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.

Understanding the mechanical behavior of a reservoir is essential for securing the safety of CO₂ storage projects. In this study, the effect of CO₂-H₂O biphasic fluid on the mechanical behavior and failure characteristics of sandstone was revealed. The preliminary results indicate that CO₂-H₂O biphasic fluid could significantly affect the mechanical behavior of sandstone, including reducing the strength, elastic modulus, Poisson’s ratio, and bulk modulus of sandstone; enhancing the bulk compaction; and postponing the onset dilatancy of sandstone. However, the strength of water-bearing sandstone is weakly increased when CO₂ is injected into the sample due to the capillary forces that develop under the unsaturated conditions. In addition, CO₂-H₂O biphasic fluid notably affects the failure characteristics of sandstone at macro- and microscales. A complex network of cracks is generated at macrostructure scale. The bonds between particles are damaged by CO₂-H₂O biphasic fluid at the microstructure scale.

Relative permeability plays a crucial role in understanding the characteristics of gas and water seepage in porous media and in establishing production schedules in practical engineering applications. However, accurately determining water saturation in the relative permeability measurement is challenging due to the minimal detectable amount of movable water in ultra-low-permeability rocks. This chapter introduces a novel method to determine water production during relative permeability measurement. We developed an apparatus that combines a separator with a high-precision differential pressure transducer (DPT) to measure the variation in water production during the experiment. The repeatability of measurements using this apparatus was tested, and the results demonstrated high stability. In addition, we used this apparatus to investigate the gas−water relative permeability in high-rank coal. The results indicate that the effect of displacement pressure on the relative permeability properties depended on the type of gas. The carbon dioxide–water system exhibited a significantly larger two-phase flow span compared to the helium–water or methane–water system at the same displacement pressure. Moreover, the relative permeability of the carbon dioxide–water system showed a higher sensitivity to the displacement pressure compared to the helium–water and methane–water systems.