Abstract
Quantitative characterization of pore structure and analysis of influencing factors of methane adsorption are important segments in shale gas reservoir and resources evaluation and have not been systematically carried out in marine–continental shale series. A series of integrated methods, including total organic carbon (TOC) contents, Rock-Eval pyrolysis, mineral composition analysis, pore structure measurement, high-pressure CH4 adsorption analysis and FE-SEM observation, were conducted on 12 transitional shale samples of well WBC-1 in the southern North China Basin (SNCB). The results indicate that TOC contents of the transitional shales range from 1.03 to 8.06% with an average of 2.39%. The transitional shale consists chiefly of quartz, white mica and clay minerals. Interparticle pore, intraparticle pore, dissolution pore and microfracture were observed in the FE-SEM images. The specific surface area (SSA) of BET for the samples ranges from 3.3612 to 12.1217 m2/g (average: 6.9320 m2/g), whereas the DR SSA for the samples ranges from 12.9844 to 35.4267 m2/g (average: 19.67 m2/g). The Langmuir volume (VL) ranges from 2.05 to 4.75 cm3/g (average = 2.43 cm3/g). There is unobvious correction between BET and DR SSA with TOC contents, which means inorganic pores are the main component of pore space in the transitional shale from the SNCB. The relationship of SSA and pore volume shows that micropore has a greater impact on the CH4 adsorption capacity than mesopore–macropore in the transitional shale. Different from shales in other petroliferous basin, clay minerals are the primary factor affecting adsorption capacity of CH4 for transitional shale in this study. The pore structure of the transitional shale for this study is characterized by higher fractal dimension and more heterogeneous pore structure compared to shale in other petroliferous basin. This study provides an example and new revelation for the influencing factors of pore structure and methane adsorption capacity of marine–continental transitional shale.
Highlights
Shale gas is considered as an important unconventional natural gas resource worldwide (Law 2002)
Five typical samples including WBC1-4, WBC1-N119, WBC1-N133, WBC1-N141 and WBC1-N159 were selected from 12 shale samples to study the microscopic visualization characteristics of pore system and minerals identification
The results indicate that the total specific surface area (SSA) and total pore volume (PV), obtained from the BJH model in N 2 adsorption experiment and density functional theory (DFT) model in CO2 adsorption experiment, are positively correlated with Langmuir volume
Summary
Shale gas is considered as an important unconventional natural gas resource worldwide (Law 2002). The shale gas can be generated by degradation of kerogen in shale matrix. Self-generation and -storage act as the specific characteristics of shale gas (Zou et al 2010). The generated shale gas was in different states, including adsorbed gas in micropore and mineral surfaces, free gas in cracking and dissolved gas in crude oil (Martini et al 1998). Previous studies reveal that the adsorption proportion in shale gas ranges from 20 to 85% (Hill and Nelson 2000). The adsorption characteristics of shale act as an important leading factor influencing the shale gas accumulation. Investigation of methane adsorption of shale can help to correct evaluation of shale gas resources
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