Studying the effects of pore structure on spontaneous imbibition (SI) is of great significance for optimizing coal seam water injection measurements. To investigate the influence of reservoir microscopic pores on imbibition, we studied pore structure in selected coal samples using Mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR), along with multifractal analysis. SI experiments were conducted on coal cores to investigate the dynamic imbibition process and quantitatively analyze imbibition at different time intervals, as well as the factors influencing imbibition. The results show that the adsorption pores primarily function as water storage spaces, while the seepage pores have dual functionality in both water conduction and storage during imbibition. Water is imbibed more quickly in seepage pores than in adsorption pores, as the effective driving force is lower in adsorption pores due to the high frictional resistance caused by strong pore heterogeneity. Due to the strong microscopic heterogeneity of coal, the imbibition process is predominantly governed by its microscopic properties. The imbibition rate of coal is mainly controlled by the pore connectivity (H), development degree of seepage pores (Dmax), concentration degree of pore size distribution (PSD) (D(1)) and uniformity of PSD (D(2)), but less effected by the adsorption pores (Dmin) and PSD heterogeneity (ΔD). The imbibition capacity is mainly affected by wettability (θ), while it is relatively weakly influenced by pore-related parameters. Considering the heterogeneity of the pore network in coals, a new model for SI was proposed, which shows better fitting performance compared to conventional models. This study can help enhance the comprehension of the underlying mechanism of SI and contribute to its practical implementation.
Naturally occurring macropores such as root channels and earthworm holes have been shown to affect water flow and solute transport processes in soils. To further our understanding of the specific processes that are involved in macropore flow, we have developed a Split Macropore Column (SMC) which consists of a semi‐cylindrical soil column with a semi‐cylindrical macropore at the main axial of its planer surface. The SMC allows one to control a number of important physical parameters such as the size, density, and continuity of the macropores for miscible displacement studies, as well as an opportunity to observe visually the macropore's impact on flow processes at the critical matrix‐macropore interface. After rigorous testing, we found that this system is a useful instrument to study macropore flow mechanisms. Using these SMCs, we performed a series of miscible displacement experiments to study the effects of soil type, flux rate, macropore size, and macropore to matrix size ratio. Our initial results show three zones of flow through the column: (i) film flow at the matrix–macropore interface, (ii) in the matrix directly adjacent to the macropore, and (iii) in the remaining matrix. The initial data suggest that macropore flow is influenced more by the matrix–macropore size ratio than by the macropore size.
The increasing significance of shale gas resources to the global energy supply has resulted in an increasing need to understand shales as gas reservoirs. Shales commonly have complex organic and inorganic compositions and ultrafine pore structures. Fluid flow and transport in shale gas reservoirs (SGRs) are scale-dependent processes, including adsorption/desorption and diffusion of gas, water imbibition, and other non-Darcy flows, which is in contrast with conventional natural gas reservoirs, such as sandstones. In recent years, the transferring of the opinion from the prior efforts on studying the source and seal potential to the purpose of the function as the gas reservoir is creating some new challenges to the conventional analysis methods for SGRs. There has been a much larger number of publications relating to "shale and method" in the last 5 years compared to previously. On the basis of a review of these publications, this review summarizes both progresses made to improve or modify conventional methods as well as the application of new, emerging methods for a description of the complex shale petrology, rock physics, and flow mechanisms. The main part of this review is divided into six sections: organic geochemical evaluation, pore properties, such as pore type, porosity, specific surface area, and pore size distribution, wettability assessment, quantification of imbibition, gas adsorption and diffusion, and permeability evaluation. In each section, the advantages and disadvantages of conventional and emerging methods are reviewed. The goal of this review is to provide a guide for the reader to help them choose appropriate methods for future work in SGRs.
Several laboratory studies have reported that the presence of moisture will significantly affect the production of coalbed methane (CBM), but the impact of a water film formed by moisture on the transport of CBM and CO2-enhanced CBM recovery (CO2-ECBM) is still poorly understood. Furthermore, there are also few studies about the formation mechanism of thin water films on the hydrophilic surface in coal. In this study, the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was applied to analyze the formation mechanism of the thin water film on the hydrophilic surface in coal. Two different pore models, the cylindrical and slit pore model, were developed to interpret the experimental water vapor adsorption (WVA) isotherms. We proposed a DLVO theory-based model combined pore size distribution functions, which can be used to analyze the differences in the moisture status in pores with different sizes under the same humidity conditions. Besides, the mechanism of methane blocked by WVA was also theoretically explored. It was found that water molecules can form capillary condensation even at low relative humidity (RH). Also, the model proposed in this work is useful for providing theoretical guidance for the study of RH and water condensation on the CBM flow and production. According to the presented research, we argue that the mechanism of CO2-ECBM is not only related to competitive adsorption but also related to the displacement of CH4 locked by water.
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