**1. Introduction**

The coal-measure formation is rich in unconventional natural gas resources, which is called coal-measure gas, including coalbed methane in coal reservoirs, tight sandstone gas in sandstone reservoirs, and shale gas in shale reservoirs [1,2]. Coal-measure gas development has attracted global attention because of its huge resource potential, especially in China [1,3,4]. Coal seams are generally mixed with sandstones, mudstones, and shales in coal-measure [5]. Coal seams and shale in the coal-measure could be the main source rocks for coal-measure gas because they are favorable for hydrocarbon generation and gas accumulation, while the conditions, such as burial depth, time, and temperature, are appropriate [1,6,7]. Sandstones in the coal-measure are one kind of reservoir that could store gas charged by the above source rocks [4,6,8,9]. Most of the sandstones in coal-measure are tight sandstones because their porosity is less than 10%, and their permeability is less than 0.1 mD [1,6,10]. The reservoir characteristics among the coal, shale, and sandstone in the coal-measure vary greatly [11]. The pore and permeability characteristics of the

**Citation:** Huang, H.; Sun, Y.; Chang, X.; Wu, Z.; Li, M.; Qu, S. Experimental Investigation of Pore Characteristics and Permeability in Coal-Measure Sandstones in Jixi Basin, China. *Energies* **2022**, *15*, 5898. https://doi.org/10.3390/en15165898

Academic Editor: Rouhi Farajzadeh

Received: 14 July 2022 Accepted: 11 August 2022 Published: 14 August 2022

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coal-measure gas reservoir could affect the gas accumulation and production [12,13]. Many studies have focused on the reservoir characteristics of coal [14,15], while the research on the pore and permeability characteristics of tight sandstone in coal-measure in the study area has been less focused upon. The tight sandstones in coal-measure are characterized by smaller pores and complex pore throat connections [16], which lead to lower permeability. The research on pore and permeability characteristics of tight sandstone reservoirs is critical for coal-measure gas development [15].

Low field nuclear magnetic resonance (LF-NMR) is a non-destructive method to characterize the pore structure parameters of complex porous media on a wide scale compared with N2 adsorption and mercury porosimetry [1,4,17–19]. The sandstone sample should be saturated with water when the pore characteristics of the sandstone are obtained by LF-NMR. The water-saturated sample is then placed in a strong static magnetic field generated by an external magnet, and a small radio frequency magnetic field is superimposed on the static magnetic field to excite the hydrogen nuclei in the water, thereby inducing nuclear magnetic resonance [20]. When the radio frequency field is turned off, a signal varying with time can be received [21]. The amplitude of the signal change can be described by the longitudinal relaxation time (T1) and the transverse relaxation time (T2). For LF-NMR, T2 carries similar information to that of T1 [20,22,23]. Therefore, T2 is generally applied to the analysis of the pore structure parameters of sandstone samples. Pore sizes, porosity, pore connectivity, and pore throat distribution for tight sandstone reservoirs could be characterized by the relaxation time T2 distribution [1,14,19,24]. However, it cannot determine the permeability of the sandstone directly [1,16]. Tight sandstone reservoirs have complex, highly heterogeneous pore structures, and poor pore connectivity [1,25]. The pore size and pore distribution vary greatly for the samples with different lithology. There is a close relationship between permeability and pore characteristics including porosity, pore-throat size, and pore volume [14,15,26,27]. The permeability of tight sandstones cannot be assessed only by total porosity because of the sandstone's complex pore-throat geometry [26,28]. However, the movable fluid porosity attained by LF-NMR might have a certain indication on permeability [1,14].

The previous reports have studied the relationship between permeability and scaling in the water injection, pore fluid pressure, confining pressure, and effective stress [29–32]. Generally, permeability decreases with the rise of the confining pressure and the effective stress [31], the permeability declines dramatically under 4–8 MPa confining stress, and permeability reduces slightly for 8–12 MPa conditions [29–31]. In a deep coal-measure reservoir, stress increases with the burial depth, which results in low permeability. The extraction of coal-measure gas would increase the effective stress on pores and fractures, which results in the compression of pores and the closure of fractures [29,31,33–37]. The permeability of the reservoir gradually decreases with the increase of the effective stress in the process of coal-measure gas extraction [38]. This kind of phenomenon is known as the stress sensitivity of the reservoir [39]. The permeability loss by the stress sensitivity will significantly affect the water and gas production rate because the original permeability of the coal-measure reservoir is generally low [15,29,38,40,41]. The stress sensitivity coefficient of reservoir permeability (Ss) for sandstones [37,42] is an index of reservoir permeability in response to effective stress variation [38], which could be affected by the plastic mineral content, pore throat size, porosity, permeability, etc. [37]. For instance, a higher the plastic mineral content could result in a bigger Ss [37].

The effect of sandstone grains on pore characteristics, permeability, and Ss is still unclear, and the relevant research about that is of grea<sup>t</sup> significance to coal-measure gas production [26,37,38,43,44]. The objective of this study was to analyze the characteristics of pore systems and permeability in tight sandstones with different grains based on Coalmeasure Gas Well HJD1 in Jixi Basin in Northeast China. The pore systems were studied based on LF-NMR data. The permeability was obtained by the tester based on the pulse transient method. The effects of pore characteristics on permeability variation were discussed. Then, permeability variation with effective stress and grain size of sandstones were

analyzed in detail. The findings will provide a better understanding of the characterization of pore structure and permeability in the process of coal-measure gas extraction, and will be useful for the efficient development of coal-measure gas.

#### **2. Geological Setting**

Jixi Basin is mainly composed of the northern depression and the southern depression, which are separated by the central Mashan fault (Figure 1a). Large-angle normal faults are well developed in the basin. The strikes of the faults are mainly east–west and northeast.

**Figure 1.** Geological background of the study area: (**a**) Location of Coal-measure Gas Well HJD1 in Jixi Basin; (**b**) geological structure of the area where Well HJD1 is located; (**c**) sampling stratigraphic column in Well HJD1.

The Quaternary in the Cenozoic and the Lower Cretaceous in the Mesozoic are the main strata in the research area where Well HJD1 is located (Figure 1). The Lower Cretaceous covers the Jixi Group and Huashan Group. The Jixi Group consists of Didao Formation, Chengzihe Formation, and Muling Formation from bottom to top (Figure 1c). The Muling Formation and Chengzihe Formation in the Lower Cretaceous are the main coal-bearing strata. Well HJD1 was constructed for coal-measure gas extraction from the Chengzihe Formation in the Jixi Basin, which is in eastern Heilongjiang province, in northeast China.

The Chengzihe Formation is the target strata in this research (Figure 1c). The buried depth of the Chengzihe Formation in Well HJD1 is 767.50–1739.05 m. The Chengzihe Formation is mainly composed of siltstone, fine sandstone, and coal seam. The sandstone samples used in this study are taken from the middle section of the Chengzihe Formation. The main coal seams involved in the sampling section are Numbers 22, 23, 25, and 28, respectively. In addition, most sandstone layers are between 2–8 m in thickness, as the sandstone layers are separated by thin mudstone layers and coal seams. The grains of sandstone also vary greatly.

#### **3. Samples and Experimental Methods**

*3.1. Sample Collection and Processing*

There are 4 sandstone samples collected from Well HJD1: medium sandstone (HJD1-01), fine sandstone (HJD1-06 and HJD1-08), and siltstone (HJD1-10). The 4 sandstone samples were lithologically classified according to the rock flake identification standard SY/T5368-2016 issued by the National Energy Administration in China. The grain size, and the percentage of clastic constituents and clay of the four samples can be seen in Table 1. The appearance of the rock samples was gray white sandstone without visible cracks.


**Table 1.** Rock thin section identification results of sandstone samples.

The collected sandstones were made into cylinders for permeability testing with a diameter of 50 mm and a height of 30 mm–100 mm. In addition, the flatness of the top and bottom surfaces of the sandstone samples was within 0.05 mm. Then, these samples were washed and dried before the test, in accordance with the requirements of Practices for Core Analysis (SY/T5336-2006).

HJD1-10 (silts sandstone), HJD1-06 (fine sandstone), and HJD1-01 (medium sandstone) are sandstones with different grain sizes. They were selected to undergo LF-NMR experiments. The LF-NMR experiments were conducted to characterize the pore system of the sampling sandstones and illustrate the effect of pore characteristics on the permeability of the sandstone.

## *3.2. Permeability Experiment*

The permeability was tested by a Pulse Decay Permeameter-PDP-200 (American Core Lab Company, Tulsa, OK, USA, Figure 2), which was based on the pulse transient method. The permeability could be obtained based on the pressure decay curve. Practices for Core Analysis (SY/T5336-2006) was the reference method for this experiment, and the test gas was nitrogen. Permeability experiments were conducted by increasing the confining pressure and fixed pore pressure. This simulates the increasing effective stress during the coal-measure gas development.

**Figure 2.** Pulse Decay Permeameter-PDP-200.

In this experiment, the test sample was put into a closed container, then the initial pressure was set, and time allowed for the initial pressure to reach the equilibrium state. Then, a pulse pressure (ΔP) was applied to the top of the sample. The gas percolated through the sample under the pressure ΔP, and this process resulted in the decrease of the pressure of the top container and the increase of the pressure of the bottom container. Finally, another equilibrium state (Pf) was reached between the top container and the bottom container; there was no pressure difference attenuation.

The intake nitrogen gas pressure, which is also known as pore pressure, was set to 700 psi, and the average pore pressure stabilized between 682 psi and 704 psi. The confining pressure for each sample was set to 1100 psi, 1200 psi, 1300 psi, 1400 psi, and 1500 psi to obtain the permeability. The temperature in the permeability tests was the indoor atmospheric temperature (about 25 ◦C).

#### *3.3. LF-NMR Experiments*

The LF-NMR experiments were performed by the MesoMR23-060H-I instrument with a main frequency of 23.400 MHz (Figure 3). The temperature of the laboratory where the NMR experiments were performed was 23.5 ◦C, the magne<sup>t</sup> temperature was constant at (35 ± 0.02) ◦C, the echo spacing was 0.1 ms, the numbers of scans were 32, and the echo numbers were 8000.

**Figure 3.** MesoMR23-060H-I instrument for LF-NMR experiments.

First, the columnar sandstone samples were dried. The sample was put into a 101 electric heating blast box for nearly 24 h until its weight remained constant. In addition, the samples were vacuumed for 2 h, and then they were saturated with water for nearly 24 h in the 2XZ-4B vacuum-saturation device. The quality difference of the sandstone sample before and after saturation was less than 0.05%. Moreover, the T2 spectrum distribution of the saturated sandstone sample was obtained by the NMR test instrument. After that, the movable water within the saturated sandstone sample was removed by the TCL-21M desktop high-speed refrigerated centrifuge. Finally, the T2 spectrum distribution of the centrifuged sandstone samples was tested by the MesoMR23-060H-I instrument.

In an LF-NMR test, the number of hydrogen atoms present within a fluid in a porous medium can be detected by the transverse relaxation time (T2) [1,14], and the T2 spectrum is generally used to characterize the physical parameters of the rock, such as pore size distribution and connectivity. It is supposed that longer T2 corresponds to larger pores, while shorter T2 corresponds to the smaller pores [4,28]. The amplitude of the T2 distribution reflects the number of pores within a certain size. The higher the amplitude, the greater the number of pores [20,21].

The application of NMR in the study of pore types in sandstone is based on the fact that T2 is positively correlated with pore size. This relationship can be expressed as [20,28,45]:

$$\frac{1}{T\_2} = \rho\_2 \left(\frac{\text{S}}{\text{V}}\right) \tag{1}$$

where T2 is the transverse relaxation time resulted from surface interactions, ms; and ρ2 is a constant representing the transverse relaxation strength, μm/ms; S is the surface area of pores (cm2); and V is the pore volume (cm3).
