Laboratory Experiment on Coalbed-Methane Desorption Influenced by Water Injection and Temperature

Abstract

Summary Traditional coalbed-methane (CBM) exploitation technologies, including ordinary drills, enhanced drills, water jets, and hydrofracture in the coalbed, are widely used in modern coal mining and involve high-pressure water. Because the CBM-desorption capacity decreases when using high-pressure water, the CBM output efficiency is lower than expected. To investigate the decrease in CBM desorption after water injection and the increase in CBM output after temperature is raised, an experimental system was built to simulate ideal conditions for one water-injection well per production well. The apparatus comprised a gas-liquid-injection system, a coal-sample container, a temperature-control device, a measurement system, and other auxiliary devices. Experiments on the effects of water injection and temperature on CBM desorption were carried out. The results revealed that the CBM-desorption capacity mainly depends on the water-injection pressure at constant temperature. The capacity decreased by at least 50% at less 2 MPa and became stable if the water pressure exceeded 8 MPa. Furthermore, the desorption capacity rapidly increased when the temperature was increased after water injection. Desorption reached a plateau once the temperature reached the boiling point of water, and the percentage desorption (PD) at 90°C was greater than that for free broken coal. The results demonstrate that heated hydrofracture is an effective technology with good efficiency for CBM exploitation.

Mechanism of Liquid-Phase Adsorption and Desorption in Coalbed Methane Systems - A New Insight into an Old Problem

“…When the pore radius is smaller, the capillary force is larger, and the hysteresis is more obvious. Similarly, Zhao (2011) [21] has carried out several experiments to study the methane desorption from coals Љconfined by waterЉ, his results show that desorption rate is related to water content, and for coals with high water content, it will be less than 13% due to the effect of capillary force, which coincides with our analysis. Therefore, in CBM reservoirs, the influence of capillary pressure on desorption should not be ignored.…”

“…(1) for three sets of experiments is only 3.36%, 4.98% and 4.21%, respectively (Fig.4). Actually, a similar experiment has been carried out by Zhao(2011) [21] , in his study, methane was firstly adsorbed into coal pore systems at a certain equilibrium pressure, and then liquid water was injected into the container with coal and adorbed gas until it was full of water. Finally, methane desorption from the aqueous environment was performed.…”

“…(18) and Eq. (19), the critical Gibbs free energy in capillary nucleation can be expressed by (20) If we assume the methane for capillary nucleation is ideal gas, the methane molar quantities of stable bubble or nucleus (bubble radius reach r*) can be expressed by (21) Where, P c is pressure difference between water and gas bubble, which can be calculated by Young-Laplace equation, MPa; and detailed derivation about V bubble see in appendix.…”

Experimental Study on Methane Desorption from Lumpy Coal under the Action of Hydraulic and Thermal

“…Thus, if high pressure water has been injected into coalbed, the desorption capacity of methane would be effected due to moisture. (Zhao et al, 2011) Some researchers have engaged in this field in order to improve coalbed permeability and amount of gas output. The two major technologies are hydraulic fracture and thermal injection.…”

“…According to the mechanism of moisture increased in coalbed, many scholars have performed studies on the influence of moisture on methane desorption (Xie et al 2011;Zhang and Sang 2009), but they wet the coal samples before the experiment. Some scholars realized that injecting water would destroy the statement for gas adsorption (Pakowski et al 2011;Xiao and Wang 2011;Zhang et al 2011a;Zhao et al, 2011;Chen et al, 2015), and they designed the ideal experimental process in which water was injected into the dry coal samples under the equilibrium state of gas absorption. All of these results showed that the maximum desorption quantity decreases gradually with the amount of injected water, but they were not considering different water pressure impacted on same or different equilibrium gas adsorption state.…”