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Frontiers of Earth Science

ISSN 2095-0195

ISSN 2095-0209(Online)

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Front. Earth Sci.    2020, Vol. 14 Issue (4) : 673-683    https://doi.org/10.1007/s11707-020-0837-x
RESEARCH ARTICLE
A study on the flowability of gas displacing water in low-permeability coal reservoir based on NMR technology
Minfang YANG1, Zhaobiao YANG2,3(), 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
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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 (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 low-pressure 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.
Keywords coalbed methane      low-permeability coal reservoir      NMR      gas displacing water      flowability      pore size     
Corresponding Author(s): Zhaobiao YANG   
Online First Date: 17 November 2020    Issue Date: 08 January 2021
 Cite this article:   
Minfang YANG,Zhaobiao YANG,Bin SUN, et al. A study on the flowability of gas displacing water in low-permeability coal reservoir based on NMR technology[J]. Front. Earth Sci., 2020, 14(4): 673-683.
 URL:  
https://academic.hep.com.cn/fesci/EN/10.1007/s11707-020-0837-x
https://academic.hep.com.cn/fesci/EN/Y2020/V14/I4/673
Fig.1  Diagram of gas displacing water in a coal core with NMR technology.
Sample No. Coal syncline Ro,max/% Proximate analysis/% Maceral composition/%
Mad Ad Cd Vitrinite Inertinite Exinite
YL1 Panguan 0.86 0.89 28.78 46.82 70.79 20.78 8.43
YCK Enhong 0.93 0.52 23.36 50.77 72.55 21.57 5.88
YL2 Panguan 0.98 0.88 26.54 44.86 69.28 27.11 3.61
DHS Laochang 2.14 0.92 15.91 73.01 89.38 10.62 0
LJ Zhuzang 2.38 1.48 27.67 56.25 88.15 11.85 0
Tab.1  Coal rank, maceral composition, and proximate analysis of the selected coal samples (M: moisture, A: ash yield; C: fixed carbon; ad: air-dried basis; d:dried basis)
Sample No. Length
/cm		 Diameter/cm fHe /% K/mD Saturation water /g Remaining water in different displacement pressure/g
0.5
MPa		 1 MPa 2 MPa 4 MPa 6 MPa 8 MPa 10 MPa
YL1 5.077 2.517 3.27 0.023 0.447 0.433 0.358 0.341 0.331 0.329 0.324 0.320
YCK 5.150 2.514 4.96 0.402 0.736 0.328 0.287 0.225 0.246 0.234 0.241 0.249
YL2 5.064 2.528 6.15 0.081 0.686 0.486 0.424 0.312 0.263 0.252 0.219 0.198
DHS 5.012 2.527 5.57 0.011 1.196 1.014 0.968 0.933 0.933 0.924 0.896 0.842
LJ 5.014 2.482 5.43 0.045 0.986 0.826 0.81 0.77 0.769 0.753 0.715 0.714
Tab.2  Basic properties and experimental results of different displacement pressures of coal samples.
Sample No. Ro,max/% Left Peak value/ms Average Pore diameter value/nm Transfer formula ρ2
YL1 0.86 0.244 30 d = 122.95T2 20.49
YCK 0.93 0.198 19 d = 95.96T2 15.99
YL2 0.98 0.198 34 d = 171.72T2 28.62
DHS 2.14 0.425 16 d = 37.65T2 6.27
LJ 2.38 1.047 14 d = 13.37T2 2.23
Tab.3  Conversion equations of T2 and d
Fig.2  T2 spectrum of coal samples after water saturation under vacuum.
Fig.3  Pore diameter spectrum of coal samples.
Fig.4  T2 spectrum of coal samples after the water displacement by gas experiment under different pressures.
Fig.5  Water content and water saturation of coal samples at different displacement pressures.
Fig.6  Correlation between the cumulative water weight displaced and the displacement pressure.
Fig.7  Correlation between P/Wc and P.
Coal number fHe/% K/mD WL/g PL/MPa Equation
YL1 3.27 0.023 0.15 1.64 Wc = 0.15P/(P+ 1.64)
YCK 4.96 0.402 0.49 0.02 Wc = 0.49P/(P+ 0.02)
YL2 6.15 0.081 0.51 0.69 Wc = 0.51P/(P+ 0.69)
DHS 5.57 0.011 0.34 0.61 Wc = 0.34P/(P+ 0.61)
LJ 5.43 0.045 0.28 0.55 Wc = 0.28P/(P+ 0.55)
Tab.4  P~Wc equation of coal samples
Fig.8  Calibrated d' corresponding to different displacement pressures of sample YCK.
Fig.9  d' corresponding to different displacement pressures of the coal samples.
Sample No. d' in different Displacement pressure/nm Fitting formula
0.5 MPa 1 MPa 2 MPa 4 MPa 6 MPa 8 MPa 10 MPa
YL1 8296 1680 1300 1300 1680 1200 1100 d' = 2971P-0.48(R2 = 0.60)
YCK 327 76 31 25 23 20 20 d' = 100.9P-0.84(R2 = 0.85)
YL2 4000 1900 773 413 254 84 83 d' = 1868P-1.29(R2 = 0.97)
DHS _ 3854 1180 389 48 41 35 d' = 4672P-2.21(R2 = 0.96)
LJ 42.52 32 29 27 25 25 25 d' = 34.46P-0.16(R2 = 0.90)
Tab.5  d' corresponding to different displacement pressures in coal samples and the fitting formula
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