Speaker
Description
The method of carbon dioxide (CO2) injection in shale reservoirs has prevailed in recent decades, attributed to CO2 injection promoting shale gas (CH4) production while being sequestrated to eliminate the greenhouse effect for an environmentally friendly society. However, the efficiency of CO2-enhanced CH4 recovery (CO2-EGR) in shale reservoirs is influenced by geological factors, including depth-induced variation in temperature and pressure and subsurface water encroachment with various salinity. The CO2-EGR process induces pore size redistribution and formation deformation, which is attributed to the reaction of CO2 adsorption and CH4 desorption from the surface of the shale slit and tiny pores inside the matrix. Meanwhile, this reaction, in turn, affects the CO2-EGR performance owing to the variation in the physical characteristics of shale. In order to reveal the fundamental mechanisms of the CO2-EGR to develop a high displacement efficiency, as well as the formation dynamic response for the CO2-EGR process, a hybrid simulation method of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) process is carried out to uncover the gas adsorption and displacement from a molecular aspect. The molecular dynamics can be established at various temperatures and pressures, and other geological factors can be quantified and added into the simulation system, which is an effort-saving and visualization-friendly way. Adsorbed gas is primarily present in the organic matter of shale, and the adsorbed gas positively relates to the organic carbon content, owing to the large specific surface area and significant pore volume. Therefore, this study employs the type II-D kerogen fragment (C175H102N4O9S2) to construct the flexible shale matrix. This work observes that, for pure gas adsorption, CH4 increases by 9.2% at 308 K, 10.8% at 338 K, and 11.9% at 368 K in the deformable model than that of the fixed one. In contrast, CO2 has a 36.5% increase at 308 K, a 39.3% increase at 338 K, and a 44.4% increase at 368 K, presenting that the flexible model preserves additional CH4 and sequestrates more CO2 than estimated. Subsurface water is also added at 0-5 wt% of the organic model with the salinity of 3-6 mol/L NaCl solution. Moisture has a noticeable pore space reduction effect, and 5 wt% moisture content leads to a 44.9% reduction in CH4 adsorption, compared to a 24.5% reduction in CO2 adsorption. Moreover, the 6 mol/L NaCl within 5 wt% moisture content further reduces CH4 adsorption by 9.8%, compared to 13.8% for CO2. Subsurface water suggests an impeded influence on CH4 adsorption and CO2 sequestration. The preferential selectivity SCO2/CH4, as a competitive determination and displacement indicator, is addressed for the equimolar binary mixture between CO2 and CH4 under the above influencing factors and observes moisture positively influences SCO2/CH4, salinity promotes SCO2/CH4, and C2H6 develops SCO2/CH4. This study sheds light on the CO2-EGR project and advances the CCUS in unconventional reservoirs.
| Country | China |
|---|---|
| Acceptance of the Terms & Conditions | Click here to agree |
