fluids are very intriguing and are described.
These include the thermodynamics of the fluids and the physical properties of density and viscosity of these fluids. The effect of compressibility and the transient effects on fluid leak off are discussed. The suggested process for proppant placement with these low viscosity, non-damaging fluids are described. This includes the density and turbulent effects. The design model considerations for the design of the fracturing jobs with these fluids are also summarized. Operational considerations of pumping these low temperature fluids in the field are detailed. A summary of the field use of these fluids is described.
Conventional fracturing fluid requirements include considerations of viscosity for proppant transport and breaking of these high viscosity fluids for fluid recovery and retained permeability considerations. Damage to the formation, filter cake build up and fracture physics are affected by the fracturing fluid leak off parameters.
Several papers have described the unique nature of liquid CO2 as a fracturing fluid.1–3 Without the presence of any other carrier fluid, gel or other chemicals, the proppant is placed in the formation without causing damage of any kind. As was described in a previous paper, "the use of a reservoir-friendly substance like liquid CO2 offers unique advantages through the elimination of capillary fluid retention and clays welling."3 However, liquid CO2 is not the obvious fracturing fluid due to its low viscosity. Since 1981 (patented in 1982) more than 1200 fracturing treatments have been completed using this low viscosity fluid. To understand the success of liquid CO2 fracturing, it is important to understand the chemistry and physics of the process which are described below. Discussion of Theories, Operations and Application History are presented later.
The physical properties of liquid CO2 makes it a unique treatment fluid. CO2 is relatively inert compound that exists in all three phases as shown in the phase diagram (Figure 1). Depending on the temperature and pressure, it exists as a solid, liquid, gas or super critical fluid. Above the critical point, it is considered to be a super critical fluid. In field operations, liquid CO2 is at 2.0 MPa and -35°C in the storage vessel (shown as 1 in the figure 1). After the addition of proppants, high pressure pumps increase the pressure to surface treating pressure (STP)(example 35 to 40 MPa), as shown in figure 1. As the fluid enters the formation, the temperature increases (shown as 4) toward bottom hole temperature (BHT). On closure, the temperature continues to increase and the pressure declines toward formation pressure. During flow back, the pressure decreases and CO2 comes to the surface (shown as 5) as a gas.
Figure 2 shows the density of CO2. For most fracturing operations, the density of the liquid is between 1 and 1.2g/cm3.
Figure 3 shows the viscosity of CO2, which ranges from 0.02 to 0.16 cP during the fracturing operations, confirming that viscosity is not the dominant means of proppant transport.