Aging characteristic of effective distance on injecting N₂ to displace CH₄ in anisotropic coal seam

ABSTRACT

Gas injection is an effective way to enhance methane drainage in a low permeability coal seam, and the reasonable distance between injection holes and drainage holes depends on effective influence distance of injection gas in a coal seam. For anisotropic permeability of coal seam, the variation of the gas-flow rate from a test hole in the anisotropic coal seam is measured as nitrogen is injected into the injection hole near the test hole. The effective distance regularity is revealed by nitrogen injection in an anisotropic coal seam, and the model of nitrogen injection is established to numerically analyze the aging characteristic of effective influence distance in the anisotropic coal seam. The results show that the permeability in coal seam reduced exponentially with the increase in gas pressure, and it is larger in the direction of parallel to bedding than that in the direction of perpendicular to bedding. The effective distance with nitrogen injection increased exponentially in the direction of parallel/perpendicular to bedding, and the effective distance in the direction of parallel to bedding is bigger than that in the direction of perpendicular to bedding. The optimum distances between injection hole and test hole, with 0.5 MPa of injection gas pressure and 8.36 h of injection gas time, are 2.05 m in parallel to bedding direction, and 1.53 m in perpendicular to bedding direction. This study will have important theoretical and practical significance for a reasonable layout of injection-drainage holes during injecting gas to enhance methane drainage.

KEYWORDS:

• Gas injection
• effective distance
• anisotropic permeability
• enhanced methane drainage
• coal seam

Acknowledgments

We thank the support of Shigang coal mine, and the workers help us to collect the coal sample. We are also grateful to Dr. Wang H., who provided the instrument and equipment to accomplish the tests. We are also indebted to the anonymous reviewers for their comments which have helped improve the manuscript.

Author’s contributions statement

Yue Gaowei conceived the experiment and analyzed the results, J.W YUE and M.M. LI carried out the test, C.L. ZENG simulated the process of gas injection. W.M Liang coordinated the study and helped draft the manuscript. All authors gave final approval for publication.

Competing interests

The authors declare no competing interests.

Additional information

Funding

Financial support came from the National Natural Science Foundation of China (No. 51274090, 41772163), and the program for Innovative Research Team (in Science and Technology) at the University of Henan Province (No. 17IRTSTHN030).