Investigation of the Methane Adsorption Characteristics of Marine Shale: A Case Study of Lower Cambrian Qiongzhusi Shale in Eastern Yunnan Province, South China
Abstract:Marine organic-rich
shale in South China has considerable exploration
potential, and the shale adsorption capacity has a great impact on
the accumulation of shale gas. To study the methane adsorption capacity
of marine shale, ten shale samples from the Lower Cambrian Qiongzhusi
Formation in eastern Yunnan province were investigated by organic
geochemical analysis (total organic carbon content, thermal maturity,
and kerogen type), X-ray diffraction (XRD) analysis, field emission
scanning electron microscopy (FE… Show more
“…It means that there is no free gas region in micropores, and absolute adsorption capacity is the same as the total methane capacity. These results agree with the past works 4 , 75 , 76 that under in-situ shale reservoir condition, methane fills micropores most, and monolayer adsorption occurs in larger pores. Figure 5 Methane density distributions in illite nanopores of varying pore sizes at 333.15 K and ( a ) MPa; ( b ) MPa.…”
In this work, we use grand canonical Monte Carlo (GCMC) simulation to study methane adsorption in various clay nanopores and analyze different approaches to characterize the absolute adsorption. As an important constituent of shale, clay minerals can have significant amount of nanopores, which greatly contribute to the gas-in-place in shale. In previous works, absolute adsorption is often calculated from the excess adsorption and bulk liquid phase density of absorbate. We find that methane adsorbed phase density keeps increasing with pressure up to 80 MPa. Even with updated adsorbed phase density from GCMC, there is a significant error in absolute adsorption calculation. Thus, we propose to use the excess adsorption and adsorbed phase volume to calculate absolute adsorption and reduce the discrepancy to less than 3% at high pressure conditions. We also find that the supercritical Dubinin-Radushkevich (SDR) fitting method which is commonly used in experiments to convert the excess adsorption to absolute adsorption may not have a solid physical foundation for methane adsorption. The methane excess and absolute adsorptions per specific surface area are similar for different clay minerals in line with previous experimental data. In mesopores, the excess and absolute adsorptions per specific surface area become insensitive to pore size. Our work should provide important fundamental understandings and insights into accurate estimation of gas-in-place in shale reservoirs.
“…It means that there is no free gas region in micropores, and absolute adsorption capacity is the same as the total methane capacity. These results agree with the past works 4 , 75 , 76 that under in-situ shale reservoir condition, methane fills micropores most, and monolayer adsorption occurs in larger pores. Figure 5 Methane density distributions in illite nanopores of varying pore sizes at 333.15 K and ( a ) MPa; ( b ) MPa.…”
In this work, we use grand canonical Monte Carlo (GCMC) simulation to study methane adsorption in various clay nanopores and analyze different approaches to characterize the absolute adsorption. As an important constituent of shale, clay minerals can have significant amount of nanopores, which greatly contribute to the gas-in-place in shale. In previous works, absolute adsorption is often calculated from the excess adsorption and bulk liquid phase density of absorbate. We find that methane adsorbed phase density keeps increasing with pressure up to 80 MPa. Even with updated adsorbed phase density from GCMC, there is a significant error in absolute adsorption calculation. Thus, we propose to use the excess adsorption and adsorbed phase volume to calculate absolute adsorption and reduce the discrepancy to less than 3% at high pressure conditions. We also find that the supercritical Dubinin-Radushkevich (SDR) fitting method which is commonly used in experiments to convert the excess adsorption to absolute adsorption may not have a solid physical foundation for methane adsorption. The methane excess and absolute adsorptions per specific surface area are similar for different clay minerals in line with previous experimental data. In mesopores, the excess and absolute adsorptions per specific surface area become insensitive to pore size. Our work should provide important fundamental understandings and insights into accurate estimation of gas-in-place in shale reservoirs.
“…On the other hand, these estimates may be low if capillary condensation is neglected 28 , but this only occurs to a significant extent for wet gas. Methane adsorption capacities reported for other shales range from 0.26 (Eagle Ford) to 1.50 mg g −1 (Barnett) for US shales (measured at 40–50 bar) 29 and between 1.00 and 4.08 mg g −1 for Qiongzhusi shale, China (measured at 140 bar) 30 . Not surprisingly, these estimates are considerably lower than for isolated type II kerogen 31 , which had an adsorption capacity of 15 mg g −1 .Fig.…”
Exploration for shale gas occurs in onshore basins, with two approaches used to predict the maximum gas in place (GIP) in the absence of production data. The first estimates adsorbed plus free gas held within pore space, and the second measures gas yields from laboratory pyrolysis experiments on core samples. Here we show the use of sequential high-pressure water pyrolysis (HPWP) to replicate petroleum generation and expulsion in uplifted onshore basins. Compared to anhydrous pyrolysis where oil expulsion is limited, gas yields are much lower, and the gas at high maturity is dry, consistent with actual shales. Gas yields from HPWP of UK Bowland Shales are comparable with those from degassed cores, with the ca. 1% porosity sufficient to accommodate the gas generated. Extrapolating our findings to the whole Bowland Shale, the maximum GIP equate to potentially economically recoverable reserves of less than 10 years of current UK gas consumption.
“…Experimental samples (Kaol, Mont, Chl, and Ill) were derived from American rock formations, as shown in Table 1, and were pure clay minerals, the purity and composition of which can be quantitatively analyzed by XRD [8,36,37].…”
Five groups of methane isotherm adsorption tests were carried out at 61 °C under a pressure range of 0-25 MPa, including four groups of pure clay minerals (illite, montmorillonite, chlorite, kaolinite, for brief they are expressed as Ill, Mont, Chl, and Kaol respectively) and a group of mixed minerals by four pure clay minerals in equal mass ratio. The experimental results showed the order of methane adsorption of pure clay minerals: Kaol > Ill > Mont > Chl. And the composition of clay minerals was obtained by XRD experiments. The result showed that the four samples were of high purity, both of which were greater than 90%. In addition, the results of low-pressure nitrogen adsorption experiments showed the order of specific surface area: Kaol > Ill > Mont > Chl, which is consistent with the order of methane adsorption. The methane adsorption is positively correlated to the specific surface area. The adsorption data was fitted to the Langmuir-Freundlich model to establish the method to calculate the adsorption of mixed minerals. The result showed that the parameters of the adsorption model of mixed clay minerals can be obtained by summing the parameters of the adsorption model of the pure minerals contained in the mixture according to their constituent mass ratio and the feasibility of this result was also verified by literature data. Overall, once the mass ratio in mixed clay minerals gets known, its theoretical adsorption can be calculated.
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