Swelling Pressure of Double-Expansive Material and Its Active Support Effect for Coal Seam Gas Drainage Borehole

Zhiming Wang,Yuning Sun

doi:10.1155/2018/8245036

Abstract

To improve coal seam gas drainage performance, we developed a double-expansive (DE) material to seal the borehole. The swelling process of this material includes an initial swelling stage and a secondary swelling stage. We studied the swelling pressure properties of the DE material under four constraint conditions using a self-made swelling test device. Further, the active support effect of the DE material on the borehole was analyzed by simulating borehole stability with COMSOL Multiphysics software. The results exhibit the following: (1) The swelling pressure of the DE material exhibits time-dependent behavior, and the mathematical relationship between the swelling pressure and time can be obtained by nonlinear fitting. (2) The radial swelling potential is principally formed during the secondary swelling stage, providing the main active support on the radial constraint. (3) The active support imposed on the hole wall can prevent the extension of plastic and damage regions around the borehole, for improved stability of the gas drainage borehole. Finally, field tests demonstrate improved gas drainage performance of the borehole sealed by the DE material compared to a conventional sealing material.

Highlights

Coalbed methane (CBM) is an unconventional natural gas and presents hazards in coal mines [1,2,3]
To improve coal seam gas drainage performance, we developed a DE material for borehole sealing. is material is composed of ordinary Portland cement (OPC), mineral powder, a solid expansion agent, bentonite, naphthalene water reducer and aluminum powder. e swelling process of the DE material includes an initial swelling stage affected by the foaming effect of the aluminum powder and a secondary swelling stage induced by the growth and formation of the crystals
The key conclusions are summarized as follows: (1) e swelling test device was used to test the swelling pressure of the DE material under four constraint conditions. e swelling pressure properties of the DE material revealed that (a) the swelling pressure of the DE material is time-dependent during its hydration, and the relationship described here of the swelling pressure and time can describe this timedependent behavior accurately and (b) the radial swelling potential is principally formed during the secondary swelling stage

Summary

Introduction

Coalbed methane (CBM) is an unconventional natural gas and presents hazards in coal mines [1,2,3]. Gas drainage via boreholes in the coal seam is the major strategy used to exploit CBM in underground coal mines in China [6,7,8,9]. Borehole sealing plays a critical role during the gas drainage process, and the quality of this sealing directly influences gas drainage performance. If the borehole is poorly sealed, the air in the tunnels of coal mines can be inhaled into the gas drainage borehole via fractures, which is considered to be an air leakage phenomenon of gas drainage borehole. When the concentration of methane drops to 5%–16%, there are hazards of fire and blasting in the drainage pipe net. Methane can only be utilized directly when its concentration is above 30%, and it is expensive for current technology to utilize methane that has been diluted to a lower concentration [13]

Objectives

Methods

Results

Conclusion