Enhanced Shale Gas Recovery by the Injections of CO2, N2, and CO2/N2 Mixture Gases

Abstract

In this paper, the experiments of enhanced shale gas recovery by the injections of CO2, N2, and CO2/N2 mixture gases were carried out in a fixed bed setup to investigate the influence of the types of displacing fluid on CH4 recovery and gas flow dynamics. Investigation results show that when taking CO2 or N2 as displacement agent, the Coats–Smith dispersion–capacitance model can give an excellent simulated result to the breakthrough curves of CO2 and N2. The injection of N2 leads to the shortest breakthrough time (tb) of injected gas and the lowest recovery of CH4 product (RCH4-product), while injecting CO2 into shale formations results in the longest tb of injected gas and the highest RCH4-product with a relatively sharp displacement front. The differences of dispersion coefficient (KD) and the flowing fraction of pore space (Fv) in the Coats–Smith dispersion–capacitance model are the underlying reasons for the distinct behaviors of CO2 injection and N2 injection. With increasing CO2 mole fraction in CO2/N2 mixture gases, RCH4-product rises. The injection of 50:50/N2:CO2 mixture gases exhibits the biggest enhancement degree of N2 concentration during the displacement process. The injection of a N2-rich mixture can significantly prolong tb of CO2 and help to sequestrate injected CO2 over a long-term. For the transport of CO2 in reservoir, Fv increases and KD and the mass transfer coefficient between mobile and immobile regions (Km) decreases with increasing N2 concentration in binary gas mixture, revealing that N2 can hinder the diffusion of CO2 into the micropore system to displace CH4. The fluctuation range of flow rate of injected gas (Finjected-gas) and the CO2 storage amount (Vstorage-CO2) enhance as CO2 mole fraction in mixture raises. In order to optimize RCH4-product, Vstorage-CO2, and CO2 sequestration time, the selection of displacing fluid and the ratio of CO2/N2 mixture gases should be taken into consideration.