Simulations Of Gas Adsorption And Chain-molecules Transportation In Microscaled Pores Of Rocks And Its Preliminary Application In Petroleum Geology

Unconventional energy (e.g. coal bed methane, shale gas) now is playing more important roles in the whole energy system. Understanding the adsorption capacity and deformation change during adsorption of methane and CO2 on coal and shale, may help the exploitation and resoure evaluation. Four coal samples and eleven shale samples were measured by N2 adsorption method. Isotherms as well as pore size distrubtions of these samples were obtained. Base on these exmperimental data, a systematic simulation work were carried out.Firstly, the quenched solid density functional theory (QSDFT) is employed to study the adsorption of methane and CO2 on coal at geological conditions. The main focus is made on coal deformation in the course of adsorption that may result either in expansion/swelling or contraction, depending on pressure, temperature, and pore size. The results of methane adsorption capacity, density profile and salvation pressure on coal in pores from 0.5nm to 50nm at 298 K and 360 K with pressure up to 100 MPa are given. Two qualitatively different types of deformation behavior are found depending on the pore width:Type I shows a monotonic expansion in the whole pressure range. This behavior is characteristic for the smallest pores<1.3σff (0.5nm) that cannot accommodate more than one layer of methane. Type II displays contraction at low pressures followed by expansion. Type II behavior is found for several groups of pores, which can accommodate dense packing with integer number (from 2 to 6) of adsorbed layers. Two typical deformation behaviors at 360 K could be summarized from the CO2 solvation pressure dependence on the external pressure. Type I behavior shows a monotonic expansion in the whole pressure range, but at high pressures it may either continue to expand or contract. This behavior is typical for the smallest pores<1.3σff (0.5 nm) that cannot accommodate more than one layer of CO2, as shown by the density profile in 0.5 nm pore. Type II behavior displays contraction at low pressures followed by expansion. At high pressure, it also may expand or contract again. Type Ha is found for 0.6 nm and 0.9 nm pores. All other pores>0.9 nm present the typeⅡb behavior.Then, the results of QSDFT model arecompared with literature experimental data, and the model is then employed to study the adsorption behavior of model coals at elevated pressures and temperatures. An instructive example of CO2 sequestration into coal bed is given. We established the relationships between the methane and CO2 capacity and the solvation pressure it exerts on the coal matrix and the depth of coal bed for pores of different sizes. The replacement process of methane by CO2 is discussed. We found that the coal deformation depends on the bed depth, and at different depths it either swell or contract depending on the pore size distribution. Through calcualtion on the replacement of methane by CO2 under geological condition, CO2 capcity and the structure change in the ECBM scenario were discussed. The difference in the volumetric strain induced by adsorption of CO2 and methane is the smallest in the case of 5nm pore (0.6%). For micropores (≤2nm), the volumetric strain difference can be as large as 1.7% in the case of 0.7 nm pore at 100 m depth, which may cause significant reduction in permeability of the reservoir through deformation.Secondly, similiar model is employed to study the adsorption capacity of methane and CO2 in shale and the adsorption induced deformation behavior. Correspondingly, the relationships between the methane and CO2 capacity and the solvation pressure it exerts on the shale matrix and the depth of shale for pores of different sizes is discussed. When the depth is small, the adsorption capacity (per unit volume) of micropore is bigger than that of mesopore. Methane adsorptin capacity in pores