The sample size or particle size of shale plays a significant role in the characterization of pores by various techniques. To systematically investigate the influence of particle size on pore characteristics and the optimum sample size for different methods, we conducted complementary tests on two overmature marine shale samples with different sample sizes. The tests included small-angle neutron scattering (SANS), gas (N2, CO2, and H2O) adsorption, mercury injection capillary pressure (MICP), and field emission-scanning electron microscopy (FE-SEM) imaging. The results indicate that artificial pores and fractures may occur on the surface or interior of the particles during the pulverization process, and some isolated pores may be exposed to the particle surface or connected by new fractures, thus improving the pore connectivity of the shale. By comparing the results of different approaches, we established a hypothetical model to analyze how the crushing process affects the pore structure of overmature shales. Our results imply that intact wafers with a thickness of 0.15–0.5 mm and cubic samples (~1 cm3) are optimal for performing SANS and MICP analyses. Meanwhile, the 35–80 mesh particle size fraction provides reliable data for various gas physisorption tests in overmature shale. Due to the intrinsic heterogeneity of shale, future research on pore characteristics in shales needs a multidisciplinary approach to obtain a more comprehensive, larger scale, and more reliable understanding.
Water uptake behavior can greatly impact gas migration in shale reservoirs. In this work, we combined water vapor sorption (including adsorption and desorption) and spontaneous imbibition (SI) to systematically investigate the water uptake behavior and influence factors of samples from the Longmaxi shales in the northern Guizhou Province, China. The model of fracturing fluid loss was divided into two stages: (i) SI stagefracturing fluid is drawn into fractures by capillary forces, then into large pores, and finally into small pores; (ii) water vapor adsorption (WVA) stagehere an increase in relative humidity (RH) causes water vapor molecules to be initially absorbed into hydrophilic pores via monolayer adsorption, followed by multilayer adsorption, and finally cluster formation and/or capillary condensation when both hydrophilic and hydrophobic pores are filled. The results also show that pore size distributions derived from WVA exhibit unimodal distributions, which are generally greater than that obtained from N 2 adsorption because of clay swelling. The illite−smectite mixed-layer mineral is the main WVA site within clay minerals. Moreover, the controlling factors of WVA vary with RH. At low RHs, clay is the main controlling factor for WVA by providing strong intermolecular bonding for monolayer vapor molecules, while at high RHs, porosity and pore volume become the main controlling factors by providing more space for capillary condensation. Specifically, the pores with sizes of around 3 nm mainly contribute to WVA under high RHs. This work should therefore improve the understanding of fracturing fluid loss process and influence factors, contributing to the optimization of shale gas production.
A petrophysical evaluation of caprock pore characteristics and seal capacity is critical for geologic storage of CO 2 . In this study, integration of X-ray diffraction, mercury injection capillary pressure (MICP), and spontaneous imbibition analyses is used to evaluate the pore characteristics and seal capacity of shale samples from the Ilaro formation, which could serve as a seal for CO 2 injected into the reservoir units of the eastern Dahomey basin. X-ray diffraction reveals the samples as clay-rich and early diagenetic, while spontaneous imbibition reveals pore connectivity heterogeneity. MICP analysis shows the samples as being characterized by a low breakthrough pressure and reveals the mesopore dominance with majority of pore volumes dominated by 5 to 10 nmsized pore throats. Furthermore, high CO 2 column heights are obtained from conversion of MICP data (at 20% saturation) for mesopore-dominated samples containing ankerite and kaolinite. The study reveals that should injected CO 2 migrate upward from the reservoir units of the onshore eastern Dahomey basin, portions of the Ilaro formation may function as a good seal due to the mesopore mineralogical composition influence on the CO 2 −brine−rock interaction. This result should therefore serve as an important insight for future caprock sealing studies conducted on the onshore eastern Dahomey basin.
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