The breakthrough of clean and efficient shale gas stimulation technology and reduction of global carbon emissions is in a critical period. The injection of supercritical carbon dioxide (SCCO 2 ) into shale gas reservoirs, fracturing the reservoir and displacing preadsorbed CH 4 , enhancing gas recovery, and completing CO 2 geological storage, is regarded as an optimum scheme due to its engineering advantages and high adsorption capacity and preference in shale. This investigation systemically summarizes the theoretical understanding and research progress on SCCO 2 fracturing technology and the displacement of CH 4 by CO 2 in shale gas development practice and theoretical simulation in the past decade and proposes the current challenges and prospects. The fracture space distribution characteristics of hydraulic fracturing and SCCO 2 fracturing and the stimulation effectiveness of the shale gas reservoir are compared. The low viscosity, high diffusion coefficient, and zero surface tension of SCCO 2 bring up its distinctive advantages for fracturing engineering. And the recovery of CH 4 is much higher than that from hydraulic fracturing. The adsorption capacity and preference for CO 2 are greater than those for CH 4 , caused by its molecular structure, properties, thermodynamics, and kinetics. Shale gas reservoirs are widely distributed and enormous resources. Several shale gas reservoirs in several basins are deemed to store CO 2 by several to tens of gigatons. However, the wide commercial application of CO 2 has been limited due to (i) the high cost of CO 2 capture and transportation, (ii) low sand carrying capacity, nonuniform distribution, and easy settlement of proppant, (iii) the complex coupling mechanism of CO 2 −CH 4 − H 2 O−shale, and (iv) the environmental threat after CO 2 storage. Therefore, it is urgent to develop a thickener and cosolvent with economical, clean, safe, and environmentally friendly characteristics that is highly compatible with SCCO 2 . High-precision reduction models of the structural characteristics and pore-fracture distribution characteristics of kerogen and mineral components are necessary to simulate competitive adsorption of CO 2 and CH 4 . Additionally, a perfect monitoring system for CO 2 leakage risk should be established near CO 2 storage sites to avoid impact and harm to the atmosphere, groundwater, and surface ecosystems.
This work presents the adsorption behavior and appearance characteristics of CH 4 and CO 2 on the Longmaxi shale at high pressure and temperature. To investigate the variation of gas adsorption patterns under the constraint of pressure and temperature, the applicability of the theories of monolayer adsorption, multilayer adsorption, and micropore filling was discussed. The preferential selection coefficient of CO 2 for CH 4 under different conditions was characterized by the absolute adsorption capacity (V abs ) ratio of CO 2 to CH 4 (αCO 2 /CH 4 ). Moreover, the implication of the CO 2 injection to enhance gas recovery and the CO 2 capture and storage (EGR−CCS) process was analyzed. The results exhibit that the excess adsorption curves of CH 4 are smooth, and the experimental temperature has no noticeable effect on the shape of curves. At the same time, a "sharp peak" is recorded in the excess adsorption curves of CO 2 at low temperatures (30 and 55 °C) near the critical pressure, which is quite distinct from the smooth curves at high temperatures (80 and 100 °C). Correspondingly, there are two "jump pressure" values in the density curves (30 and 55 °C) of the adsorption system and the density curves are divided into three stages. The Dubinin−Astakhov and Brunauer−Emmett−Teller (BET) models show an optimum degree of fit for CH 4 and CO 2 adsorption curves under all experimental temperature and pressure conditions. The Langmuir model fits the adsorption curves of 80 and 100 °C better, while the BET model is appropriate for 30 and 55 °C. The adsorption affinity of CO 2 is higher than CH 4 , with the value of αCO 2 /CH 4 in the range of 2.47−12.16. The value of αCO 2 /CH 4 increases with a rise in pressure but is inhibited by high temperatures, while the inhibition is negligible when the experimental temperature exceeds 80 °C. The adsorption preferential of CO 2 is stronger in the shallow reservoir (αCO 2 /CH 4 > 10.5), and the application prospect of the EGR process is promising. In contrast, the adsorption preferential is slightly weakened in the deep reservoir (αCO 2 /CH 4 < 4.5), which can be considered for CO 2 capture, utilization, and storage. Results from this investigation provide novel insights on the adsorption characteristics of CH 4 and CO 2 on the shale matrix at high pressure and temperature. They are also expected to give certain enlightenment for the EGR−CCS process.
Adsorbed gas is one of the crucial occurrences in shale gas reservoirs; thus, it is of great significance to ascertain the adsorption capacity of shale and the adsorption characteristics of CH4. In this investigation, the Taiyuan–Shanxi Formations’ coal-measure shale gas reservoir of the Carboniferous–Permian era in the Hedong Coalfield was treated as the research target. Our results exhibit that the shale samples were characterized by a high total organic carbon (TOC) and over to high-over maturity, with an average TOC of 2.45% and average Ro of 2.59%. The mineral composition was dominated by clay (62% on average) and quartz (22.45% on average), and clay was mainly composed of kaolinite and illite. The Langmuir model showed a perfect fitting degree to the experimental data: VL was in the range of 0.01 cm3/g to 0.77 cm3/g and PL was in the range of 0.23–8.58 MPa. In addition, the fitting degree depicted a linear negative correlation versus TOC, while mineral composition did not exhibit a significant effect on the fitting degree, which was caused by the complex pore structure of organic matter, and the applicability of the monolayer adsorption theory was lower than that of CH4 adsorption on the mineral’s pore surface. An apparent linear positive correlation of VL versus the TOC value was recorded; furthermore, the normalized VL increased with the growth of the total content of clay mineral (TCCM), decreased with the growth of the total content of brittle mineral (TCBM), while there was no obvious correlation of normalized VL versus kaolinite, illite and quartz content. The huge amount of micropores and complex internal structure led to organic matter possessing a strong adsorption capacity for CH4, and clay minerals also promoted adsorption due to the development of interlayer pores and intergranular pores.
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