Anisotropic coal permeability estimation by determining cleat compressibility using mercury intrusion porosimetry and stress–strain measurements

https://doi.org/10.1016/j.coal.2019.02.011Get rights and content

Highlights

  • An economical and time effective method to reasonably estimate anisotropic permeability of the coals.

  • Laboratory experiments and field data are not required to fit cleat compressibility using permeability models.

  • Cleat compressibility calculated using stress-strain and mercury intrusion porosimetry measurements.

  • Cleat compressibility validated using Seidle’s model (1992) to compare with measured permeability.

Abstract

This paper presents a novel method to calculate the anisotropic and stress-dependent coal permeability by determining cleat compressibility using the Mercury Intrusion Porosimetry (MIP) and stress–strain measurements. Cleat compressibility is often assumed isotropic and constant in the literature and is usually obtained by numerical fitting to a matchstick permeability model e.g. the model by Seidle et al. (1992) despite the gross simplification of this representation of the coal pore network. This paper provides a method to calculate anisotropic cleat compressibility using only MIP and stress–strain measurements which are easy to conduct, and without permeability information, which is harder to come by, requiring laboratory experiments on the core or through fitting field data. We report the measured stress–strain behaviour of a coal sample, with hydrodynamic loading/unloading over the range 0.5–4.0 MPa, and the permeability in face cleat (kF) and butt cleat (kB) directions using the Triaxial Stress Permeameter (TSR). The stress–strain measurement is used to calculate the anisotropic modulus of elasticity (EF, EB, and EV) in face cleat, butt cleat and bedding plane directions, and cleat compressibility in the face cleat (CfF) and butt cleat (CfB) directions using the fractal dimension analysis with MIP measurement. Finally, the cleat compressibilities are used to calculate the anisotropic coal permeability by Seidle et al. (1992) permeability model and compared with the measured permeability of the coal sample.

Introduction

The anisotropic elastic behaviour of coal has been discussed in coal literature for several decades, for example, Szwilski (1984) reported anisotropic elastic moduli of coal measured in two mutually perpendicular directions, reporting values as high as 9.50 GPa and 3.61 GPa. However, most of the commonly used models to predict permeability in coal (e.g., Palmer and Mansoori (1998), Seidle et al. (1992), and Shi and Durucan (2005)) apply uniform modulus of elasticity, and often also uniform cleat compressibility, using the same value in all directions and neglecting the anisotropic nature of the mechanical properties of the coal. Palmer and Mansoori (1998) found that permeability, as calculated with their model, is strongly dependent on the modulus of elasticity (E), and this was found to lie in the range E = 0.85 GPa to E = 3.0 GPa (E = 1.24 × 105 to 4.35 × 105 psi). In this and many other research papers, the modulus of elasticity and cleat compressibility are assumed to be constant and independent of the effective stress applied to the coal (Liu et al., 2012). Experimental evidence, e.g., as presented by Connell and colleagues (Connell et al., 2016; Zheng et al., 2012) suggests this assumption is not representative of coal behaviour.

Most of the coal permeability models include several key parameters to describe the physical and mechanical properties of the coal, including typically Poisson's ratio (ν), modulus of elasticity (E), porosity (ϕ), swelling and strain coefficients (εL and β), and cleat compressibility (Cf). Commonly, the cleat compressibility is found by regression of the model to measured permeability data (k) (Connell et al., 2016; Peng et al., 2017; Zheng et al., 2012). A more useful approach would be to determine cleat compressibility independently since a primary objective in many cases is to predict permeability, rather than use it as a measured value to extract coal properties. The sensitivity of the permeability to the input cleat compressibility values has been documented for the common models by Zheng et al. (2012).

It has been argued that cleat compressibility is hard to measure directly and that cleat compressibility tests are costly, lengthy, and often yield uncertain results (Seidle et al., 1992); and that it is actually easiest to obtain by fitting to permeability data (Zheng et al., 2012). However, a rigorous experimental program to measure the anisotropic permeability of coal across a relevant range of stress and pore pressure conditions is also a costly exercise.

In this paper, a new method to determine both the anisotropic elastic moduli and cleat compressibility of coal is presented, without the need to make permeability measurements and tested to show that it can be applied to reasonably predict the coal permeability.

Section snippets

Mechanistic approach

The method proceeds by assuming the measured bulk strain of a coal sample is essentially all taken up as cleat/fracture strain since the coal matrix is very stiff and the cleats/fracture comparatively soft. This assumption is supported by the literature which shows the matrix compressibility is three to four orders of magnitude lower than cleat compressibility: hence matrix strain can be assumed negligible under all but the most severe stresses. For example, Guo et al. (2014) reported the

Coal sample preparation

Industrial partner supplied a coal core collected from the ‘Lower Jundah’ formation of Surat Basin at a depth of 178.32 m to 178.50 m. The dimensions of the received core, as shown in Fig. 1(a), were ~180 mm length and 63.5 mm diameter. The core was cut into two pieces each about 90 mm long and then machined into 40 mm cubes as shown in Fig. 1(b). Because this coal is relatively brittle, the cutting broke some edges of the coal cubes, which were rebuilt using plastic bond (Selleys Plasti bond).

Stress–strain (σ–ε) measurement

Fig. 6 shows the strain response of the coal sample across the face cleat (εF), butt cleat (εB), and bedding-plane (εV) directions through 25 hydrodynamic loading-unloading cycles at a loading rate of 0 .1MPa/min from effective stresses of σeff = 0.5 MPa to σeff = 4.0 MPa. The maximum loading stress of 4.0 MPa was selected as this is well below the uniaxial compressive strength of Surat Basin coal (UCS ~18 MPa from Fig. 7 by Minaeian and Rasouli (2011)), well inside the elastic limits, as

A novel method to calculate the cleat compressibility (Cf) using MIP and stress–strain measurements

The anisotropic cleat compressibility of the coal sample in the face cleat (CfF) and the butt cleat (CfB) directions may be determined from the stress–strain data (Fig. 6) together with pore size data from mercury porosimetry (MIP) (Fig. 3). The main idea is to assign the measured bulk strain of the sample (Fig. 6) to the cleat/crack strain only, with the assumption that the coal matrix virtually incompressible as compared to cleats/cracks. This assumption is based on the knowledge that the

Cleat compressibility validation

The permeability of naturally fracture reservoirs can be described by a nine-component permeability tensor (Crosdale et al., 1998; Faiz et al., 2007), commonly simplified for coals (Wang et al., 2009) to include permeability only in mutually perpendicular directions. In this study, these mutually perpendicular directions represent the face cleat and butt cleat directions (i.e. k = [kFkB]T). Permeability changes may thus be used to validate the calculated cleat compressibility. Seidle's

Conclusions

An inexpensive, simple and time effective method to estimate the anisotropic permeability coal and its change with net stress, is outlined based on estimating cleat compressibility from two easy measurements: stress–strain data and mercury intrusion porosimetry. The method provides a straightforward way to estimate cleat compressibility and avoids the normal and much more difficult way this is normally obtained, that is, deduced from changes in field or laboratory measured permeability.

Acknowledgements

The authors gratefully acknowledge the funding and support from Centre for Coal Seam Gas at the University of Queensland and its industry members (Arrow Energy, APLNG, Shell Australia (QGC) and Santos). The authors also thank Shell Australia (QGC) for the provision of the core used in this study. The work was partly supported by ARCDP160103896.

References (43)

Cited by (33)

  • Accurate characterization of coal pore and fissure structure based on CT 3D reconstruction and NMR

    2021, Journal of Natural Gas Science and Engineering
    Citation Excerpt :

    The principle of this method is to use mercury intake/adsorption and mercury removal/desorption curves to study the pore and fissure structure indirectly and obtain structural parameters such as coal porosity and pore size distribution. Syed et al. (2019) proposed a method to estimate the permeability of anisotropic coal using MIP and stress-strain measurements to determine the cleat compressibility. Liu et al. (2020a, 2020b) put forward a method for characterizing the full-scale pore structure of coal by controlling the injection pressure and the injection rate of mercury.

  • The role of sorption-induced coal matrix shrinkage on permeability and stress evolutions under replicated in situ condition for CBM reservoirs

    2021, Fuel
    Citation Excerpt :

    In recent years, coal permeability has been comprehensively investigated through experimental measurements, mathematical models, numerical simulations and field observations. Specifically, the dynamic coal permeability models has been continuously improved and established [44,48,52,54–60]. Even many improved permeability models were proposed to incorporate the impact of sorption and pore-elastic properties of coal on permeability change in those studies, the coupled effects of the mechanical compression/expansion, the matrix swelling/shrinkage induced by ad/desorption, and the compressibility of cleat volume [20,61,62] are still not fully incorporated.

  • A permeability model for anisotropic coal masses under different stress conditions

    2021, Journal of Petroleum Science and Engineering
    Citation Excerpt :

    CBM production is directly related to the permeability in the coal mass, which characterizes how well the gas flow can pass within the coal mass through internal fractures and pores (Chatterjee et al., 2019; Ali et al., 2017). The coal mass is often found to be highly anisotropic and heterogeneous in nature because its complex pore distribution (Raza et al., 2019; Mukherjee and Misra., 2018; An et al., 2015) and hence the permeability varies strongly in different directions (Wang et al., 2009; Zou et al., 2019). The permeability of a coal mass, under reservoir conditions, can be greatly affected by the process of gas adsorption, pore pressure, slippage effect, and stress conditions (Meng et al., 2018).

  • Effects of permeability anisotropy on coal mine methane drainage performance

    2021, Journal of Natural Gas Science and Engineering
    Citation Excerpt :

    An experiment was conducted by testing the same coal samples before and after liquid nitrogen super-cooling, and the results showed that the permeability of coal samples increased by 48.89%–93.55% (Cai et al., 2015). The methane drainage efficiency of in-seam boreholes is related to permeability, and the direction of permeability (Zhang et al., 2015, 2017; Raza et al., 2019). The permeability of a coal seam has a significant anisotropic property, and its direction is mainly affected by the original fracture structure and in-situ pressure (Ding et al., 2019; Ren et al., 2015; Liu et al., 2019).

View all citing articles on Scopus
View full text