Abstract
The fluids in coal reservoirs mainly consist of different gases and liquids, which show different physical properties, occurrence behaviors, and transport characteristics in the pore-fracture system of coal. In this study, the basic characteristics of fluids in coal reservoirs are firstly reviewed, consisting of coalbed methane (CBM) components and physical properties of CBM/coalbed water. The complex pore-fracture system mainly provides the enrichment space and flow path for fluids, which have been qualitatively and quantitatively characterized by various methods in recent years. Subsequently, this study has summarized CBM adsorption/desorption behaviors and models, the CBM diffusion-seepage process and models, and gas-water two-phase flow characteristics of coal reservoirs. Reviewed studies also include the effects of internal factors (such as coal metamorphism, petrographic constituents, macroscopic types, and pore structure) and external factors (such as pressure, temperature, and moisture content) on CBM adsorption/desorption and diffusion behaviors, and the relationship between three main effects (effective stress, gas slippage effect, and coal matrix shrinkage effect) and the CBM seepage process. Moreover, we also discuss in depth the implication of fluid occurrence and transport characteristics in coal reservoirs for CBM production. This review is aimed at proposing some potential research directions in future studies, which mainly includes the control mechanism of the microscopic dynamics of fluids on CBM enrichment/storage; enhancing CBM desorption/seepage rate; and the synergistic effect of multiple spaces, multilevel flow fields, and multiphase flow in coal reservoirs. From this review, we have a deeper understanding of the occurrence and transport characteristics of fluids in pore-fracture structures of coal and the implication of fluid performance for CBM production. The findings of this study can help towards a better understanding of gas-water production principles in coal reservoirs and enhancing CBM recovery.
Highlights
Coal is one of the most widely available energy sources with a huge world consumption, which is formed from the remains of ancient organisms through deep burial and coalification under the conditions of incredible heat and pressure (Figure 1(a)) [1,2,3]
Since 2002, a breakthrough of coalbed methane (CBM) development has been made in the medium–high-rank coal reservoirs, and several large-scale CBM fields have been built in the southern Qinshui Basin and eastern margin of Ordos Basin, China [12,13,14,15]
During the CBM production process, the reservoir pressure gradually decreases as the water in the coalbed is drained, and CBM desorbs from the pore surface and diffuses to the cleat/fracture system accompanied by an increase in effective stress and coal matrix shrinkage as gas desorbs [5, 22]
Summary
Coal is one of the most widely available energy sources with a huge world consumption, which is formed from the remains of ancient organisms through deep burial and coalification under the conditions of incredible heat and pressure (Figure 1(a)) [1,2,3]. During the process of coalification, complex physical and chemical changes occur in coal-forming materials, which demonstrates the decrease of volatile fraction and water content, the increase of calorific value and fixed carbon content, and the generation of fluids accompanied with methane as main components in coals (Figure 1(b)) [16, 17]. During the CBM production process, the reservoir pressure gradually decreases as the water in the coalbed is drained, and CBM desorbs from the pore surface and diffuses to the cleat/fracture system accompanied by an increase in effective stress and coal matrix shrinkage as gas desorbs [5, 22]. The occurrence and flow characteristics of fluids (CBM and water) in the pore-fracture system of coal reservoirs have a significant implication for understanding the CBM production characteristics and enhancing CBM recovery. The effects of fluid performance on CBM production characteristics have been thoroughly summarized
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