Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Characterization and Evolution of Nanoporosity in Superdeeply Buried Shales: A Case Study of the Longmaxi and Qiongzhusi Shales from MS Well #1, North Sichuan Basin, China Kun Jiao, Yuehao Ye, Shugen Liu,* Bo Ran, Bin Deng, Zhiwu Li, Jinxi Li, Ziquan Yong, and Wei Sun State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China ABSTRACT: The nanopore characteristics of shale samples from the superdeeply buried Longmaxi Shale (drillcore recovered from 6604−6920 m below ground level), Wufeng Shale (6920−6926 m), and Qiongzhusi Shale (7960−8044 m) were studied from MS Well #1, Sichuan Province, China, which was completed in March 2016 and is the deepest onshore well yet drilled in Asia. To gain a better understanding of the influence of burial depth on the pore system of shales and to aid in the study of nanopore characteristics, the samples were analyzed by FESEM and N2 gas adsorption. Samples of Sichuan Basin shales recovered from depths ranging from 0 to 5000 m were selected as a control group. The results show similar nanopore characteristics in all 32 superdeeply buried shale samples from the three formations. The dominant pore types in the superdeeply buried shales are organic matter pores and interparticle pores, along with minor intraparticle pores. The dominant pore morphology is slit-like in shape. Low-pressure N2 adsorption analysis shows that the isotherms of all samples are type IV with an H3 hysteresis pattern. The quenched solid density functional theory (QSDFT) pore size distribution is dominantly in the range of 4−16 nm, and the BET surface area ranges between 8.63 and 16.13 m2/g. In comparison with nonsuperdeeply buried shales, superdeeply buried shales in MS Well #1 have a more dispersed pore-size distribution, lower micropore volume and micropore surface area, and higher mesopore volume and mesopore surface area. Thus, the mesopore/micropore volume and mesopore/ micropore surface area ratios of the superdeeply buried shales are several orders of magnitude higher than those of the nonsuperdeeply buried shales. Compaction related to burial depth may compress the pores to reduce the pore sizes and change the pore shapes from round or elliptical-shaped to slit-shaped. Given their relatively small pore sizes, micropores are most easily destroyed during the superdeep burial stage.
1. INTRODUCTION
form, and gas production rates. Consequently, more research is required to address these insufficiencies in our knowledge. The results of academic research and commercial exploration have shown that the Lower Cambrian Qiongzhusi Shale within the Sichuan Basin and its periphery has experienced the geological and geochemical conditions necessary for shale gas development.16,17 Within the Qiongzhusi Formation, the total organic carbon (TOC) and brittle mineral contents, thermal maturation, and effective thickness are commonly at the levels required for shale gas generation.18,19 However, few shale gas wells currently yield commercial quantities of gas from the Qiongzhusi Shale. The Jinye Well #1-HF from the Jingyan− Qianwei area of the southern Sichuan Basin produces 8 × 104 m3 of gas per day, making it the most productive of all the wells that penetrate the Qiongzhusi Shale. In contrast, the Jiaoye Well #1 from the Fuling area and the Yang Well #201-H2 in the Luzhou area are classic shale gas wells penetrating the lower Silurian Longmaxi Shale, producing 2.03 × 105 and 4.3 × 105 m3 of gas per day, respectively. Therefore, at present, the shale gas potential and production of the lower Cambrian Qiongzhusi Shale is significantly lower than that of the lower Silurian Longmaxi Shale from the Sichuan Basin.20−22 The hydrocarbon generating ability of the Qiongzhusi Shale has
Prospective gas shales in the Sichuan Basin, China, record a complicated tectonic evolution, meaning that an understanding of their evolution is key to determining the shale gas enrichment of black shales in the basin.1−4 The temperature and pressure conditions of potential gas shales are strongly influenced by the regional tectonic evolution. Slight physical and chemical changes related to temperature and pressure may greatly change the pore and fracture system in shales.1,5−7 Fluid behavior related to temperature and pressure, especially hydrocarbon generation and expulsion during key periods such as peak oil generation, peak gas generation, and periods of uplift and erosion, may strongly affect the shale gas potential of organic-rich shales.5,8 Furthermore, adsorption and desorption processes related to the tectonic evolution can influence geological conditions and engineering requirements when extracting gas from shale.5,9−11 Current gas-producing shales and prospective gas shales of the Sichuan Basin are lower Paleozoic black shales that have been subjected to a complex tectonic evolution, influenced by great burial followed by deformation, uplift, and erosion.12,13 Substantial burial is an important characteristic of potential gas shales in the Sichuan Basin and is well-known as a key factor in their development.14,15 However, less well understood is the influence of burial on the dynamic evolution of pressure within the reservoir, the physical reservoir characteristics, the reservoir © XXXX American Chemical Society
Received: September 28, 2017 Revised: November 26, 2017 Published: November 28, 2017 A
DOI: 10.1021/acs.energyfuels.7b02932 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Location map and tectonic setting of MS Well #1 (revised from Jiao et al.34).
been validated previously;23−25 however, the levels of thermal maturity and the preservation conditions of the pore and fracture system are generally believed to be less favorable than in the Longmaxi Shale.3 The main difference between the Qiongzhusi and Longmaxi shales is burial depth. The depth of the base of the Longmaxi Shale in the Sichuan Basin and its periphery is mainly
