Abstract The use of CO2-miscible, viscosified hydrocarbon fracturing fluids has been proven to be a very effective gas-well stimulation technique in Canada and the United States. Fluid recovery is enhanced after stimulation by this process. Anderson and Gruber presented the original concepts used in paper CIM 95–45, entitled "Carbonated Hydrocarbons for Improved Gas Well Fracturing Results."
This paper provides results of wells recently stimulated. Simplifications to the fluid and job design processes, and additional design considerations are introduced. New CO2 solubility data generated at 50°C, 85°C, and 120°C with 30%, 40%, and 50% CO2 in four different fracturing fluids are presented to provide a wider range of design data. Compositional data for a number of specialty fracturing fluids are introduced to illustrate their enhanced fluid-recovery capability compared to heavier fluids such as kerosene and diesel.
Introduction The objective of this paper is to describe in simple terms an effective and proven gas-well stimulation technology that has global application. Oil-based fracturing fluids provide an ideal solution for reservoirs that are water-sensitive. However, one must recover the oil to prevent relative-permeability damage, a problem that has limited the use of oil-based fluids in the past. The technology first disclosed in CIM 95–45, and expanded upon in this paper, presents a solution to the relative-permeability limitation by providing a means to rapidly recover most or all of the fracturing fluid. A methane-drive displacement mechanism is achieved by which the produced methane effectively displaces the fracturing fluid from the reservoir into the wellbore.
Theory The theory of fracturing gas wells with carbonated hydrocarbons was presented by Norm Gruber and Hal Anderson in paper CIM 95–45, "Carbonated Hydrocarbons for Improved Gas Well Fracturing Results." The Gruber/Anderson paper demonstrates that methane will effectively displace CO2-saturated, aromatic hydrocarbon fluids, provided that a sufficient volume of carbon dioxide (CO2) gas exists between the methane and oil. By using the produced methane as the drive mechanism for miscible displacement of the fracturing fluid, rapid fluid recovery is realized, even in low-pressure, dry-gas reservoirs.
Application To enable methane to act as the fluid-drive mechanism, one must create the necessary volume of CO2 gas between the produced natural gas and CO2-saturated hydrocarbon fracturing fluid. The volume is created in two ways, as described below.Cause an evolution of CO2 gas from the fracturing fluid at downhole temperature and pressure.
Use a 100% CO2 pre-pad to help ensure that adequate CO2 gas exists between the produced methane and fracturing fluid for effective methane gas-drive fluid recovery.
Evolution of CO2 Gas.
The objective is to exceed the solubility limit (bubble point) of CO2 in the fracturing fluid under downhole conditions. A calculated excess of CO2 is added to evolve the necessary volume of CO2 gas. However, while pumping, the pressures are high enough to ensure complete miscibility of the CO2 and the hydrocarbon fracturing fluid. Once pumping stops, the fracturing fluid temperature increases and its pressure decreases to a level that exceeds the CO2 bubble point, causing CO2 gas to evolve. The gas will follow the pressure gradient from the fracture into the formation, where it is placed between the fracturing fluid and produced natural gas, as desired.
Evolution of CO2 Gas.
The objective is to exceed the solubility limit (bubble point) of CO2 in the fracturing fluid under downhole conditions. A calculated excess of CO2 is added to evolve the necessary volume of CO2 gas. However, while pumping, the pressures are high enough to ensure complete miscibility of the CO2 and the hydrocarbon fracturing fluid. Once pumping stops, the fracturing fluid temperature increases and its pressure decreases to a level that exceeds the CO2 bubble point, causing CO2 gas to evolve. The gas will follow the pressure gradient from the fracture into the formation, where it is placed between the fracturing fluid and produced natural gas, as desired.