Influence of Pore Water on the Gas Storage of Organic-Rich Shale

Abstract

Previous studies that use dry samples have been conducted on the nanoporosity of organic matter in shale. However, underground shales contain pore water that may greatly influence their pore structures and adsorbed gas content. To address this, a typical organic-rich, marine, gas-bearing shale in China was analyzed in this study. The water content and isothermal adsorption under different saturation conditions were determined, and the adsorbed and free gas contents were quantified. Across scales, water occurs within clays, between grains, and in fissures. In micro- and mesopores, gas is mainly adsorbed onto the surface of kerogen, because clay surfaces are water-saturated. In macropores and fractures, gas mainly exists in a free state. Adsorbed and free gas contents decrease with increasing water saturation in shale reservoirs. In high-production reservoirs, a limited amount of adsorbed gas is stored within micropores, while a relatively large amount of free gas is stored in larger pores and natural fractures. Because shale gas occurs in low concentrations, high total organic carbon may not be the only necessary factor; high porosity and many natural fractures may be more favorable for high reservoir productivity. The results provide valuable insights for understanding gas storage and finding zones of high productivity in shale reservoirs and have potential applications in engineering conditions.

Porosity and Water Saturation Estimation for Shale Reservoirs: An Example from Goldwyer Formation Shale, Canning Basin, Western Australia

“…However, after applying the kerogen corrections, the corrected porosity ranging from 5 to 10% gives more accurate results that can be well-compared with core porosity (Table 2 and Figure 7). Moreover, the clay minerals also affect the pore structure of shale that directly affects the water saturation [37,38]. The Goldwyer shale also consists interparticle pores influenced by illite that may change the water saturation (Figure 6).…”

Effect of Pre-Adsorbed Water on Methane Adsorption Capacity in Shale-Gas Systems

“…Meanwhile, at high water saturation, the micropores of both organic matter and clay minerals are blocked predominantly by water molecules and therefore barely capable of adsorbing methane molecules, while for mesopores and macropores, water molecules occupy the majority of adsorption sites forming water film, and yet clay minerals pores have higher water molecules concentration compared to organic matter counterparts. In addition, since organic matter tends to absorb more methane molecules than water molecules (Tian et al, 2020;Sun et al, 2021), its methane molecules concentration at low water saturation is higher than its water molecules concentration at high water saturation. Likewise, as clay minerals are more prone to adsorb water molecules (Zolfaghari et al, 2017;Wang T. et al, 2019;Chen J. et al, 2021), its water molecules concentration at high water saturation is higher than its methane molecules concentration at low water saturation.…”

“…Thus, methane adsorption experiments on moisture-equilibrated shale samples were produced. Some researchers carried out equilibrium moisture treatment on rock samples prior to the experiments (Ross and Bustin, 2009;Gasparik et al, 2013;Merkel et al, 2015;Wang et al, 2018;Tian et al, 2020). For example, Ross and Bustin (2009) studied the methane adsorption capacity of shale under moistureequilibrated condition and reported a 20-90% reduction compared to the value under dry condition.…”

“…In the process of increasing the water content, the relationship between the adsorption amount and the moisture content is negatively correlated. Tian et al (2020) investigated the influence of pore water on the gas storage of organic-rich shale and pointed out that the adsorption space of methane onto clay mineral sufaces in microand mesopores is occupied by water, which results in a decrease of adsorbed gas, and the occurrence of adsorbed gas is dominated by organic matter adsorption. In addition, some scholars have also investigated the influence of water on methane adsorption through molecular simulation and found that water has an important effect on methane adsorption (Huang et al, 2018;Li et al, 2019;Chen G. et al, 2021).…”

Reservoir Porosity Measurement Uncertainty and its Influence on Shale Gas Resource Assessment

“…How to evaluate the porosity measurement uncertainty of a shale reservoir accurately has become an important problem. Conventional reservoir porosity testing methods (Bowker, 2007;Loucks et al, 2007;Sondergeld et al, 2010;Bustin et al, 2008;Tian et al, 2020) are used in experiments, however, the measurement deviation caused by the specific characteristics of a shale is neglected and the quantitative description of the accuracy of porosity measurement is lacking, which seriously restricts the evaluation of the accuracy of shale resources.…”