A study on the flowability of gas displacing water in low-permeability coal reservoir based on NMR technology

Abstract

Flowability of gas and water through low-permeability coal plays crucial roles in coalbed methane (CBM) recovery from coal reservoirs. To better understand this phenomenon, experiments examining the displacement of water by gas under different displacement pressures were systematically carried out based on nuclear magnetic resonance (NMR) technology using low-permeability coal samples of medium-high coal rank from Yunnan and Guizhou, China. The results reveal that both the residual water content (W_r) and residual water saturation (S_r) of coal gradually decrease as the displacement pressure (P) decreases. When P is 0–2 MPa, the decline rates of W_r and S_r are fastest, beyond which they slow down gradually. Coal samples with higher permeability exhibit higher water flowability and larger decreases in W_r and S_r. Compared with medium-rank coal, high-rank coal shows weaker fluidity and a higher proportion of irreducible water. The relationship between P and the cumulative displaced water content (W_c) can be described by a Langmuir-like equation, W_c = W_LP/(P_L + P), showing an increase in W_c in coal with an increase in P. In the low-pressure stage from 0 to 2 MPa, W_c increases most rapidly, while in the high-pressure stage (P>2 MPa), W_c tends to be stable. The minimum pore diameter (d') at which water can be displaced under different displacement pressures was also calibrated. The d' value decreases as P increases in a power relationship; i.e., d' the coal gradually decreases with the gradual increase in P. Furthermore, the d' values of most of the coal samples are close to 20 nm under a P of 10 MPa.

Keywords

coalbed methane / low-permeability coal reservoir / NMR / gas displacing water / flowability / pore size

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