Insights into matrix compressibility of coals by mercury intrusion porosimetry and N2 adsorption

doi:10.1016/j.coal.2018.11.007

International Journal of Coal Geology

Volume 200, 1 December 2018, Pages 199-212

https://doi.org/10.1016/j.coal.2018.11.007 Get rights and content

Highlights

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Coal matrix compressibility shows an exponential decrease with increasing coal rank.
•
An antithetical relation between inertinite and coal matrix compressibility.
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The wetting action of water molecules weakens the link between coal particles.
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Mineral appearance has significant impact on coal matrix compressibility.

Abstract

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 (R_o,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.

Introduction

The decrease in fluid pressure during coalbed methane (CBM) production results in volume changes of both reservoir fluids and coal reservoirs (Liu and Harpalani, 2014). The volumetric response to these pressure changes (Palmer and Mansoori, 1998; Pan et al., 2010) and/or stress variation (Li et al., 2013) influences CBM production behavior (Clarkson and Qanbari, 2015). Thus, during drilling, production or injection of fluids during enhanced CBM production, and/or CO₂ sequestration there is a dynamic coal response. Unfortunately, volumetric data changes are not typically available due to the complexity of coal reservoirs (Liu and Harpalani, 2014), it can however be estimated by the matrix compressibility.

Mercury intrusion porosimetry (MIP) is widely used for determining the pore size distributionof porous materials and has applicability to conventional and unconventional reservoirs (e.g., tight sand, shales, and coals) (Labani et al., 2013; Lan et al., 2017; Liu et al., 2016; Song et al., 2018). However, during the experiment the coal matrix will be compressed and potentially damaged (Friesen and Mikula, 1988; Harpalani, 1999; Spitzer, 1981; Suuberg et al., 1995; Toda and Toyoda, 1972; van Krevelen, 1981). This volume reductions impacts the reservoir permeability and hense the CBM production (Meng et al., 2011). The apparent pore volume increase in MIP at >10 MPa is due to the coal compressibility (Toda and Toyoda, 1972; Cai et al., 2013; Guo et al., 2014). Multiple techniques have established that coal can have a range of pore sizes and a complex pore size distribution (Gan and Nandi, 1972) composed of macropores (>50 nm), mesopores (2–50 nm) and micropores (<2 nm) by IUPAC (1982). There is a rank (and maceral/lithotype) influence on this pore size distribution and hence there is an expectation that the compress extent is rank dependent.

Coal matrix deformation can be divided into elastic deformation related to mechanical decompression of the solid matrix, and non-elastic swelling induced by adsorption (Liu and Harpalani, 2014). As mercury is non-absorbing the non-elastic swelling can be neglected. Both experimental and theoretical methods have been developed to understand the compressibility of coals (Liu et al., 2015). As the applied mercury pressure increases, synchronous pore filling and compression may occur. Thus, determining the matrix compressibility of coals ranks capturing the effects of coal compressibility across the pressure range should be examined. Here, we suggest a theoretical approach to evaluate the compressibility of a coal rank range and establish the interrelationships between coal composition, moistures, ash yields, and pore structure to coal matrix compressibility. The distinguishing features of this work include 1) the evaluation of compressibility for different rank coals, 2) pore structure assessment using MIP and the N₂ adsorption isotherm at 77 K, and 3) factors affecting the coal matrix compressibility for different rank coals.

Section snippets

Coal sampling, and analyses

Here 39 coal blocks (30 × 30 × 30 cm³), were selected from seven coal basins capturing: 7 low-rank (LRC, 0.49–0.65% R_o,m), 27 medium-rank (MRC, 0.66–1.90% R_o,m) and 5 high-rank coals (HRC, 2.00–2.95% R_o,m). Maximum vitrinite reflectance (R_o,m) (immersion in oil) and maceral composition were conducted with a microscope photometer (MPV-III, Leitz Company of Germany) following the GB/T 6948–1998 at China University of Geosciences at Beijing (CUGB). The R_o,m varies from 0.49 to 2.95%, as listed in

Coal characteristics

The 39 coal samples were divided into three coal ranks: low-rank coals (R_{o, m} < 0.65%), medium-rank coals (0.65% < R_{o, m} < 1.9%) and high-rank coals (R_{o, m} > 1.9%). These coals vary markedly in their maceral and minerals contents, which represent a wide range in composition, as shown in Table 1. The volume contents for vitrinite, inertinite and exinite are in the range of 11–92.04%, 0.6–78.08% and 0–18.2%, respectively. Exinite disappears when the maximum vitrinite reflectance is over 1.19%.

Effects of coal rank on coal matrix compressibility

Coal matrix compressibility shows a negative exponential relationship as coal rank increases, as shown in Fig. 4(a). The coal matrix compressibility values of low-rank coals emerge as a rapid decrease from 13.26 × 10⁻⁴ MPa⁻¹ to 1.44 × 10⁻⁴ MPa⁻¹ as coal rank increases from 0.49% to 0.65% R_o,m as listed in Table 2. The average coal matrix compressibility for low-rank coals is 6.93 × 10⁻⁴ MPa⁻¹. For medium-rank coals (0.65%–1.9% R_o,m), the coal matrix compressibility decreases slowly from

Conclusions

Mercury intrusion porosimetry can be used as an effective means to evaluate pore structure, provided the matrix compressibility of coals associated with this method can be evaluated through N₂ adsorption/desorption at 77 K. Factors that affect coal matrix compressibility were discussed herein. We determined that the coal matrix compressibility varies from 0.24 × 10⁻⁴ MPa⁻¹ to 13.56 × 10⁻⁴ MPa⁻¹ as coal rank changes. The following conclusions are made:

1)
Coal matrix compressibility follows an

Acknowledgements

This research was funded by the National Natural Science Foundation of China (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 Development Projects of the Xinjiang Uygur Autonomous Region (grant no. 2017B03019-01) and the Research Program for Excellent Doctoral Dissertation Supervisor of Beijing (grant no. YB20101141501).

Nomenclature

P_c: The pressure at which mercury enters pores (MPa).
σ: The surface tension (N/m).
θ: The wetting contact angle (°).
r_c: The capillary radius at the corresponding pressure (μm).
C_m: Coal matrix compressibility (MPa⁻¹).
V_m: The coal matrix volume (cm³/g).
dV_m/dP: The volume change of coal matrix as a function of pressure.
ΔV_mercury: The observed increase in mercury volume (cm³/g).
ΔV_pore: Pore filling volume (cm³/g).
ΔV_compaction: Solid compression volume (cm³/g).

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Insights into matrix compressibility of coals by mercury intrusion porosimetry and N2 adsorption

Highlights

Abstract

Introduction

Section snippets

Coal sampling, and analyses

Coal characteristics

Effects of coal rank on coal matrix compressibility

Conclusions

Acknowledgements

Nomenclature

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Insights into matrix compressibility of coals by mercury intrusion porosimetry and N₂ adsorption