Spontaneous imbibition (SI) is an important method to improve oil recovery in tight sandstone reservoirs. Commonly, the physical simulation of SI is performed at atmospheric pressure but the characteristics of spontaneous imbibition under confining pressure (SIUCP) is often neglected. In this study, oil distribution in tight cores was obtained in combination of high pressure mercury intrusion (HPMI) measurements and low-field nuclear magnetic (LF-NMR) measurements. After that, oil recovery for SI and SIUCP of tight core samples with all faces open (AFO) were obtained using LF-NMR measurements. Finally, a new scaling law for SIUCP was proposed to predict shut-in time in field scale. The results showed that 95.94–98.12 wt % of the oil was distributed in nanopores (0.1 ms < T 2 < 100 ms) of core samples, and the average amount of oil in nanomicro-pores, nanomeso-pores and nanomacro-pores were 34.04, 40.15, and 22.75 wt %, respectively. Ultimate oil recovery for core samples were 22.41, 44.41, 57.27, 61.84, and 62.82 wt %, respectively, as confining pressure increased from 0 to 2175 psi. The improved oil recovery for SIUCP was associated with the decline of effective pore radius as a function of confining pressure, which results in the effect of enhanced SI and compaction. Finally, a modified dimensionless time model was proposed in combination of Mason’s dimensionless time model and effective pore radius as a function confining pressure.
The lower Silurian Longmaxi Formation hosts a highly productive shale gas play in the Zhaotong region of southern China. According to core observation, X-ray diffraction analyses, and scanning electron microscopy (SEM) observations, the shale comprises primarily quartz, carbonate minerals, and clay minerals, with minor amounts of plagioclase, K-feldspar, and pyrite. The clay mineral content ranges from 15.0% to 46.1%, with an average of 29.3% in the Zhaotong region. Organic geochemical analyses show that the Longmaxi Formation has good potential for shale gas resources by calculating total organic carbon, vitrinite reflectance, and gas content. Scanning electron microscope images demonstrate that reservoir pore types in the Longmaxi shale include organic pores, interparticle pores, intercrystalline pores, intraparticle pores, and fractures. Reservoir distribution is controlled by lithofacies, mineral composition, and geochemical factors. In addition, we investigated the relationships between reservoir parameters and production from 15 individual wells in the Zhaotong region by correlation coefficients. As a result, the brittleness index, total organic carbon (TOC), porosity, and gas content were used to define high-quality reservoirs in the Longmaxi shale. Based on these criteria, we mapped the thickness and distribution of high-quality reservoirs in the Longmaxi Formation and selected highlighted several key sites for future exploration and development.
As a national shale-gas demonstration zone in China, the Zhaotong area has great gas resource potential. However, the nanopore structure characteristics, methane adsorption capacity, and their affecting factors of the Lower Silurian Longmaxi Shale in this area remain unclear. To address these puzzles, we conducted a series of experiments, such as X-ray diffraction, field emission scanning electron microscopy, low-pressure [Formula: see text] adsorption, and high-pressure methane adsorption, and we calculated the relevant characteristic parameters, such as pore volume (PV), specific surface area (SSA), fractal dimension, and Langmuir parameters by using the nonlocal density functional theory method, Frenkel-Halsey-Hill theory, and Langmuir model, respectively. The results indicate that the nanopores of the Lower Longmaxi Shale in the Zhaotong area are composed of micropores and mesopores, which mainly exist as organic matter (OM) pores. The pore surface exhibits a high degree of heterogeneity as indicated by the fractal dimensions ranging from 2.845 to 2.866. The nanopore structure characteristics (i.e., SSA and PV) and methane adsorption capacity are mainly controlled by the total organic carbon (TOC) content. In addition, the mineralogical composition (i.e., the quartz and clay content) also contributes significantly to the micropore PV and gas content. The external provenance has a significant effect on the mineralogical composition, TOC content, and methane adsorption capacity. With the increasing influence of the external provenance, the biogenic quartz content decreases and the relationship between the quartz content and TOC content becomes more discrete, which indicates the change of depositional environment, and the clay content increases, which can dilute the OM concentration during the deposition and enhance the compaction potential, and it can eventually result in less gas content. The results of this study reveal the nanopore system characteristics of the Longmaxi Shale in the Zhaotong area and provide further insight into the influence of external provenance on reservoir characteristics and gas content variability of the Lower Longmaxi Shale in the southern Sichuan Basin.
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