The relative permeability of coal to gas and water is an essential parameter for characterizing coalbed methane (CBM) reservoirs and predicting coal seam gas production, particularly in numerical simulations. Although a variety of studies related to the relative permeability of coals have been conducted, the results hardly meet the needs of practical engineering applications. To track the dynamic development of relative permeability measurements in the laboratory, discover the deficiencies, and discuss further work in this field, this paper investigates the relative permeability measurement preparation work and laboratory methods and summarizes the development of techniques used to determine the water saturation during experimentation. The previously determined relative permeability curves are also assembled and classified according to coal rank and the absolute permeability. It is found that the general operations in the relative permeability measurement process are still not standardized. The techniques applied to determine the water saturation of coal in experiments have been refined to some extent, but no optimal technique has been recognized yet. New techniques, such as the incorporation of high-precision differential pressure gauges, can be used to determine the water production during relative permeability measurement. In addition, the existing relative permeability data are limited, and no study has focused on supercritical carbon dioxide-water and mixed gas (methane and carbon dioxide)-water relative permeability measurements. To meet the requirements of actual projects, further research on this topic must be conducted.

The permeability of coal is an indispensable parameter for predicting the coalbed methane (CBM) and enhanced CBM (ECBM) production. Considering the low permeability characteristics of coal, the permeability is usually measured by the transient technique in the laboratory. Normally, it is assumed that the calculated permeability will not greatly vary if the pulse pressure applied in the experiment is small (less than 10% of pore pressure) and previous studies have not focused on the effect of the pulse pressure on the measurement permeability. However, for sorptive rock, such as coals and shales, the sorption effect may cause different measurement results under different pulse pressures. In this study, both nonadsorbing gas (helium) and adsorbing gas (carbon dioxide) were used to investigate the adsorption effect on the gas permeability of coal measurement with the pulse-decay technique. A series of experiments under different pore pressures and pulse pressures was performed, and the carbon dioxide permeability was calculated by both Cui et al.’s and Jones’ methods. The results show that the carbon dioxide permeability calculated by Jones’ method was underestimated because the adsorption effect was not considered. In addition, by comparing the helium and carbon dioxide permeabilities under different pulse pressures, we found that the carbon dioxide permeability of coal was more sensitive to the pulse pressure due to the adsorption effect. Thus, to obtain the accurate permeability of coal, the effect of adsorption should be considered when measuring the permeability of adsorptive media with adsorbing gas by the transient technique, and more effort is required to eliminate the effect of the pulse pressure on the measured permeability.

The relative permeability is essential for understanding porous media's gas and water seepage characteristics and establishing production schedules in practical engineering applications. However, the movable water is too small to be detected for ultra-low permeability rocks, and it is difficult to determine the water saturation in the relative permeability measurement accurately. In this study, a differential pressure transducer (DPT) was applied to a self-developed apparatus to quantify displaced water precisely. Results indicate that (1) Both the permeability and the relative permeability measurement results show high stability in repeatability tests with the application of DPT. (2) The final cumulative water flow data measured by the DPT is reliable. The relative error of the electronic balance and DPT value was less than 4%. (3) This self-developed instrument can obtain the relative permeability curve for ultra-low permeability rocks, such as tight sandstone and anthracite coal. Although there are limitations, this technique provides an economical and reliable pathway for studying the seepage characteristics of the gas and water in ultra-low permeability rocks.

Water saturation, displacement pressure, and gas type are essential factors affecting the seepage characteristic of fluid in coal due to the swelling induced by gas adsorption. However, the understanding of the seepage behavior of water, CH 4 , and CO 2 in coal seams is still poor owing to the lack of research. In this work, a series of gas permeability and gas−water relative permeability experiments were conducted by our self-developed device. The results can be summarized as follows: (1) with the increase in water saturation, the gas permeability decreased rapidly at first and then reduced sharply again after a period of slow decline; (2) the two-phase flow span showed no significant difference for He−water and CH 4 −water systems under different displacement pressures, but it increased for CO 2 -water system. The different behaviors of the two-phase flow span suggested that the effect of displacement pressure on relative permeability properties is dependent on the gas type. (3) The CO 2 −water relative permeability showed a sizeable two-phase flow span compared to the He−water or CH 4 −water system under the same displacement pressure, and the gas relative permeability decreased obviously in the following sequence: He > CH 4 > CO 2 . The concept of relative permeability surface was proposed based on the experimental results. A parametric interpolation method was applied to determine the relative permeability surface, which could be used to accurately describe the flow characteristics of fluids in a mixed gas−water system.