Permeability evolution of coal subjected to triaxial compression based on in-situ nuclear magnetic resonance

doi:10.1016/j.ijrmms.2022.105213

International Journal of Rock Mechanics and Mining Sciences

Volume 159, November 2022, 105213

https://doi.org/10.1016/j.ijrmms.2022.105213 Get rights and content

Abstract

A knowledge of permeability evolution with stress is important for the enhancement of coalbed methane production. In this study, a series of compaction-permeability tests with in-situ nuclear magnetic resonance (NMR) measurement were conducted by using a designed NMR testing system equipped with a loading device. Based on the NMR results, the permeability variation mechanism is investigated during the complete stress-strain process. The stress damage and its influence on permeability are analyzed. Experimental results illustrate that the permeability curve shows a slow decrease first, followed by a stable change, then a slow increase, and finally a sharp rise during the complete stress-strain process. Permeability evolution is closely related to coal pores. Adsorption pores have little contribution to the coal permeability while seepage pores contribute over 99% to the permeability. The permeability growth is found to lag behind the coal pores growth due to the poor connection of new pores. With the increase of damage, coal permeability increases nonlinearly. A power function can be used to describe the relationship for the tested samples.

Introduction

Coalbed methane (CBM) is an important clean energy resource as well as a hazard source.¹ It is reported that CBM has proven reserves of more than 229 trillion m³ in the world, and has accounted for 7%, 10%, and 3% of the annual natural gas production in America, Australia, and China, respectively.² Meanwhile, CBM explosion accidents happen when the methane is not well controlled. Over 40,000 outbursts have been recorded in the world which seriously threaten miner's lives.³^,⁴ Permeability, as a key parameter for reservoir evaluation, depends on the size of pores and cracks. These pores and cracks evolve with in-situ stress and gas pressure during gas extraction from coal seams, resulting in a change in coal permeability. A knowledge of permeability evolution in coals with stress is essential to quantitatively analyze the gas migration and to further enhance CBM production.⁵^,⁶

Extensive studies have been performed on permeability evolution with stress through experimental work, theoretical derivation, and numerical simulation. In recent decades, researchers have proposed abundant theoretical models to predict the evolution of coal permeability with stress. These models can account for stress effect,7, 8, 9 sorption-induced swelling,10, 11, 12 thermal damage,¹³^,¹⁴ anisotropy,¹⁵ and so on. These models are derived under the assumption of idealized coal fracture system (such as the matchstick conceptual model¹⁶), which cannot consider the effects of new fractures induced by stress damage. To reveal the permeability evolution rules during the rock deformation, researchers have carried out a large number of permeability tests to investigate the permeability evolution during the loading process. These experimental studies revealed that the permeability evolution curve underwent a decrease stage, stability stage, slight increase stage, and dramatic increase stage during the progressive failure process.17, 18, 19 The permeability evolution of rocks has been preliminarily investigated, but the effect of microstructural evolution on the permeability variation mechanism has not been elucidated. Although many non-destructive testing methods have been developed to investigate the microstructure of rocks (such as scanning electron microscopy, nitrogen adsorption, mercury intrusion porosimetry, X-ray computed tomography),20, 21, 22 the application of these methods to detect microstructure evolution with stress is rare because most of these testing systems are not equipped with a loading device. To reveal the permeability variation mechanism, it is necessary to develop an apparatus to perform compression-permeation tests with real-time microstructure measurement.

In recent years, the NMR method has been successively applied to measure the microstructure and physical parameters (permeability and porosity) of CBM reservoirs.23, 24, 25 Compared with commonly used nondestructive testing methods, the NMR method has the ability to characterize the pore size at the micro-scale (which can detect pores as small as 0.1 nm). In this paper, an NMR testing system equipped with a loading device is designed to conduct compression-permeation tests with real-time NMR measurement. The dynamic change of permeability, as well as microstructure, are investigated during the complete stress-strain process. The stress damage and its influence on permeability are analyzed.

Section snippets

Coal core samples

Coal core samples were extracted from the Jinsha Coal Mine located in Guizhou Province, China. They were first drilled out from a coal block and then were cut into cylinders with dimensions of 25 mm in diameter and 50 mm in height. The coal samples belong to anthracite coal. Fig. 1 shows the pore structure in coal observed by SEM. As shown in the figure, the SEM results of original coal show that there are amygdaloidal pores, line pores, slit-shaped pores, and polygonal pores. Table 1 list the

Perfigmeability evolution and T₂ distributions

Fig. 4, Fig. 5, Fig. 6 shows the curve of stress-strain-permeability of samples under varied confining pressure. As shown in these figures, the permeability-stain curve underwent a slow decrease of stage Ⅰ, a stable change of stage Ⅱ, a slow increase of stage Ⅲ, and a sharp rise of stage Ⅳ. The four stages correspond to the compaction deformation stage, linear-elastic deformation stage, strain-softening stage, and post-peak stage of the stress-strain curve. At the compaction deformation stage,

Conclusions

The permeability evolution mechanism of coal subjected to triaxial compression is investigated through the real-time NMR measurement. Based on the study, the following conclusions can be drawn.

(1)
Using the time-domain NMR technique, the dynamic evolutions of pores and permeability during the loading process are obtained. Both of them exhibited a downward trend and then an upward trend with the increase of loading stress. Meanwhile, the growth of permeability lags behind the coal pores growth due

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by National Natural Science Foundation of China (No. 52264006, No. 52004072, No. 52064006, and No. 52164001), the Guizhou Provincial Science and Technology Foundation (No. [2020]4Y044, No. [2021]292, No. GCC[2022]005-1, and [2021]N404), the Research Fund for Talents of Guizhou University (Grant No. 201901), and Specialized Research Funds of Guizhou University (Grant No. 201903). The authors declare no conflict of interest.

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Permeability evolution of coal subjected to triaxial compression based on in-situ nuclear magnetic resonance

Abstract

Introduction

Section snippets

Coal core samples

Perfigmeability evolution and T2 distributions

Conclusions

Declaration of competing interest

Acknowledgments

Int J Coal Geol

J Nat Gas Sci Eng

Fuel

J Petrol Sci Eng

Int J Coal Geol

Fuel

Int J Coal Geol

J Nat Gas Sci Eng

Int J Rock Mech Min Sci

Int J Coal Geol

Int J Coal Geol

J Petrol Sci Eng

Fuel

Powder Technol

Cement Concr Compos

Powder Technol

Geothermics

Int J Rock Mech Min Sci

J Pet Sci Eng

Fuel

Int J Rock Mech Min Sci

Int J Rock Mech Min Sci

Perfigmeability evolution and T₂ distributions