Characteristics of the physical parameters and the evolution law of anthracite around the coalification jump: A case of the Jincheng and Guxu mining area, China

Abstract

To investigate the physical properties of anthracite reservoirs with different metamorphic grades (Ro,max), 38 anthracite samples with an Ro,max value between 2.5 and 4.2% from the Guxu and Jincheng mining areas in China were selected to conduct proximate analysis and determine the macerals, pore size distribution, adsorption capacity, in-situ gas content and permeability. The results showed that a coalification jump at Ro,max=3.7% existed in the anthracite stage and greatly influenced the evolution of physical properties. Before the coalification jump point, a series of micropores and transition pores was formed during metamorphism, which increased the specific surface area of the reservoir. This phenomenon further increased the adsorption capacity and gas content. The formation of methane during metamorphism also provided a material basis for the enrichment of methane. After the coalification jump point, a decreased specific surface area was observed due to compaction of micropores and transition pores, thereby reducing the adsorption capacity and gas content. The end of gas generation also caused the gas content to increase with increasing Ro,max. The burial depth (overlying geostress) and metamorphism degree were key factors in controlling the permeability. The change in the metamorphic degree controlled the formation of microfractures, and the burial depth controlled the degree of fracture closure. The physical properties of reservoirs changed notably before and after the coalification jump point of high-rank coals. Therefore, more attention should be paid to coalification in the process of reservoir evaluation and development of high-rank coal reservoirs.

Permeability experiments with overburden pressure of coal measure reservoirs and numerical analysis of its stress sensitivity: A case study of Qinshui Basin, China

“…Coal measures in China are rich in unconventional energy resources, with the amount of coalbed methane (CBM), shale gas, and tight sandstone gas being as high as 60–70 × 10 12 m 3 (Cao et al., 2014). Among these resources, tight sandstone gas, which forms in coal measure strata, features close sources and reservoirs as well as short-distance migration and accumulation, while also boasting of abundant gas sources and effective source–reservoir allocation (Guo et al., 2017; Kang et al., 2019; Li et al., 2012). An in-depth study of the coexistence, accumulation, superimposition, and joint exploitation of the three kinds of unconventional coal measure gases is conducive to reducing the cost of exploration and development, increasing the amount of resources that can be recovered by CBM technology, and effectively improving the single-well utilization rate (Liang et al., 2016).…”