Modeling Of Shales And Displacement Study Of Shale Gas By CO₂ At Geological Depths

The rising worldwide energy demands and the difficulty in developing novel clean energies have greatly stimulated the exploitation of shale gas. In addition, the using of fossil energies will lead to the emmission of CO2, which is the major contributor of global warming. To stabilize the atmospheric CO2 emissions at a level that could minimize the impact on the global climate, CO2 capture and sequestration (CCS) provides a bridging strategy to the development of carbon-free energy systems. The replacement of shale gas by CO2 injection can not only realize the purpose of CO2 permanent storage, but also enhance shale gas production.Understanding adsorption and diffusion of shale gas under different geological depths is an important issue. In the first work, we used grand canonical Monte Carlo and molecular dynamics simulations to investigate adsorption and diffusion behavior of shale gas (main component is methane) in a modeled shale in different burial depths up to 6 km. To examine the diffusion of shale gas, the equilibrium configuration of GCMC simulation was used as initial inputs for further MD simulations. The results indicate that capacity of shale gas increases slightly with the depth, while the diffusion coefficient of shale gas in the shale matrix decreases with the increase of pressure. Interestingly, a maximum diffusion coefficient of methane appears in burial depth of 5 km. By cooperatively considering adsorption and diffusion results, we propose that the optimum operating condition is under depth of 3-5 km. Moreover, we found when the basal spacing increases to 100 A, the diffusion coefficients get an improvement of 80 times compared to the case with basal spacing of 8 A, which provides useful guidance for exploitation of shale gas.In the second work, we investigate the effects of suface functional groups on the adsorption of CO2, CH4, CO2/CH4. We firstly have built five layered-pillared models with different suface functional groups. Then Bader charge analysis, electronic structure calculations are performed to provide the optimized configuration and corresponding charge information required carry out statistical-based molecular simulations. Monte Carlo simulations were performed to calculate the isotherms of methane in modeled shales. It is found that the simulated isotherms fit well with the experimental ones when the spacing of the layered-pillared pore is 13.9 A. Then we used the model built here for further simulations. It has been observed that, due to the induced suface functional groups, the selectivity of CO2 over CH4 is highly promoted. Moreover, the selectivity of CO2 over CH4 drops by 50% when the depth increases from lkm to 3km, and then comes to be more stable between the depth of 3km and 6km. Nevertheless, the storage capacity of CO2 decreases uniformly with depth. By cooperative consideration, we propose that the optimum depth for displacement of shale gas and strorage of CO2 is 1～3km.