Shale gas has become an important natural gas resource in recent years as the conventional oil and gas resources are depleting. Shale gas content is one of the most important parameters for reserve calculation and sweet-spot prediction. The traditional core recovery method is widely used to determine gas content. However, the estimation of lost gas content is the main factor of error and difficulty. Large errors and uncertainties occur when using the widely used methods, such as the United States Bureau of Mines (USBM) method. Hence, a more accurate method is required. In this work, a full-process model is developed in COMSOL Multiphysics to describe the lost gas with time during the core recovery process as well as the desorption stage after the core is covered. In this method, by setting the initial gas pressure and flow parameters and matching the desorbed gas volume and considering variable diffusivity with respect to temperature, the initial gas content and the gas lost with respect to time are calculated. Overall, 10 field data are tested using this full-process model, and the USBM method is also applied to compare the results. It is found that if the ratio of lost gas volume estimated using the USBM method to the desorbed gas volume of the field data is lower than 2.0, the USBM method underestimates the lost gas compared to the full-process method; if the ratio is about 2.0, the results from the USBM and the full-process methods are comparable; and if the ratio is close to 3.0, the USBM method tends to overestimate the lost gas. The modeling results indicate that this proposed full-process method is more theoretically sound than the USBM method, which has high uncertainties depending on the number of desorbed gas data points used. Nevertheless, this proposed method requires a large number of parameters, leading to the difficulty in finding true parameters. Therefore, an optimization algorithm is required. In summary, this study provides theoretical support and a mathematical model for the inversion calculation of lost gas during shale core recovery. It is helpful to evaluate the resource potential and development economics of shale gas more accurately.
The recent discovery of deep and ultra-deep gas reservoirs in the Permian Changxing Formation reefs, northeastern Sichuan Basin is a significant development in marine carbonate oil & gas exploration in China. Reef dolomites and their origins have been major research topics for sedimentologists and oil & gas geologists. The petrography, trace element and isotope geochemistry of 13 18 O values and very different 87 Sr/ 86 13 87 Sr/ 86 Sr ratios which were close to coeval seawater also did not support the possibility of the mixture of deep-burial circulated 13 C values from the dissolution of widely distributed Triassic evaporites during the burial diagenetic processes (including dehydration of water-bearing evaporites) could have been the
The free radicals concentration (N g ) of organic matter evolves with the temperature, time, kerogen types and other factors for the individual kerogens. This paper focuses on the N g evolution of immature type I and II kerogens and a type III coal under the laboratory pyrolysis temperature and heating time. The experiments were carried out in a closed system pyrolysis at an isothermal reactor in the temperature range from 300 to 500°C over 30 to 480 minutes. A temperature time index (TTI) is applied and the TTI values were calculated in order to indicate the pyrolysis temperature and time at which the kerogen had experienced. Some equations were established to show the relationship between the TTI and N g of type I, II and III kerogens, based on the tested data from laboratory anhydrous pyrolysis experiments of three kerogen types and geological type I kerogen samples. However, the evolution of free radicals concentration with temperature and time shows some differentials between different types of kerogen samples. This paper also discusses the different TTI-N g relation of type I kerogen between laboratory pyrolysis experimental and geological samples, which are collected from core samples with depth interval of 1137 to 4090.6 m. The TTI-N g equation from pyrolysis experimental samples should be calibrated by the data from geological samples when the equation is to be extended to the geological timescales. As the free radical concentration in organic matter is strongly maturity dependent, the relationship between TTI-N g may prove to be a valuable method to study paleotemperature of sedimentary basins.Keywords: Free radicals, Vitrinite reflectance (R o ), Kerogen, Temperature Time Index (TTI), Thermal history, Pyrolysis experiment INTRODUCTIONFree radicals are units with an unpaired electron in organic matter. Kerogen, the portion of sedimentary organic matter not soluble in organic solvents, is believed to have analogous structures in which numerous aromatic rings form angularly conjunctive aromatic sheets, with substituents (such as paraffinic side chains) attached at the sheet margins (Tisssot and Welte, 1984). Thermal cracking of paraffinic chains from the kerogen molecules initially forms alkyl and kerogen free radicals (Wang and Chen, 1988). Smaller alkyl radicals are highly unstable and are thus rapidly quenched by hydrogen extracted from the surrounding environment. However, the larger kerogen free radical is stabilized by the extended aromatic system and can be stable through geological time. Although there have been various structure models for different types of kerogen at different level maturity, the main chemical structure of kerogen include the core structures and linkages. Kerogen is defined to be a complex geopolymer and a heterogeneous macromolecule comprised of randomly cross-linked core structures that may be aliphatic, naphthenic, aromatic and/or heteroatomic . The chemical structures of kerogen and the free radicals reactions have been studied widely (Siskin et al., 1995;Sun and P...
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