Experimental and Analytical Study on Fracture Structural Evolution

In this study, we conducted laboratory experiments on coal cryogenic immersion freezing to investigate its fracturing mechanism. The ultrasonic method was employed to thoroughly monitor the seismic response of coal under the cryogenic condition. A theoretical model was proposed and established to determine fracture stiffness of coal from measured seismic velocity data. Using the analytical solution for fracturing stiffness, the observed macroscopic scattered wavefield can be linked with the changes in fracture properties, which can directly inform flowability modification due to cryogenic treatment.

The seismic interpretations of fracture stiffness of coal under freezing conditions can directly predict the change in coal flowability and accessing the effectiveness of cryogenic fracturing.

6.6.1 Background of Ultrasonic Testing

Because of the importance of cleats/fractures on coal permeability, active monitoring techniques need to be employed to quantify the changes in cleat frequency and distribution induced by cryogenic fracturing. Rock mass characterization with seismic wave monitoring provides an instant evaluation of the physical properties of the fractured

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rock mass. In the laboratory, a few previous studies have been devoted to measuring the seismic responses of various types of rocks subject to liquid nitrogen. Experimental evidence showed that the acoustic wave velocities and amplitudes decreased after cryogenic stimulation (Cai et al., 2016; Cha et al., 2017; Cha et al., 2014; Qin et al., 2017a;

Qin et al., 2018a, 2018b; Qin et al., 2016; Zhai et al., 2016). Cha et al. (2009) indicated that the mechanical characteristics of fractures exert predominant effects on the elastic wave velocity of cracked rock masses. Fractures as mechanical discontinuities are potential pathways for fluid flow that play an important role in gas production. If seismic techniques could be used to locate and characterize fractures or fracture networks, then such non-instructive geophysical techniques can probe fluid flow through fractured rock masses and ascertain the effectiveness of formation stimulation. A simple air- or fluid-filled fracture may not be a realistic representation. In fact, a fracture often comprises of two rough surfaces that do not exactly conform (Pyrak-Nolte et al., 1990). They are partially in contact, and in between the contacts are the void spaces or cracks controlling fluid flow behaviors. Fracture properties such as surface roughness, contact area, and aperture distributions directly govern the flowability of fractured rocks, but these geometric parameters are hard to be accurately quantified. Goodman et al. (1968) introduced a concept of fracture stiffness that measures fracture closure under the stress condition to quantify the complicated fracture topology without conducting a detailed analysis of fracture geometry. Although many studies (Hedayat et al., 2014; Myer, 2000; Pyrak-Nolte et al., 1990; Sayers and Han, 2002; Verdon et al., 2008) have estimated fracture stiffness from elastic waves propagation within fractured media with a single artificial fracture, very

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little fracture stiffness data have been reported in the literature for naturally fractured rocks such as coal.

6.6.2 Coal Specimen Procurement

Cylindrical coal specimens of 100 mm in length and 50 mm in diameter were taken from one CBM well in Qingshui basin, Shanxi, China. The coal specimens were initially cut by a rock saw and then abraded to satisfactory accuracy using a water jet. The cores were prepared in a way that the axial direction of each coal specimen is perpendicular to its bedding plane. For seismic measurements, intact cores with smooth and complete surfaces were selected. Figure 6-18 is an example of a tested coal core (M-2), and basic information on the studied coal specimens is summarized in Table 6-6. The permeability of the virgin coal samples in Qingshui basin is ultra-low with values less than one mD (Zhang and Kai, 1997). This low permeability cannot provide economic gas flow rates without stimulation. Thus, massive stimulation treatments such as hydraulic fracturing are required in the field. But the routine hydraulic fracturing in Qingshui basin does not always give the expected gas productivity (Zhu et al., 2015). As the fracturing fluid is imbibed into the formation, this elongates water drainage period, and the interaction between extraneous water and methane molecules reduces gas desorption pressure and prevents gas from being produced. Because of the associated water usage, hydraulic fracturing may not be the most effective stimulation technique for CBM exploration. Cryogenic fracturing using an anhydrous fluid that eliminates these water-related issues may substitute hydraulic fracturing. In this study, we tried to study the effectiveness of cryogenic treatment through

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the characterization of fracture stiffness which is inherently related to the change in permeability.

Figure 6-18: An intact coal specimen (M-2) before freezing.

Table 6-6: Physical properties of two coal specimens used in this study.

Sample Height Diameter Density Porosity Moisture Content

(mm) (mm) (g/cm³) (%)

M-1 99.96 49.89 1.39 0.036 0

M-2 100.07 50.17 1.38 0.048 0.58

6.6.3 Experimental Procedures

The two coal specimens were dried in an oven with a constant temperature of 80⁡℃

for 24 hrs to remove the moisture content. Figure 6-19 depicts the test systems used to investigate the velocities and attenuations of shear and compressional pulses propagated

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through the fractured coal specimens when subjected to a low-temperature environment.

Frost shattering and thermal shock are the two dominant mechanisms underlying cryogenic fracturing. To examine these mechanisms separately, the measurements of transmitted compressional and shear waves made with a dry specimen (no moisture content) would be compared with a saturated coal specimen. One of the coal specimens (M-2) was saturated with water in a vacuum water saturation device for 12 hrs with the other one (M-1) being a dry sample. The physical properties and moisture content of the dry and saturated coal specimens were listed in Table 6-6. Initial ultrasonic measurements of the intact coal specimens were made with a pair of platens aligned in the axial direction. The tested coal specimens were frozen in the thermal bottle filled with LN2 for up to 60 mins, and seismic measurements were made in between the freezing process over a range of time intervals from 5 mins to 15 mins. Followed by the freezing process, the coal specimens were thawed at room temperature for a complete freezing-thawing cycle. Waveforms of seismic pulses were then collected for the treated coal specimens. As coal is a highly attenuating material, the employed seismic transducers have low center frequency yielding strong penetrating signals. In this experiment, the center frequency of the P-wave transducer is 50 kHz, and it of the S-wave transducer is 100 kHz.