Frontiers of Earth Science最新文献

The permeability and its horizontal anisotropy induce a critical influence on staged CH₄ output inhibition process. However, a quantitative evaluation of this influence has been rarely reported in the literature. In this work, the impact of horizontal anisotropic permeability on CO₂-ECBM was numerically investigated. The variation in the staged CH₄ output inhibition was analyzed. The ideal displacement profile of the CO₂-ECBM process was established for the first time. Moreover, the variation in CH₄ output of different wellbores was discussed. The results showed that 1) low-permeable or weak-anisotropic reservoirs were not conducive to enhanced CH₄ recovery owing to long inhibition time (> 1091 days) and high inhibition level (> 36.9%). As permeability and anisotropy increased, due to the accelerated seepage of free water, the hysteresis time and inhibition time could decrease to as short as 5 days and 87 days, respectively, and the inhibition level could weaken to as low as 5.00%. Additionally, the CH₄ output and CO₂ injection could increase significantly. 2) Nevertheless, high permeability and strong anisotropy easily induced CO₂ breakthrough, resulting in lower CH₄ production, CO₂ injection and CO₂ storage than expected. While maintaining high efficiency of CO₂ storage (> 99%), upregulating CO₂ breakthrough concentration from 10% to 20% might ease the unfavorable trend. 3) Along the direction of fluid flow, the ideal displacement profile consisted of CO₂ enriched bank, CO₂ and CH₄ mixed bank, CH₄ enriched bank, and water enriched bank, whereas a remarkable gap in the displacement profiles of the dominant and non-dominant seepage directions was observed. 4) The potential of CH₄ output might vary greatly among different wellbores. The producers along the dominant seepage direction held more potential for CH₄ recovery in the short-term, while those along the non-dominant seepage direction avoided becoming invalid only if a long-time injection measure was taken for the injectors. These findings pave the way to understand fluid seepage in real complex reservoirs during CO₂-ECBM and conduct further field projects.

The discovery of Loulan ancient city (LA) in the early 20th century has important significance for understanding the history of Western regions and the Silk Road civilization. The current academic community still has disputes on whether LA was the capital of Loulan Kingdom, the time of its rise, peak and decline, and the process, rate and driving mechanism of human activity change. This study uses the radio carbon dates (¹⁴C) database of LA to reconstruct the history of the rise and fall of human activity, and finds that LA experienced more than ∼500 years from its rise to its peak and then to its decline: 1) the city rose rapidly, and the population increased rapidly from ∼A.D. 0 to 230; 2) the city was prosperous and flourishing, and the intensity of human activity reached its peak from ∼A.D. 160 to 340, especially in ∼A.D. 230, when the population reached its peak; 3) the city accelerated its decline, and the intensity of human activity decreased significantly, and the population shrank rapidly from ∼A.D. 230 to 500; 4) LA was completely abandoned after ∼A.D. 560. The results of the ¹⁴C dating database do not support that LA was the early capital of the Loulan Kingdom. By comparing the human activity record of LA with the existing high-resolution palaeoclimate records in the surrounding mountainous areas of the Tarim Basin and South Asia, it is found that the superposition of centennial-scale westerly circulation strength events and the ∼500-year cycle of the Indian monsoon jointly controlled the precipitation and meltwater (snow) supply of the mountains in the Tarim Basin, affecting the changes of surface runoff and oasis area in the basin, which is one of the important factors causing the rise and fall of LA.

Carbon capture, utilization, and storage (CCUS) is considered one of the most effective measures to achieve net-zero carbon emissions by 2050, and low-rank coal reservoirs are commonly recognized as potential CO₂ storage sites for carbon sequestration. To evaluate the geological CO₂ sequestration potential of the low-rank coal reservoirs in the southern margin of the Junggar Basin, multiple experiments were performed on coal samples from that area, including high-pressure mercury porosimetry, low-temperature N₂ adsorption, overburden porosity and permeability measurements, and high-pressure CH₄ and CO₂ isothermal adsorption measurements. Combined with the geological properties of the potential reservoir, including coal seam development and hydrodynamic characteristics, the areas between Santun River and Sigong River in the Junggar Basin were found to be suitable for CO₂ sequestration. Consequently, the coal-bearing strata from Santun River to Sigong River can be defined as “potentially favorable areas for CO₂ eequetfraiion” To better guide the future field test of CO₂ storage in these areas, three CO₂ sequestration modes were defined: 1) the broad syncline and faulted anticline mode; 2) the monoclinic mode; 3) the syncline and strike-slip fault mode.

Overmature continental shale is commonly developed, but few studies have given insight into its pore structure and sorption capacity. Various techniques, including SEM, helium porosity and permeability, N₂/CO₂ adsorption, MICP, and NMR, were used to detect the pore structure of shale from the Shahezi Formation, Xujiaweizi Fault, Songliao Basin. The excess methane adsorption volumes were measured by the volumetric method and modeled by the Langmuir model. Based on the findings, the most developed pores are intraparticle pores in clay minerals, followed by the dissolution pores in feldspar, but organic pores are uncommon. The selected shales have low helium porosity (mean 1.66%) and ultralow permeability (mean 0.0498 × 10⁻³µm²). The pore throats are at the nanoscale, and the pore-throat size distributions are unimodal, with most less than 50 nm. The studied shales are characterized by the lower specific surface area (SSA) and pore volume (PV) but the larger average pore diameter. The total SSA is contributed by the micro- and mesopores, while the PV is dominated by meso- and macropores. The pore structures are more complex and controlled by multiple factors, such as mineral compositions and diagenesis, but organic matter is not critical. The maximum absolute adsorption methane volume (V_L) is 0.97–3.58 cm³/g (mean 1.90 cm³/g), correlating well with the total SSA, SSA, and pore volume of micropores, which indicates that methane is mainly adsorbed and stored in micropores, followed by mesopores.

CO₂ flooding can significantly improve the recovery rate, effectively recover crude oil, and has the advantages of energy saving and emission reduction. At present, most domestic researches on CO₂ flooding seepage experiments are field tests in actual reservoirs or simulations with reservoir numerical simulators. Although targeted, the promotion is poor. For the characterization of seepage resistance, there are few studies on the variation law of seepage resistance caused by the combined action in the reservoir. To solve this problem, based on the mechanism of CO₂, a physical simulation experiment device for CO₂ non-miscible flooding production manner is designed. The device adopts two displacement schemes, gas-displacing water and gas-displacing oil, it mainly studies the immiscible gas flooding mechanism and oil displacement characteristics based on factors such as formation dip angle, gas injection position, and gas injection rate. It can provide a more accurate development simulation for the actual field application. By studying the variation law of crude oil viscosity and start-up pressure gradient, the characterization method of seepage resistance gradient affected by these two factors in the seepage process is proposed. The field test is carried out for the natural core of the S oilfield, and the seepage resistance is described more accurately. The results show that the advancing front of the gas drive is an arc, and the advancing speed of the gas drive oil front is slower than that of gas drive water; the greater the dip angle, the higher the displacement efficiency; the higher the gas injection rate is, the higher the early recovery rate is, and the lower the later recovery rate is; oil displacement efficiency is lower than water displacement efficiency; taking the actual core of S oilfield as an example, the mathematical representation method of core start-up pressure gradient in low permeability reservoir is established.

CO₂ immiscible flooding is an environmentally-friendly and effective method to enhance oil recovery in ultra-low permeability reservoirs. A mathematical model of CO₂ immiscible flooding was developed, considering the variation in crude oil viscosity and starting pressure gradient in ultra-low permeability reservoirs based on the non-Darcy percolation theory. The mathematical model and numerical simulator were developed in the C++ language to simulate the effects of fluid viscosity, starting pressure gradient, and other physical parameters on the distribution of the oil pressure field, oil saturation field, gas saturation field, oil viscosity field, and oil production. The results showed that the formation pressure and pressure propagation velocity in CO₂ immiscible flooding were lower than the findings without considering the starting pressure gradient. The formation oil content saturation and the crude oil formation viscosity were higher after the consideration of the starting pressure gradient. The viscosity of crude oil considering the initiation pressure gradient during the formation was higher than that without this gradient, but the yield was lower than that condition. Our novel mathematical models helped the characterization of seepage resistance, revealed the influence of fluid property changes on seepage, improved the mathematical model of oil seepage in immiscible flooding processes, and guided the improvement of crude oil recovery in immiscible flooding processes.