Adsorption Characteristics and Controlling Factors of CH4 on Coal-Measure Shale, Hedong Coalfield

Abstract

Adsorbed gas is one of the crucial occurrences in shale gas reservoirs; thus, it is of great significance to ascertain the adsorption capacity of shale and the adsorption characteristics of CH4. In this investigation, the Taiyuan–Shanxi Formations’ coal-measure shale gas reservoir of the Carboniferous–Permian era in the Hedong Coalfield was treated as the research target. Our results exhibit that the shale samples were characterized by a high total organic carbon (TOC) and over to high-over maturity, with an average TOC of 2.45% and average Ro of 2.59%. The mineral composition was dominated by clay (62% on average) and quartz (22.45% on average), and clay was mainly composed of kaolinite and illite. The Langmuir model showed a perfect fitting degree to the experimental data: VL was in the range of 0.01 cm3/g to 0.77 cm3/g and PL was in the range of 0.23–8.58 MPa. In addition, the fitting degree depicted a linear negative correlation versus TOC, while mineral composition did not exhibit a significant effect on the fitting degree, which was caused by the complex pore structure of organic matter, and the applicability of the monolayer adsorption theory was lower than that of CH4 adsorption on the mineral’s pore surface. An apparent linear positive correlation of VL versus the TOC value was recorded; furthermore, the normalized VL increased with the growth of the total content of clay mineral (TCCM), decreased with the growth of the total content of brittle mineral (TCBM), while there was no obvious correlation of normalized VL versus kaolinite, illite and quartz content. The huge amount of micropores and complex internal structure led to organic matter possessing a strong adsorption capacity for CH4, and clay minerals also promoted adsorption due to the development of interlayer pores and intergranular pores.

Review of the Effect of Diagenetic Evolution of Shale Reservoir on the Pore Structure and Adsorption Capacity of Clay Minerals

“…V L is used to characterize the theoretical saturated adsorption capacity of CH 4 in unit mass shale. 13,134,135 The contribution of clay minerals to the extent of CH 4 adsorption at each stage is discussed respectively, and the effects of diagenetic stages and diagenesis on clay pore structure and adsorption capacity are clarified.…”

“…In period B of the middle diagenetic stage and the late diagenetic stage (Figure 3c−e), no obvious correlation is recorded between V L and the clay content (R 2 = 0.0102, 0.0202, and 0.0002, respectively), which show that at these stages, the clay content does not have a clear influence on the adsorption capacity of CH 4 in shale, and the pore structure and adsorption capacity of shale are dominated by organic matter. 13,21,93 This is interpreted by the strong mechanical compaction and chemical compaction with diagenetic evolution, whereby the transformation of smectite to Illite is completed, which represents the attenuation process of interlayer water and interlayer pores in clay minerals. The primary intergranular pores are lost at this stage, and only a small number of intragranular pores and dissolution pores at the nanoscale occurred, so the adsorption capacity of CH 4 decreases dramatically.…”

“…22−24 Comparatively, the contribution of brittle minerals in shale to pore structure and adsorption capacity is weak, although certain dissolution pores are found in feldspar and others. 13,20 Bhowmik and Dutta 25 concluded that CH 4 in clay minerals occupies mainly the widely developed micropores (pores with diameter less than 2 nm in the classification of International Union of Pure Application and Chemistry). Wang et al 26 and Xie et al 27 energy and their pore volume and specific surface area contribute significantly to the adsorption capacity of shale.…”

“…However, only a few countries such as United States, Canada, and China have successfully realized large-scale commercial exploitation. Due to the limitation of accumulation mechanism and development technology, most countries are still in the exploration process. − Shale gas occurs mainly in pores and fractures of reservoirs, in adsorbed and free states, of which adsorbed gas accounts for 20%–85%. − The efficient desorption, migration, diffusion, and production of adsorbed gas is an important aspect to enhance shale gas recovery. − Organic matter structure and pores are regarded as the primary occurrence sites of shale gas, in which total organic matter content (TOC) and thermal maturity control the CH 4 storage capacity of the reservoirs. − As another significant carrier for CH 4 adsorption, clay content also presents an apparent impact on shale gas content. − Shale is mainly composed of clay minerals. Clay content is generally higher than 20% in commercial shale gas reservoirs, and sometimes as high as 80%. − Clays are layered porous inorganic minerals with pores developed between crystal layers, inside minerals, and among particles. − The size, morphology, specific surface area, surface adsorption potential, and roughness of pores determine the adsorption capacity of clay minerals for CH 4 . − Comparatively, the contribution of brittle minerals in shale to pore structure and adsorption capacity is weak, although certain dissolution pores are found in feldspar and others. , …”

Gas Migration Patterns with Different Borehole Sizes in Underground Coal Seams: Numerical Simulations and Field Observations

“…The widely used model that describes this diffusion process is Fick's diffusion law. The gas concentration in the coal matrix is the gas molecules' moving driving force [14,15]. In the center of the coal matrix, the gas concentration is high.…”