Matrix compressibility and pore properties (pore size distribution) of a rank range of coals was investigated using mercury intrusion porosimetry (MIP) on coal cores with the pore size distribution also being determined using low temperature at 77 K nitrogen adsorption/desorption isotherms for crushed samples. The coal matrix compressibility is significant when the pressure of MIP is from 0.0074–35 MPa. Mathematical models were developed (based on MIP and nitrogen adsorption/desorption isotherms) to establish the porosity/pore size distribution relationships with matrix compressibility. For coal ranks, the matrix compressibility was between 0.24 × 10−4 to 13.56 × 10−4 MPa−1, and had a negative exponential relationship with the vitrinite reflectance (Ro,m%). Lignites have the maximum matrix compressibility due to their structural open structure having limitied compaction during coalification. In addition to the pore structure relationship the composition, moisture, and ash yields impacts on compressibility were also examined. Inertinite-rich coals however had a low matrix compressibility across the rank range, which may be due to the interinhibitive relationships between the mesopores, macropores and minerals. The wetting action of high moisture (water molecules) weakens the link between the coal particles of the lignites and the subbituminous coals, which causes abnormally high compressibility. Observations here relate to hydrofracturing or CO2 injection behaviors during enhancing coalbed methane (CBM) recovery.
Bibliographical noteThis research was funded by the National Natural Science Fund (grant nos. 41830427, 41602170 and 41772160), the National Major Research Program for Science and Technology of China (grant no. 2016ZX05043-001), the Key Research and DevelopmentProjects of the Xinjiang Uygur Autonomous Region (grant no. 2017B03019-01) and the Research Program for Excellent Doctoral Dissertation Supervisor of Beijing (grant no. YB20101141501).
- Matrix compressibility
- Mercury intrusion porosimetry
- Low-temperature nitrogen adsorption