Geochemical modeling of carbon isotope fractionation during methane transport in tight sedimentary rocks

Li, Wenbiao; Lu, Shuangfang; Li, Junqian; Wei, Yongbo; Feng, Weijia; Zhang, Pengfei; Song, Zhaojing

doi:10.1016/j.chemgeo.2020.120033

“…CFSR is strongly affected by the sediment source and sedimentary environments [29,30]. Consequently, CFSR is highly heterogeneous, which is primarily manifested in the mineral types, rock compositions, structural characteristics, pore space, organic matter (OM) characteristics, and hydrocarbon mobility [31][32][33][34][35][36]. Some scholars have studied this issue.…”

Section: Introductionmentioning

confidence: 99%

Coupling Relationship between Lithofacies and Brittleness of the Shale Oil Reservoir: A Case Study of the Shahejie Formation in the Raoyang Sag

Song

¹

,

Li

²

,

Li

³

et al. 2022

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Analyzing the characteristics of rock brittleness in low-permeability mudstone and shale (MS) formations is imperative for efficient hydraulic fracturing stimulation. Rock brittleness depends on the mineral composition, organic matter abundance, and bedding structure. Based on the MS from Shahejie Formation mineral composition (clay mineral, felsic mineral, and calcareous mineral contents), total organic content, and bedding structure (laminated, laminar, and massive), six types of lithofacies were identified: clay-rich MS, felsic-rich MS, calcareous-rich MS, clay MS, felsic MS, and calcareous MS. The quartz, feldspar, calcite, and dolomite of the Shahejie Formation are brittle minerals. Consequently, lithofacies with high felsic and calcareous mineral contents are more brittle. In addition, laminated and laminar MS are also conducive to hydraulic fracturing. Therefore, laminated, organic-rich, and calcareous-rich MS are the dominant lithofacies for hydraulic fracturing in the Shahejie Formation. The lithofacies and brittleness index were predicted by the response characteristics between mineral compositions and logging curves. The 3521–3552 m section of well B11x is dominated by calcareous-rich MS with developed laminae, representing a favorable section for hydraulic fracturing. Fragile minerals and oil are widely developed in the lower part of the lower 1st member of the Shahejie Formation (Es1L) in the southwestern part of Zhaohuangzhuang-Suning, where hydraulic fracturing can be used to increase shale oil production.

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“…Meanwhile, shale oil has also been proven to have considerable resources and is considered a future worldwide energy source. Shale oil is widely distributed in continental petroliferous basins in China, and more emphasis has been gradually placed on shale oil exploration and exploitation in recent years, stimulated by insufficient shale gas reserves [2][3][4].…”

Section: Introductionmentioning

confidence: 99%

Pore Structure and Fractal Character of Lacustrine Oil-Bearing Shale from the Dongying Sag, Bohai Bay Basin, China

Zhang

¹

,

Lu

²

,

Zeng³

et al. 2021

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To better understand the pore structure and fractal character of lacustrine shales and their influence on liquid hydrocarbon occurrences, in this study, a total of 29 lacustrine oil-bearing shale samples collected from the Shahejie Formation in the Dongying Sag, Bohai Bay Basin, were investigated based on nitrogen adsorption (NGA) analysis combined with TOC, Rock-Eval pyrolysis, X-ray diffraction (XRD), and field emission-scanning electron microscopy (FE-SEM) experiments. The relationships among the compositions (TOC, minerals, and oil content), pore structures, and fractal dimensions of the lacustrine shale samples were discussed. The results showed that the adsorption and fractal characteristics of lacustrine oil-bearing shales differ at relative pressures of 0-0.1 and 0.5-1. Two corresponding fractal dimensions D 1 and D 2 were determined by the FHH model according to the nitrogen adsorption branches. Specifically, D 1 varies from 2.4292 to 2.6109 (mean 2.5245), and D 2 varies between 2.4680 and 2.8535 (mean 2.6889). The specific surface area (SSA) ranges from 1.512 m2/g to 34.002 m2/g, with an average of 13.656 m2/g, the total pore volume is between 6.0 × 10-3 cm3/g and 48.4 × 10-3 cm3/g (mean 24.5 × 10-3 cm3/g), and the average pore diameter is in the range of 4.22 nm to 19.57 nm (mean 9.35 nm). Both D 1 and D 2 increase with increasing SSA and increase with decreasing average pore diameters but have no correlation with pore volume. Moreover, D 1 and D 2 exhibit positive relationships with clay minerals and negative correlations with carbonate minerals (calcite and dolomite). The relationship between fractal dimensions ( D 1 and D 2 ) and TOC contents is expressed as a U-shaped curve, characterized by the minimum D values at approximately 3% TOC. The shale oil content is controlled by the pore structures and fractal dimensions, and lacustrine shales with lower SSAs and smaller fractal dimensions would have more free oil. Therefore, lacustrine shales in the oil window with TOC contents ranging from 2% to 4% are probably the preferred shale oil exploration target in the Shahejie Formation, Dongying Sag, Bohai Bay Basin. The results indicate that fractal analysis can provide insight into the pore structure characteristics and oil storage capacity of lacustrine shales.

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“…4 Moreover, isotope fractionation may also result from the dissolution of methane isotope gas into an aqueous phase due to higher solubility for 13 CH 4 compared to that of 12 CH 4 , but the contribution of dissolution to fractionation can be ignored in shale gas production. 6,11 In addition to adsorption/desorption and dissolution, the discrepancy between transport capacity of isotopic gases during transport through shale pores also results in isotope fractionation. 19−21 Multiple flow mechanisms (e.g., viscous flow, slip flow, Knudsen diffusion, and surface diffusion) coexist in shale reservoirs because of the multiscale flow paths [micropores (<2 nm), mesopores (2−50 nm), and macropores (>50 nm)] and the complex pore structures in them.…”

Section: Introductionmentioning

confidence: 99%

“…Shale gas production has grown rapidly in recent years with the aid of core technologies such as horizontal-well volume fracturing and multi-well pad-based exploitation. , The gas-in-place content in shale reservoirs is a key parameter to evaluate shale gas resource potential and identify the “sweet spots” . Recently, carbon isotope fractionation patterns have been proposed as new methods for evaluating the gas-in-place content. − Compared with other direct and indirect methods for the evaluation of gas content, the isotopic composition of methane can provide a more precise calculation method for the gas-in-place content in shale reservoirs and reflect the stage characteristics of gas production. , In addition, carbon isotopic compositions are also important agents in identifying the origin of shale gas, gas migration patterns, and maturity of shale. , …”

Section: Introductionmentioning

confidence: 99%

Carbon Isotope Fractionation during Shale Gas Transport through Organic and Inorganic Nanopores from Molecular Simulations

Wang

¹

,

Ma

²

,

Zhao

³

et al. 2021

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Carbon isotope fractionation is a promising method to predict gas-in-place content and evaluate the shale gas production stage. In this study, molecular simulations are conducted to investigate fractionation characteristics of 12CH4 and 13CH4 in high-Kn (Knudsen number) flows (Kn > 0.1) within organic and inorganic pores under shale reservoir conditions (353 K, 5–25 MPa). The results show that isotope fractionation is more obvious (i.e., the difference in transport capacities between 12CH4 and 13CH4 is larger) in organic pores than in inorganic pores. Methane adsorption capacity and surface roughness of pore walls are two major reasons. High-coverage adsorption in organic pores reduces the effective pore size, and the Knudsen diffusion becomes significant. Moreover, the specular reflection of molecules occurs frequently on the smooth surfaces of organic pores, which enlarges the difference in isotope diffusion capacity. Indeed, the difference in energy (specific enthalpy) transport of methane isotopes in organic and inorganic pores is the intrinsic reason for fractionation. Furthermore, the fractionation level is positively correlated with Kn due to the enhanced contribution of Knudsen diffusion and surface diffusion in high-Kn flows. In addition, the isotope fractionation level decreases as pore size increases because Kn and the contribution of the adsorbed phase to the total molar flux reduce in a large pore. Our findings and related analyses may help us to understand isotope fractionation in different pore types and sizes from the atomic level and assist future applications in engineering.

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Geochemical modeling of carbon isotope fractionation during methane transport in tight sedimentary rocks

Cited by 38 publications

References 39 publications

Coupling Relationship between Lithofacies and Brittleness of the Shale Oil Reservoir: A Case Study of the Shahejie Formation in the Raoyang Sag

Coupling Relationship between Lithofacies and Brittleness of the Shale Oil Reservoir: A Case Study of the Shahejie Formation in the Raoyang Sag

Pore Structure and Fractal Character of Lacustrine Oil-Bearing Shale from the Dongying Sag, Bohai Bay Basin, China

Carbon Isotope Fractionation during Shale Gas Transport through Organic and Inorganic Nanopores from Molecular Simulations

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