CO2 Injection for Enhanced Gas Production and Carbon Sequestration

Abstract

Abstract Analyses suggest that carbon dioxide (CO2) can be injected into depleted gas reservoirs to enhance methane (CH4) recovery for periods on the order of 10 years, while simultaneously sequestering large amounts of CO2. Simulations applicable to the Rio Vista Gas Field in California show that mixing between CO2 and CH4 is slow relative to repressurization, and that vertical density stratification favors enhanced gas recovery. Introduction Although the idea of injecting carbon dioxide (CO2) into depleted natural gas reservoirs for carbon sequestration with enhanced gas recovery (CSEGR) has been around for more than ten years1,2, and independent analyses have been carried out that suggest the feasibility of the process3,4, CSEGR has yet to be implemented commercially or even tested in the field. Among the reasons for this is concern about mixing of CO2 with native methane (CH4) gas and the corresponding degradation of value of the remaining natural gas. Our previous analysis of the physical processes involved in CSEGR suggested that mixing would be limited because of the high density and viscosity of CO2 relative to CH4.4 Furthermore, our simulations suggested that CSEGR could enhance gas production by a factor of five over 20 years relative to continued primary recovery over the same period for the large Rio Vista Gas Field in California. These prior simulations were done in idealized reservoirs using simple relations for gas mixture properties. The purpose of this paper is to show additional and more detailed analyses that extend and amplify our prior findings to provide a broader scientific foundation for pilot testing and ultimate large scale deployment of CSEGR. These analyses include a discussion of physical properties of gas mixtures in the system CO2-CH4, and simulations of the effects of permeability heterogeneity and vertical stratification in a three-dimensional five-spot CSEGR scenario. Background and Prior Work As shown schematically in Figure 1 for a single power plant and gas reservoir, CSEGR is the injection of CO2 into depleted natural gas reservoirs for carbon sequestration with enhanced gas recovery. Because they have held large quantities of natural gas over geologic time scales, depleted gas reservoirs offer a proven integrity against gas escape and large available capacity for carbon sequestration, estimated at 140 GtC (Gigatonnes Carbon) worldwide5, and 10–25 GtC in the United States6. There do not seem to be any technical barriers to CO2 injection, although there are certainly costs associated with the injection of a highly corrosive gas such as CO27. Despite the vast potential for carbon sequestration in depleted gas reservoirs, CSEGR has not been tested in the field due apparently to the high present cost of CO2 and infrastructure, concerns about excessive mixing, and the high primary recovery rates of many gas reservoirs8. These arguments notwithstanding, CSEGR may offer other benefits including pressure support in the reservoir to prevent subsidence and water intrusion. Furthermore, in the future companies may be paid to dispose of CO2 as a greenhouse gas as opposed to buying it as a commodity, thus reversing the economic barrier. As for mixing, the injection of CO2 enhances gas recovery through both displacement, analogous to a water flood in oil recovery, and by repressurization of the remaining CH4. Repressurization of gas effectively concentrates the CH4 mass at distances far removed from the injection location without contamination by the injected gas. Furthermore, as discussed in our prior publication4 and below, the high density and viscosity of CO2 relative to CH4 can be exploited to avoid excessive mixing in the reservoir. In addition, the schematic of CSEGR for a coupled gas-fired power plant and gas reservoir shown in Figure 1 emphasizes that comanagement between the producing reservoir and the power plant may allow for greater acceptance of mixed CO2-CH4 gases as powerplant feedstock, in addition to allowing flexibility in gas production in response to fluctuating power demands.