Evaluation of gas-in-place content and gas-adsorbed ratio using carbon isotope fractionation model: A case study from Longmaxi shales in Sichuan Basin, China

Li, Wenbiao; Li, Junqian; Lu, Shuangfang; Chen, Guohui; Pang, Xiaoting; Zhang, Pengfei; He, Taohua

doi:10.1016/j.coal.2021.103881

“…Due to the existence of a large amount of organic matter and micropores in shale, in addition to the existence of free gas, some gases also exist in the form of adsorbed gas (Pan et al, 2015;Wu et al, 2015;Yu et al, 2016). At present, the proportion of adsorbed gas in the developed shale gas fields in China is about 30%∼70% (Curtis, 2002;Mengal et al, 2011;Li et al, 2022), and the gas volume of this part cannot be ignored. Therefore, the accurate evaluation of adsorbed gas capacity of deep shale has important implications for determining subsurface gas reserves and designing development plans.…”

Section: Introductionmentioning

confidence: 99%

Predicting adsorbed gas capacity of deep shales under high temperature and pressure: Experiments and modeling

Zhou

¹

,

Wang

²

,

Li

³

et al. 2022

Adv. Geo-Energy Res.

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“…The shale gas-bearing property is an essential index for shale gas exploration potential evaluation and sweet spot prediction 8 , 9 . The evaluation objects in regard to the shale gas-bearing property can be classified into three types, namely, total gas content evaluation, evaluation of the adsorbed gas or free gas content, and gas saturation evaluation 10 , 11 . The evaluation methods for the shale gas-bearing property can be classified into three types, including the experiment/field testing method 12 – 16 , geophysical method 17 – 19 , and mathematical statistical method 20 , 21 .…”

Section: Introductionmentioning

confidence: 99%

Application of mathematical statistics to shale gas-bearing property evaluation and main controlling factor analysis

Li

¹

,

Pang

²

,

Li

³

et al. 2022

Sci Rep

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Gas-bearing property evaluation and main controlling factor analysis have remained a concern in shale gas research. The application of principal component analysis, an important mathematical statistical method, in gas-bearing property evaluation and main controlling factor analysis of the Longmaxi shale in the Weirong area, Sichuan Basin, was examined. The Longmaxi shale exhibits high heterogeneity, manifested in the organic matter abundance, mineral composition, and pore structure. Seven geological factors, including the temperature, pressure, TOC content, clay content, brittle mineral content, pore volume, and specific surface area (SSA), were selected in principal component analysis. Four principal components with geological significance, such as mineral composition, formation condition, pore structure, and organic matter abundance, were extracted through principal component analysis, and further constituted a comprehensive factor. Shale gas-bearing properties were evaluated according to the score of the comprehensive factor. The Longmaxi shale could be identified as exhibiting good, medium, and poor gas-bearing properties based on the comprehensive factor scores of these samples. According to each geological factor’s contribution to the comprehensive factor, combined with geological analysis, it could be considered that gas-bearing properties are primarily controlled by pore volume, SSA, and clay content, followed by TOC content, brittle mineral content, temperature and pressure.

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“…However, there is an obvious defect with this method, because this method often overestimates the shale gas content under actual geological conditions. As a con-stant, in situ gas desorption is a direct mean to acquire the gas content of a shale well, which has been more and more used to evaluate the gas-in-place content and free/sorbed gas ratio for shale reservoir [31,32]. During the in situ desorption process, shale gas contains three forms of gases: lost gas, desorption gas, and residual gas in terms of desorbed time [33][34][35].…”

Section: Introductionmentioning

confidence: 99%

“…Many scholars have adopted isothermal adsorption experiment jointly combined with in situ desorption measurement to predict shale gas reserves and free/sorbed gas ratio. For in situ gas desorption measurement, it cannot give free gas and sorbed gas content, but it can be evaluated based on desorption data [31]. It is extremely important to confirm time zero and lost gas time, because these variables have significant influences on lost gas content calculation [37].…”

Section: Introductionmentioning

confidence: 99%

Marine Shale Gas Occurrence and Its Influencing Factors: A Case Study from the Wufeng-Longmaxi Formation, Northwestern Guizhou, China

Cao

¹

,

Liu

²

,

Pan

³

et al. 2022

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Organic-rich shales were found in the Ordovician to Silurian Wufeng-Longmaxi Formation in northwestern Guizhou province, China, which has high shale gas content revealed by field measurement. Shale gas occurrence, free gas/sorbed gas ratio, and their influencing factors are crucial for shale gas exploitation strategy. Results indicated that the Wufeng-Longmaxi shales are dominated by type I kerogen, with total organic carbon (TOC) and equivalent vitrinite reflectance (eqvRo) of 0.77%–6.98% and 2.37%–2.53%, respectively. The total porosity and permeability are in the range of 1.23%–8.43% and 3 × 10 − 4 – 2.23 × 10 − 1 mD, respectively. FE-SEM observation and correlation analysis show shale porosity is dominated by organic matter (OM) pores, followed by interparticle (interP) pores related to brittle minerals. CH4, derived from oil cracking, is the main component of shale gas, but its proportion is lower than that in Fuling and Weiyuan areas, probably due to the weak preservation condition. Desorption gas and lost gas determined by in situ desorption test are 0.42–1.54 cm3/g and 1.9–7.14 cm3/g, respectively, and Langmuir volume ( V L ) from isothermal adsorption experiment is 1.63–4.78 cm3/g. Shale gas content is positively correlated with micropore volume, mesopore volume and TOC content but negatively correlated with macropore volume and clay mineral content, indicating that methane is preferentially stored in micropores (<2 nm) and mesopores (2–50 nm) related to OMs. By comparing actual total gas content with theoretical gas content, shale gas is considered to exist primarily in sorbed state, and the free gas proportion can increase with increased TOC content, due to that OM pores with larger sizes are also main space for free gas. Combined with the two methods, it can result in accurate calculations of shale gas reserves and free/sorbed gas ratio. Based on this understanding, a model of shale gas occurrence was proposed, which can provide a reference for shale gas exploitation in normal pressure areas.

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Evaluation of gas-in-place content and gas-adsorbed ratio using carbon isotope fractionation model: A case study from Longmaxi shales in Sichuan Basin, China

Cited by 49 publications

References 41 publications

Predicting adsorbed gas capacity of deep shales under high temperature and pressure: Experiments and modeling

Predicting adsorbed gas capacity of deep shales under high temperature and pressure: Experiments and modeling

Application of mathematical statistics to shale gas-bearing property evaluation and main controlling factor analysis

Marine Shale Gas Occurrence and Its Influencing Factors: A Case Study from the Wufeng-Longmaxi Formation, Northwestern Guizhou, China

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