Liquid CO ₂ high‐pressure fracturing of coal seams and gas extraction engineering tests using crossing holes: A case study of Panji Coal Mine No. 3, Huainan, China

Abstract

Summary Owing to the low coal permeability coefficients and poor gas extraction efficiencies associated with the extraction of coal in the Huainan coal mining area, the aim of this study was to design, for the first time, a high‐pressure low‐temperature liquid CO2 (LCO2) pump for engineering tests with respect to the fracturing of coal seams and the improvement of CH4 recovery in China. To reasonably calculate the initiation pressure for the coal seam fracturing as well as the gas flow parameters, theoretical analysis with field testing were integrated in the design. In addition, considering the interim gas extraction standard, the gas extraction equivalent radius based on this standard was calculated. The test results revealed a maximum LCO2 pressure of 20.6 MPa, which surpasses the theoretical maximum pressure (19.5 MPa) required for the fracturing of coal seams. During the entire gas extraction process, the CO2 concentrations in boreholes K1 and K3 decrease exponentially with time, showing attenuation coefficients of 0.0205 and 0.0231, respectively. The attenuation coefficients of the three attenuation stages of the original coal seam gas flow rate were 0.0314, 0.2142, and 0.1198, which were higher than those corresponding to the test area. The CH4 concentration in boreholes K1 and K3 in the test area were both higher than that in the original coal seam, and the maximum CH4 flow rates in these boreholes were 1.31‐ and 1.72‐fold higher than that in the original coal seam, respectively. The comparative analysis of CH4 concentrations and flow rates indirectly showed an improvement in the permeability of the coal seam. Further, a quantitative equivalent gas extraction evaluation method based on LCO2 fracturing of coal seam and gas displacement was proposed. Furthermore, based on a comprehensive evaluation, the effective displacement radius caused by the LCO2 fracturing test was 9 m, with a displacement efficiency of 23.95%, and a displacement volume ratio of 0.21. This method offered the possibility of realizing the quantitative evaluation of the LCO2 injection amount and the gas extraction effect, and provides a basis for optimizing the LCO2 fracturing of coal seams as well as the gas extraction process.

Desorption Characteristics of CH4–C2H6 Mixed Gas in Heavy Hydrocarbon-Rich Coal Seams

“…The fault-structure development in the coalfield is dominated by longitudinal oblique faults. The faults lie parallel to the anticline axis, and the angle between the fault intersection line and coal seam strike is small. , The complex tectonic stress is relatively concentrated because the minefield is located at the turning–overturning end of the anticline. The content of heavy hydrocarbon gas in the coal seam ranges from 5.58 to 17.07% and is higher in the deep part of the mine (<1000 m) than in the shallow part.…”

Permeability-Enhancing Technology through Liquid CO2 Fracturing and Its Application

“…The geostress, gas pressure and gas content in coal seams also constantly grow with the increasing mining depth of coal mines [12][13][14]. Moreover, the mining becomes increasingly difficult due to high gas content and the risk of coal and gas outburst, which presents a great challenge for gas control in mining areas [15][16][17]. Therefore, the use of coal seam anti-reflection measures is of great significance for improvement, resource utilization, the recovery rate of CBM, and coal mine safety production [18][19][20][21].…”

Characteristics of Pressure Relief Gas Extraction in the Protected Layer by Surface Drilling in Huainan

“…e field results show that the technology has a significant effect on coal reservoir reconstruction and coal seam methane recovery, but the range of influence is small. Wen et al [16][17][18][19][20][21][22][23][24] proposed liquid CO 2 permeability enhancement and gas displacement technology in coal mines, carried out several underground engineering tests, and achieved good results. However, these studies had problems such as small test areas and difficulty of CO 2 transportation.…”