Pore-Fractures of Coalbed Methane Reservoir Restricted by Coal Facies in Sangjiang-Muling Coal-Bearing Basins, Northeast China

Abstract

The pore-fractures network plays a key role in coalbed methane (CBM) accumulation and production, while the impacts of coal facies on the pore-fractures network performance are still poorly understood. In this work, the research on the pore-fracture occurrence of 38 collected coals from Sangjiang-Muling coal-bearing basins with multiple techniques, including mercury intrusion porosimetry (MIP), micro-organic quantitative analysis, and optic microscopy, and its variation controlling of coal face were studied. The MIP curves of 38 selected coals, indicating pore structures, were subdivided into three typical types, including type I of predominant micropores, type Ⅱ of predominant micropores and macropores with good connectivity, and type Ⅲ of predominant micropores and macropores with poor connectivity. For coal facies, three various coal facies were distinguished, including lake shore coastal wet forest swamp, the upper delta plain wet forest swamp, tidal flat wet forest swamp using Q-cluster analysis and tissue preservation index–gelification index (TPI-GI), and wood index–groundwater influence index (WI-GWI). The results show a positive relationship between tissue preservation index (TPI), wood index (WI), and mesopores (102 nm–103 nm), and a negative relationship between TPI, WI, and macropores/fractures. In addition, groundwater level fluctuations can control the development of type C and D fractures, and the frequency of type C and D fractures show an ascending trend with increasing groundwater index (GWI), which may be caused by the mineral hydration of the coal. Finally, from the perspective of the pore-fractures occurrence in CBM reservoirs, the wet forest swamp of upper delta plain is considered to be the optimization areas for Sangjiang-Muling coal-bearing basins by a comparative study of various coal facies.

Coal Facies and Its Effects on Pore Characteristics of the Late Permian Longtan Coal, Western Guizhou, China

“…paleoenvironment governs the degree and feature of pore development by affecting the coal lithotype and macerals composition [8,9,12,13,16,32], and some macerals are porous (Figure 6). Previous studies indicate that there is close connection between coal facies parameters and different sizes pores of medium-rank coal [10,19], while there are no data to support the correlation of high-rank coal and the dual control from coal facies and coal rank to pore In this study, mercury intrusion porosimetry was conducted to recognize the pore features of various coal facies and coal ranks. As shown in Figure 7, moist forest peat swamp facies samples have much higher mercury volume saturation than shallow-water covered forest peat swamp facies samples and dry forest peat swamp facies samples, implying that the degree of pore development follows the following order: moist forest peat swamp facies coal > shallow-water covered forest peat swamp facies coal > dry forest peat swamp facies coal.…”

“…According to macerals compositions, coal facies can be subdivided into five types, and they are shallow-water covered forest peat moor, wetland herbaceous moor, shallow-water covered wetland herbaceous moor, moist forest peat moor, and low peat moor coal facies [13]. The composition and spatial distribution of coal macerals during peatification stage are important factors leading to interbedded, intrabedded, and planar heterogeneous of coal reservoirs, and coal facies has significant controls to reservoir pore structure [11,[16][17][18][19][20][21][22]. However, the effect of coal facies on pore characteristics of coal reservoirs and its mechanism are still unclear.…”

Fluid Performance in Coal Reservoirs: A Comprehensive Review

“…Characterization of Pore-Fracture Structure. For over decades, characterization techniques of the pore-fracture structure have evolved from the observation of orientation/spacing to the understanding of spacing, size, orientation, connectivity, and porosity for both pores and fractures/cleats [35,36]. The quantitative characterization and quantitative analysis techniques of the pore-fracture structure have been gradually developed as well as widely used [37], which are divided into two types: the fluid injection method and the nonfluid injection method (Figure 4).…”

Analysis of Gas Content and Permeability Change Pattern of a Coal Reservoir in the Tectonic Positions Based on a THM Coupled Model

“…Therefore, it is of great theoretical significance and engineering application to study the characteristics and changes of coalbed methane (CBM) distribution under geotectonic settings. The published research results show that the "generation, accumulation, and preservation" of CBM is not only closely related to the ground stress, coal seam buried depth, and paleogeothermal and gas generation history under the control of tectonic evolution [1][2][3][4][5][6] but also influenced by the joint effect of formation pressure, lithology of surrounding rock, and microstructural characteristics of a coal reservoir [7][8][9][10][11][12][13], which means that when the environmental conditions such as temperature, stress, and seepage of coal reservoirs change due to tectonism, the gas content and its distribution characteristics in coal seams will change. Therefore, the deposition, generation, and preservation of CBM under the tectonism are the key criteria for CBM enrichment and reservoir formation, which is an important guideline for the efficient development of CBM.…”