A study on the flowability of gas displacing water in low-permeability coal reservoir based on NMR technology
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RESEARCH ARTICLE
A study on the flowability of gas displacing water in low-permeability coal reservoir based on NMR technology Minfang YANG1, Zhaobiao YANG (✉)2,3, Bin SUN1, Zhengguang ZHANG2,3, Honglin LIU1, Junlong ZHAO2,3 1 Department of Unconventionals, Research Institute of Petroleum Exploration and Development, PetroChina, Langfang 065007, China 2 Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process (Ministry of Education), China University of Mining and Technology, Xuzhou 221008, China 3 School of Resource and Geosciences, China University of Mining and Technology, Xuzhou 221116, China
© Higher Education Press 2020
Abstract Flowability of gas and water through lowpermeability 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 (Wr) and residual water saturation (Sr) of coal gradually decrease as the displacement pressure (P) decreases. When P is 0–2 MPa, the decline rates of Wr and Sr are fastest, beyond which they slow down gradually. Coal samples with higher permeability exhibit higher water flowability and larger decreases in Wr and Sr. 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 (Wc) can be described by a Langmuir-like equation, Wc = WLP/(PL + P), showing an increase in Wc in coal with an increase in P. In the lowpressure stage from 0 to 2 MPa, Wc increases most rapidly, while in the high-pressure stage (P > 2 MPa), Wc 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.
Received June 11, 2020; accepted September 28, 2020 E-mail: [email protected]
Keywords coalbed methane, low-permeability coal reservoir, NMR, gas displacing water, flowability, pore size
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Introduction
The flowability of gas and water through coal, particularly in the micropores of coal, plays crucial roles in coalbed methane (CBM) recovery from coal reservoirs. This is even more critical for coal reservoirs with low permeability and medium-high coal ranks (Tao et al., 2019b and 2019c). Western Guizhou and eastern Yunnan are important coal and CBM resource areas in southern China, where the Upper Permian CBM resources account for approximately 10% of the total CBM in China (Strategy Research Center of Oil and Gas Resources Department, 2006). However, the coal reservoirs a
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