Pore Structure in Coal: Pore Evolution after Cryogenic Freezing with

Jun 17, 2016 - It was found that LN2 freeze–thaw cycles can cause structural deterioration in the coal so as to improve both fracture density and ov...
2 downloads 0 Views 7MB Size
Download PDF
Article pubs.acs.org/EF

Pore Structure in Coal: Pore Evolution after Cryogenic Freezing with Cyclic Liquid Nitrogen Injection and Its Implication on Coalbed Methane Extraction Cheng Zhai,†,‡,§ Lei Qin,*,†,‡ Shimin Liu,§ Jizhao Xu,†,‡ Zongqing Tang,†,‡ and Shiliang Wu†,‡ †

Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China ‡ State Key Laboratory of Coal Resources and Safe Mining, Xuzhou, Jiangsu 221116, China § Department of Energy and Mineral Engineering, G3 Center and Energy Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Freezing and thawing cycles occur with cyclic liquid nitrogen (LN2) injection in coal. The freeze−thaw treatment damages the pore structure of coal and thus increases its permeability. In this study, NMR and strain monitoring were employed to investigate the changes in coal structure when the coal specimens were under cryogenic treatment using LN2. We classified freeze−thaw process into four stages; stages I and III are dominated by seepage pore development, and stages II and IV are dominated by adsorption pore development. It was found that LN2 freeze−thaw cycles can cause structural deterioration in the coal so as to improve both fracture density and overall permeability. The results demonstrate that the rate of increase of both the effective porosity and total porosity of the coal are positively correlated with the LN2 freezing time and the number of freezing cycles but negatively correlated with the residual porosity. For the same absolute LN2 freezing time, cyclic freeze−thawing has a greater effect on the rate of growth of pore spaces and reduction of P-wave velocity in the coal compared with single freeze−thaw treatment. It was also found that the number of freeze−thaw cycles is a very important factor for the creation of larger pores, pores that can connect the fracture network. The results suggest that appropriate control of the number of freeze−thaw cycles can result in effective fracturing of coal. extraction with soluble organic materials, and others.15−20 These methods can favorably promote the desorption of gases but fail to effectively increase the fracture permeability. The volume of water increases when it freezes with phase change. Thus, the expansion of the water in the pores and fractures during freezing can result in pore size enlargement and creation of new pores. When the water-bearing rock undergoes cyclic freeze and thaw events, the pore architecture and fracture network is modified and the porosity and permeability increases. In geotechnical engineering, this is viewed as erosion and is commonly referred to as “freeze− thaw” erosion. Cyclic freezing and thawing can result in damage, disturbance, and transformation of the fractures and pores in the rock. Freeze−thaw erosion can be applied to treat coal to increase the permeability. Liquid nitrogen (LN2) boils at −196 °C under ambient conditions. When LN2 vaporizes at 21 °C, its expansion ratio at atmospheric pressure is 696. Therefore, the phase change from liquid to gaseous nitrogen can generate enormous pressures in a confined space. The latent heat of vaporization of LN2 is 5.56 kJ/mol; therefore, LN2 can absorb a considerable amount of heat from its surroundings during vaporization. Considering its other advantages, including simple preparation, wide availability, nontoxicity, and being harmless to the environment, LN2 can

1. INTRODUCTION Coalbed methane (CBM) is regarded as an alternative and relatively clean energy source in many countries.1−3 On the basis of the most recent national statistics for China, the volume of CBM at a burial depth of 1−2 km is about 22.54 × 1012 m3; this is approximately 61% of the total CBM resources at depths of less than 2 km in China.4 Concurrent extraction of coal and gas is currently the country’s major method of CBM production. As mining depths increase, the gas content and gas pressure in virgin coal seams also increases. At the same time, the permeability of the coal progressively decreases with the increase of burial depth.5,6 Unlike conventional gas reservoirs, coal generally exhibits a multiporous network structure in which the majority of the gases are adsorbed. Owing to the low permeability of virgin coal,7 coal seam stimulation for permeability enhancement becomes a major challenge for deep coal mine gas drainage. Currently, the methods most commonly used for extracting CBM from deep and individual coal seams include hydraulic fracturing, hydraulic seam cutting, and loose blasting.8−14 Although these methods are satisfactory at some locations, the conventional hydraulic fracturing requires a large amount of water and can create environmental concerns such as water pollution. It can also cause extensive near borehole damage which will badly influence the overall effectiveness of the fracturing work. To overcome these limitation, a number of innovative seam stimulation methods have been proposed including supercritical CO2 injection, electrochemical treatment, microwave fracturing techniques, © XXXX American Chemical Society

Received: April 18, 2016 Revised: June 14, 2016

A

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels thus serve as a waterless fracturing fluid to stimulate coal reservoirs. In this study, a cyclic LN2 injection-based permeability enhancement technique was proposed to enhance coal permeability. Most coal cleats contain water, and the water in the cleats will freeze and generate about 9% volume expansion if LN2 is injected. Therefore, freeze damage will result in an increase of coal porosity. Because of the overall porosity increase due to the cryogenic treatment, the coal permeability will increase. At the same time, frost-heave forces caused by formation water freezing will create extrusion damage in the coal block. Volume expansion during cleat water freezing and ice will wedge into coal along the cleats, thus resulting in fracturing and extension of the pre-existing fractures and cleats. Migration of formation water during freezing also leads to damage of coal. Due to the nonmobility of ice and the expansion of water as it freezes,21,22 LN2-based freeze−thawing has advantages that conventional water-based-fracturing techniques do not have. Therefore, the proposed method has good potential for application in the field of CBM extraction. Although LN2 injection is promising, the details of the fracturing mechanisms and the cryogenically induced changes in coal structure are not well understood. To provide a scientific basis for using LN2 and freeze−thawing to extract CBM, this study investigates the underlying mechanisms concerning the evolution of the pores and fractures during LN2-based freezing.

cycles and the length of those cycles. They concluded that the effects of different water contents in the rocks on freeze−thawinduced damage should be considered for practical engineering applications. Matsuoka28 carried out low-temperature freezing tests on impermeable and saturated andesites at different freezing rates. The experimental results demonstrate that the average strain in these rocks using a freezing rate of −6 °C/h is 1.4 times that of a freezing rate of −2 °C/h. Furthermore, more rapid rates of freezing exhibited greater frost-heave-induced damage in the rocks. According to a study by Yao et al.,29 low-field nuclear magnetic resonance (NMR) can be used to measure the porosity and pore structure in coal effectively, and Li et al.30 later analyzed coals with different ranks using NMR. Cai et al.31,32 also used NMR to study rock and coal that had been frozen using LN2. They discovered that LN2 causes greater freezing damage to coal compared to other rocks. At the same time, the porosity and permeability of coal with LN2 cryogenic treatment were enhanced and the mechanical strength was reduced. In addition, their study found higher water content resulted in an increase in the damage caused by freezing.33 The previous studies focused on the effectiveness before and after only a single LN2 freezing treatment. The systematic and intensive experimental studies with different LN2 freezing times and freeze−thaw cycles have not been done, and it is necessary to obtain detailed understanding on the fracturing mechanisms and the cryogenically induced changes in coal structure.

2. PREVIOUS STUDIES The LN2 cryogenic fracturing technology has been commercially used in the United States CBM reservoir since the 1990s. McDaniel et al.23 used liquid nitrogen as a fracturing fluid, and the results were proved to be effective for increasing permeability in the reservoir. Ishikawa et al.24 monitored the widths and internal temperatures of fractures in the bedrock of a cliff near a mountaintop in northern Japan for 1 month. They discovered that the transformation from water to ice at the tips of the fractures was the primary factor leading to fracture expansion and damage to the rocks. Freeze−thaw erosion is a common phenomenon in both natural outcrops and humanmade constructions subjected to great temperature variations between seasons. After observing rocky landslides in Swiss mountains over an extended period, Hasler et al.25 found that shear expansion of rocks mainly occurs in the thawing seasons. In addition, there was more intensive rock fracture propagation in the cold season than in the warm season. Davidson and Nye26 precast narrow grooves on the surfaces of plastic sheets. Using a photoelastic technique, they subsequently found the maximum pressure of the frost-heave induced by the ice in the grooves to be 1.1 MPa. They also found that the frost-heave pressure varied almost linearly with the freezing rate. For saturated rocks, the transformation from water to ice produces a volume expansion of approximately 9%. Theory shows that it also generates a frost force of up to 207 MPa if the pressure-bearing capacity of the rock is ignored.22 Sandström et al.22 carried out freeze−thaw experiments on rocks and reported that the number of freeze−thaw cycles had a significant effect on the water absorption in porous media and also had effects on the fracture development, water migration, and the mechanical strength of the rock. Mcgreevy and Whalley27 discovered that the initial water content in the rocks determined the amount of frost-heave damage. They also found that the water content changes with the number of freeze−thaw

3. EXPERIMENTAL WORK 3.1. Coal Sample Collection and Preparation. A large block of coal was collected from the Shengli coal mine, Inner Mongolia, China. A total of 14 core samples were drilled and prepared from the block; each core was ∼2.5 cm in diameter and 5 cm in length. The petrophysical and proximate analyses data are listed in Tables 1 and 2. The specimens were tagged according to the experimental conditions (Table 1).

Table 1. Sample Parameters and Numbersa sample

porosity (vol %)

saturated water content (wt %)

sample

porosity (vol %)

saturated water content (wt %)

T1 T5 T10 T20 T30 T40 T50 T60

17.1 17.1 16.8 18 12.4 17 15.9 13.9

13.5 13.3 12.9 14.2 9.6 13.7 12.6 11.0

C5 C10 C15 C20 C25 C30 − −

17.9 13.6 17.4 16.1 16.1 11.9 − −

14.2 11.3 14.1 13.0 13.0 10.2 − −

a T1 represents a coal sample that was frozen in LN2 for 1 min then thawed at room temperature for 1 min (one freeze−thaw cycle); C10 represents a coal sample that was frozen in LN2 for 5 min then thawed at room temperature for 5 min (10 freeze−thaw cycles); wt., weight percentage; vol., volume fraction.

Table 2. Maceral Analysis and Proximate of Raw Coala maceral analysis (vol %)

proximate (wt %)

V

I

E

M

Ro,max (%)

80.5

14.5

3.7

1.3

0.331

Mad

Aad

Vdaf

FCad

10.67

14.53

43.5

68.7

a V, vitrinite; I, inertinite; E, exinite; M, minerals; Mad, moisture, airdrying basis; Aad, ash yield, air-drying basis; Vdaf, volatile matter dry ash-free basis; FCad, fixed carbon content, air-drying basis.

B

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. Experimental system and procedure.

Figure 2. Diagrammatic illustration showing the LN2-based freeze−thaw test system. 3.2. Experimental System and Analytical Instruments. There are three main subsystems in the experimental system for investigating the LN2 freeze−thaw properties of coals. The major components of the experimental system are shown in Figure 1, and the complete system consists of the following three major subsystems. (1) The freeze−thaw test system comprises a test chamber (DN300) for freeze−thaw cycling with LN2, a dual-channel temperature monitor (TM201-2), a static stress−strain test and analysis system (DH3818-1), and a self-pressurized LN2 jar (YDZ-50). (2) The NMR-based core analyzer and MR-60 magnetic resonance imaging (MRI) analysis system was manufactured by Shanghai Niumag Electronic Technology Co., Ltd. The strength of the main magnetic field in the MRI was 0.51 T. The proton resonance frequency and the radio frequency (RF) pulses used were 21.7 and 1.0−49.9 MHz, respectively. The magnet field had a uniformity of 12.0 ppm, and the temperature of the magnet was controlled to 25−35 °C; the RF power was 300 W. (3) A vacuum drying oven (DZF-6020), a vacuum water-saturation device, and a centrifuge for rock samples were also used. An acoustic parameter tester (HS-YS4A) was used to measure P-wave velocities of ultrasonic waves in the coal.

3.3. Experimental Procedures. The experimental system is illustrated in Figure 1, and the experimental procedure is described below. The coal specimens were tagged and then dried in the vacuum drying oven (−0.1 MPa, 60 °C) to a constant weight. “Constant weight” means the differences between two consecutive weighings did not exceed 0.1%. After the samples were dried, the size and weight of the specimens were measured and recorded. The specimens were immersed in water in a beaker. The beaker was then placed in the vacuum water-saturating device. The vacuum watersaturation device was sealed by petrolatum and kept at −0.1 MPa for 12h. After sealing, the air was sucked out from pores and water will enter the pores to achieve the water saturation condition. Then, the size and weight of each specimen was measured and recorded again. Coal specimens were frozen using the LN2 freeze−thaw test system, and the evolution of coal properties were continuously monitored and recorded. The LN2 chamber was sealed. The internal pressure was limited to a maximum of 0.3 MPa to ensure the safe operating conditions. As shown in Figure 2, the coal specimens were thawed at room temperature after being frozen; thus, each specimen goes through a complete LN2 freeze−thaw cycle. C

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 3. Variations of the temperature and strain in a coal sample being freeze−thawed using LN2.

εv = εa + 2εr

Specimens were then placed into the vacuum water-saturating device (−0.1 MPa for 12 h) to achieve water saturation, after which the size and weight of each sample was measured and recorded again. Then, the MR-60 MRI analysis system was used to determine how the pore structure had been altered by the freeze−thaw cycle. Finally, each core specimen was placed in the centrifuge and swung (at 1.38 MPa centrifuge pressure) for 90 min to remove the reducible water. Then, the specimen size and weight were again measured and recorded before the MR-60 MRI analysis system was again employed to analyze the pore structure.

(1)

where εa and εr represent the axial and hoop strains, respectively. Figure 3 shows the strains (axial, hoop, and volumetric strains in percnet), temperature (K) of the coal, and the chamber pressure (kPa) during the freeze−thaw test. In Figure 3, the freezing time was 60 min, and the time after that represents the thaw period (at room temperature). According to the sign of the volumetric strain and as shown in Figure 3, the freeze−thaw time can be divided into several subperiods: a freezing shrinkage subperiod (I), a frost-heave subperiod (II), another freezing shrinkage subperiod (III), and the final frost-heave subperiod (IV). The times and temperatures corresponding to each subperiod are listed in Table 3. The volumetric strain, εv, has two extrema in the two freezing

4. EXPERIMENTAL RESULTS 4.1. Temperature and Strain. The change in volume per unit volume of the coal treated with LN2, or the volumetric strain, εv, is expressed as34 D

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

4.2. NMR Characterization and T2 Spectral Analysis. The fully water-saturated cores were subjected to Carr− Purcell−Meiboom−Gill pulse sequences in the NMR spectrometer. The attenuation signal of the echo trains, that is, the superposition of the water signals from the pores of different sizes, was recorded. The attenuation amplitude of the echo trains can be fitted accurately using the sum of a group of exponential decay curves, each curve having a unique attenuation constant. The collection of all the attenuation constants forms the distribution curve for the transverse relaxation time, T2. Subsequently, the distribution and connectivity of the micropores, mesopores, and macropores in the pore system of the coal samples can be obtained using the transverse relaxation times. The relationship between the transverse relaxation time, T2, and the pore size, r, can be expressed by eq 2:38,39

Table 3. Times and Temperatures for Four Freeze−Thaw Subperiods for a Coal Sample Subjected to Freeze−Thaw Cycling subperiods freezing shrinkage subperiod frost heave subperiod freezing shrinkage subperiod frost heave subperiod

subperiod time (min)

time of εv extremum (min)

subperiod temperature (K)

temperature of εv extremum (K)

[0, 7]

4.83

[274, 243]

253

[7, 24]

18.35

[243, 173]

198

[24, 170]

80.28

[173, 76] [76, 295]

247

[170, −]

−

[295,−]

−

I II III IV

1/T2 = ρ(S /V ) = FS(ρ /r )

shrinkage subperiods, one at 4.83 min in the freezing stage and the other at 80.28 min in the thawing stage. The corresponding temperatures are 253 K in the freezing stage and 247 K in the thawing stage. An extremum in the volumetric strain occurs in the frost-heave subperiod at 18.35 min into the freezing stage at a temperature of 198 K. Thus, it can be seen that the coal experiences an alternating pattern during these freeze−thaw cycles, namely freezing shrinkage followed by frost-heave followed by another interval of freezing shrinkage. Therefore, the coal’s structure deterioration can be realized through the cyclic freezing and thawing. These cycles fracture the coal when the water- and ice-induced swelling stress exceeds the tensile strength of the coal. Taber35,36 and Sass37 suggested that the damage induced by frost heave in rock or earth masses was caused by water migration and by the frost force of the transformation of water to ice. According to our liquid-nitrogen freeze−thaw experiments, the pattern “freezing shrinkage−frost heave−freezing shrinkage” occurs cyclically. To simplify the mechanical model for the fractures in coal, a theoretical analysis was performed on a single fracture in the coal. The factors that influence the expansion and extension of the fractures include the following: I, the frost force, Pi, caused by the transformation of water into ice in the fractures; II, the expansive force, Pn, induced by the vaporization of LN2 to gaseous nitrogen; III, the solid-state cryoturbation in the coal, DLN2, caused by the extremely low temperatures from the use of LN2; and IV, the in situ stress, σ. These factors are illustrated in Figure 4.

(2)

where T2 is the transverse relaxation time (ms) and ρ indicates the strength of the transverse surface relaxation (μm/ms); S, V, and Fs denote the surface area (cm2), volume (cm3), and shape factor of the pores, respectively; r signifies the pore size. The shape factors (Fs) of spherical pores, columnar pores, and fractures are 3, 2, and 1, respectively. The distribution of the transverse relaxation times reflect the size of the pores: a smaller T2 value corresponds to a smaller pore size, and the greater the amplitude of the T2 curve, the greater the number of pores corresponding to that pore size. Therefore, the T2 distribution also reflects the distribution of the pore volumes.40 According to eq 2 and the conventions used to classify pore size, the first peak in the T2 spectrum corresponds to micropores in the coal (pores with diameters smaller than 0.1 μm). These pores belong to the family of pores that hold the adsorbed gas in coal seams and are called “adsorption pores” in this paper. The second peak represents meso- and macropores (pores with diameters in the range 0.1− 100 μm), and the third peak respresents fractures in the coal (openings >100 μm).41 Studies by Li et al.42 and Yao et al.43 classified pores corresponding to the second and third peaks in the T2 spectra as “seepage pores” (i.e., the spaces responsible for gas seepage). Therefore, as shown in Figure 5, the pores in the coal specimen were divided into adsorption and seepage pores. Figure 5 shows the T2 curves for the water-saturated coal specimens representing the distribution of all the water-bearing

Figure 4. Stress analysis for the fractures in coal (a) and a schematic showing freeze−thaw damage caused by LN2 (b). E

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 5. T2 spectra for coal samples subjected to different LN2 freezing times and cycles: (a) LN2 freezing time and (b) number of cycles.

space in the coal samples. Similarly, the T2 curves for the centrifuged coal samples indicate only the space occupied by irreducible water. The distribution of the space occupied by free water can therefore be obtained by subtracting the irreducible water space from the total water-bearing space. Figure 5a,b shows the T2 curves of water-saturated and centrifuged coal before and after freeze thawing for different LN2 freezing times and cycles. It can be seen that both the amplitude and width of the T2 curves for the water-saturated coals are positively

correlated with LN2 freezing time. It can also be seen that the size and number of pores in the coal increase as the freezing time increases. It was also found that the total number of pores and freely connected pores increases with LN2 freezing times and cycles. More interestingly, the interconnectivity of the coal pore network was also improved under cryogenic LN2 treatments, as shown in Figure 5. Note that the amplitudes of the T2 curves for the centrifuged coal exhibit a negative correlation with the LN2 freezing time; the widths of the F

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 6. Porosity spectra for coal sample TN-1 before (a) and after (b) a 1 min freeze−thaw cycle.

Figure 7. Change in the porosity owing to different LN2 freezing time and number of cycles.

can quantify the irreducible water porosity. The effective porosity φNF40 of the coal can then be acquired by subtracting the residual porosity from the total porosity:

relaxation time intervals remain essentially unchanged. This shows that the number of closed pores decreases as the freezing time increases. These findings coincide with the conclusions drawn from the physical map in Figure 5: with an increase in the LN2 freezing time and cycle number, the fracture network becomes increasingly more complicated, and this is accompanied by a gradual increase in the width of the fractures. Comparing panels a and b of Figure 5, one must conclude that the increase in amplitude of the T2 curves is influenced more by the number of freeze−thaw cycles than by a single freeze−thaw. In particular, P2 and P3 are significantly higher, which corresponds well with the larger fractures and more abundant fracture networks in the coal, as demonstrated in Figure 5b. 4.3. Porosity Evolution and Quantification after Freeze−Thaw Cycling. The porosity of coal is an important property used to quantify the pore structure of coal and is a factor that directly affects the sorption capacity and gas transport properties in CBM reservoirs. Low-field NMR T2 spectra can be analyzed to accurately determine the porosity of tested coal specimens.29 By analyzing the water-saturated coal samples, T2 spectra representing the variations of porosity with relaxation time were obtained. Then, a maximum porosity value was obtained by accumulating the porosity component over time. The maximum cumulative porosity from the T2 spectra represents the total porosity, φN, of the coal. The same method was used to analyze the centrifuged coal samples. The residual porosity, φNB, of the coal specimen was obtained, and this value

φNB = φN × BVI/(BVI + FFI)

(3)

φNF = φN × FFI/(FFI + BVI)

(4)

where φNB, φNF, and φN are the residual, effective, and total porosities of the coal specimen, respectively. BVI stands for the bound fluid index and is calculated using the fraction of the area under the spectral curve corresponding to the residual water state; FFI denotes the free fluid index. The sum, BVI + FFI, signifies that all fluids are included and is calculated using the fraction of the area under the curve corresponding to the watersaturated state, as demonstrated in Figure 6. Figures 6a and 6b show the spectra for the porosity for a coal sample before and after a 1 min freeze−thaw cycle. By comparing the two figures, it can be seen that the T2 cutoff value and residual porosity decrease as the total and effective porosity increase. The difference in the porosities before and after freeze−thaw is consistent with the variations in the amplitudes of the T2 curves for the water-saturated and centrifuged coal samples. The effective porosity φNF indicates the proportion of free-flowing fluid space in the coal, the space that is available to transport gas. The increase in total porosity φN represents the total increase of coal fissures, and the effective porosity φNF increase represents the increase of gas G

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 8. Evolution of the differently sized pores owing to the effect of using different LN2 freezing time.

free-migration pathways, and the residual porosity φ NB reduction represents the decrease of gas adsorption space. This indicates that the fracture network in the coal expanded after LN2 freeze−thaw action, and the increased pore space allowed the CBM to desorb from the coal. To quantify the changes in the porosity of the coal, a rate of increment for porosity, Δφ%, can be defined as the rate of change of the porosity under different freeze−thaw conditions:

Δφ = φpost − φpre

(5)

Δφ% = Δφ /φpre

(6)

As shown in Figure 8, the rate of growth of the area under the T2 curve representing seepage pores (black circles) shows an indication of decelerated growth followed by accelerated growth during a 1 h freeze−thaw cycle. At the same time, the growth rates in the T2 areas corresponding to adsorption pores (blue triangles) and full pores (red squares) exhibit a tendency for rapid growth, then stable growth, then rapid growth again during the 1 h freeze−thaw cycle. When the growth rates of the T2 spectrum areas for the adsorption and seepage pores are compared, the spectrum after a freezing time of 60 min could be divided into four stages based on pore development. Those stages are stages I (0−12.5 min) and III (30−48 min) for seepage pores and stages II (12.5−30 min) and IV (48−60 min) for adsorption pores. In stage I, the micro- and mini-pores develop rapidly, and the larger seepage pores can be regarded as the extended space of the adsorption pores within the limited space available. In this stage, the newly developed adsorption pores begin to impinge on the seepage pores, leading to a reduction in the increase in the seepage pore growth rate. The growth rate for the adsorption pores exceeds that of the seepage pores in stage II. During this interval, adsorption pores increase gradually and connect to form seepage pores with large sizes (where the number of adsorption pores is high enough). Because adsorption pores are being transformed into seepage pores, the adsorption pore growth slows and rapid seepage pore growth begins. During stage III, the rate of growth of seepage pores is greater than that of adsorption pores, and both sizes of pore grow rapidly. The rate of growth of adsorption pores is greater than that of seepage pores in stage IV; therefore, during this time both types of pore exhibit a gradually increasing growth rate. The trends in the changes of the amount of free water and bound water space in the coal with LN2 freezing time can be determined by examining the different water-bearing pores and fractures in the coal, as shown in Figure 9. The free water space represents the fracture space where water can flow freely, and if the fractures are mutually connected, they provide favorable channels for CBM movement. The bound water space is the pore space in which water cannot flow freely, and most of these pores are not connected and therefore do not contribute to CBM movement. It can be seen from Figure 9 that the rate of growth in the free water space is positive and is positively correlated with the LN2 freezing time. In contrast, the growth rate for bound water space is negative and is negatively correlated with the LN2 freezing time. At the same time, the

where Δφ represents the increment in porosity change and φpre and φpost indicate the porosities before and after the coals were freeze thawed, respectively. The increment rate, Δφ%, was analyzed statistically. It was found that the rate of change for both total porosity, ΔφN%, and effective porosity, ΔφNF%, are positively correlated with the LN2 freezing time and number of cycles. The increment for the residual porosity is negatively correlated with these variables, as demonstrated in Figure 7. As can be seen from Figure 7a, the incremental rates for all three porosities are clearly linearly related to the LN2 freezing time. In contrast, in Figure 7b, the rates are nonlinear with respect to the number of freeze−thaw cycles. More specifically, as the number of freeze− thaw cycles increases, the change in the amplitude of the porosity increment rate becomes larger.

5. DISCUSSION 5.1. Pore Growth Rates and Liquid Nitrogen Freezing Times. A larger area under the curve in a T2 spectrum represents a greater number of pores of that pore size. To quantitatively describe the changes in the number of coal pores as a function of the freeze−thaw variables, a growth rate, Dt, in the T2 spectrum area was defined to indicate the rate of change in the number of pores per unit volume of coal. Where S is the area under the T2 curve: ΔS = Spre − Spost

(7)

and Dt = ΔS /Spre = (Spre − Spost)/Spre

(8)

with Spre and Spost being the relevant areas in the T2 spectra of the coal before and after being freeze−thawed, respectively. H

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

cycles. The rate of growth for the seepage pores resulting from an increased number of freeze−thaw cycles is greater than that caused by a single cycle. This suggests that multiple freeze− thaw cycles are very important for generating seepage pores with larger sizes. 5.3. Pore and Fracture Enhancement Efficiency. In this research, we compared the free water space, bound water space, full pores, seepage pores, and adsorption pores using NMR T2 spectra. The aim was to quantitatively compare the fracturing efficiency with respect to the LN2 freezing time and number of cycles. We now consider the P-wave velocities of ultrasonic waves in coal and how they are affected by freeze−thawing. The “absolute value of the freezing time” with LN2 is defined as an index representing the contact time between the LN2 and the coal (that is, it corresponds to the LN2 freezing time). For the same absolute LN2 freezing time, the growth rates of the pores and longitudinal wave velocities of the ultrasonic waves were compared for different LN2 freezing times and cycles. The differences in the growth rates as a result of freeze−thaw are highlighted by the arrows in Figure 11. It was found that the Pwave velocity is closely linked to the number of fissures and fractures. The more numerous the fissures and fractures, the lower the P-wave velocity.44,45 Wang et al.46 also found that the P-wave velocity exhibits a decreasing trend with increasing fracture density and porosity. The P-wave velocity is negatively correlated with the LN2 freezing time and number of cycles. That is to say, the number of fractures in the coal increases with LN2 freezing time and the number of cycles, leading to a reduction in P-wave propagation velocities. For the same absolute LN2 freezing time, cyclical freeze− thaw has a greater effect on the rate of growth of the seepage, adsorption, and full pores and the free water and bound water spaces than does a single freeze−thaw cycle. The rate of reduction of the P-wave velocity in the coal follows the same pattern. The change in the pore and fracture growth rates and P-wave velocity as a result of freeze−thaw compared with the absolute LN2 freezing time is illustrated in Figure 12. Figure 12 shows the differences in efficiency between multiple freeze− thaw cycles and a single freeze−thaw for the same absolute LN2 freezing time. As can be seen, the efficiency differences of the pore space and the P-wave velocity first increase and then decrease with absolute LN2 freezing time. A maximum point appears where the freeze−thaw cycle reaches maximum efficiency. The maximum difference in the rate of increase of free water space is at the 25 min point. In other words, the freeze−thaw cycles are most efficient at generating free water

Figure 9. Evolution of the water-bearing space owing to different LN2 freezing times.

rate at which the growth rate in the free water rises is greater than the rate at which the growth rate in the bound water falls. Therefore, it may be concluded that LN2-based freeze−thawing does provide favorable channels for the flow and extraction of CBM. 5.2. Freeze−Thaw Cycles. Through examination of the statistics for the different pore sizes and the different waterbearing spaces in coal treated using different numbers of freeze−thaw cycles, the trends in the evolution of the different pores may be derived. These trends are summarized in Figure 10. The rate of growth of the seepage pores tends to follow a growth sequence that is rapid−stable−rapid as the increase of freeze−thaw cycles. However, the trends in the rates of growth of adsorption pores, full pores, and free water space exhibit a pattern of exponential growth (representing growth that is rapid, then stable). The rate of growth of bound water space is negative and is negatively correlated with the number of freeze−thaw cycles. During the cyclic liquid nitrogen-induced freezing and thawing, the rate of growth of the free water space shows a positive correlation with the number of cycles, but the bound water space shows a negative correlation. Free water space increases rapidly during the first five cycles and more slowly in the later cycles. The rates of growth of the adsorption, seepage, and full pores are positively correlated with the number of freeze−thaw cycles. Adsorption and full pores show a rapid growth in the first five cycles but slower growth in the later

Figure 10. Evolution of the pores and water-bearing space in coal owing to the use of different numbers of freeze−thaw cycles. I

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 11. Graphs comparing the pore space and P-wave velocities in coal subjected to different freeze−thaw regimes: (a) free water space, (b) bound water space, (c) full pores, (d) seepage pores, (e) adsorption pores, and (f) P-wave velocity of ultrasonic waves.

space after five cycles. The maximum difference in the value for the growth rate of full pores is found at the 30 min point; therefore, six cycles are most efficient for full pores. Similarly, the maximum difference in growth rate for adsorption pores appears at 35 min; thus, the maximum efficiency of the freeze− thaw process for adsorption pores is seven cycles. The maximum difference in growth rate for seepage pores appears at 50 min; therefore, seepage pore maximum efficiency requires 10 cycles. The absolute difference of the growth rate of bound water space increases with increasing absolute LN2 freezing time. The maximum difference in the rate of the P-wave velocity reduction was found at 35 min. That is to say, the greatest difference in the rate of generation of fractures was with seven freeze−thaw cycles.

These results show that by suitably controlling the number of freeze−thaw cycles, one can realize very effective fracturing of the coal.

6. CONCLUSIONS Based on this experimental work and the analysis of the results, the main findings are the following: According to our strain analysis, the freezing (60 min) caused by LN2 was divided into four subperiods: a freezing shrinkage subperiod I, a frost heave subperiod II, a freezing shrinkage subperiod III, and a frost heave subperiod IV. According to the way the pores evolve, the LN2 freezing time (60 min) was divided into two dominant seepage-pore development stages (I and III) and two dominant adsorptionpore development stages (II and IV). The rate of growth of the seepage pores follows a pattern of rapid growth−stable J

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

(3) Mitra, A.; Harpalani, S.; Liu, S. M. Laboratory measurement and modeling of coal permeability with continued methane production: Part 1 − Laboratory results. Fuel 2012, 94, 110−116. (4) Zhang, S. H.; Tang, S. H.; Qian, Z.; Pan, Z. J.; Guo, Q. L. Evaluation of geological features for deep coalbed methane reservoirs in the Dacheng Salient, Jizhong Depression, China. Int. J. Coal Geol. 2014, 133, 60−71. (5) Wang, G. D.; Ren, T.; Wang, K.; Zhou, A. T. Improved apparent permeability models of gas flow in coal with Klinkenberg effect. Fuel 2014, 128, 53−61. (6) Fu, X. H.; Qin, Y.; Wang, G. G. X.; Rudolph, V. Evaluation of coal structure and permeability with the aid of geophysical logging technology. Fuel 2009, 88 (11), 2278−2285. (7) Tian, L.; Cao, Y. X.; Chai, X. Z.; Liu, T. J.; Feng, P. W.; Feng, H. M.; Zhou, D.; Shi, B.; Oestreich, R.; Rodvelt, G. Best practices for the determination of low-pressure/permeability coalbed methane reservoirs, Yuwu Coal Mine, Luan mining area, China. Fuel 2015, 160, 100−107. (8) Javadpour, F.; McClure, M.; Naraghi, M. E. Slip-corrected liquid permeability and its effect on hydraulic fracturing and fluid loss in shale. Fuel 2015, 160, 549−559. (9) Li, Q. G.; Lin, B. Q.; Zhai, C. The effect of pulse frequency on the fracture extension during hydraulic fracturing. J. Nat. Gas Sci. Eng. 2014, 21, 296−303. (10) Lin, B. Q.; Yan, F. Z.; Zhu, C. J.; Zhou, Y.; Zou, Q. L.; Guo, C.; Liu, T. Cross-borehole hydraulic slotting technique for preventing and controlling coal and gas outbursts during coal roadway excavation. J. Nat. Gas Sci. Eng. 2015, 26, 518−525. (11) Xu, B. X.; Li, X. F.; Haghighi, M.; Ren, W. N.; Du, X. Y.; Chen, D.; Zhai, Y. Y. Optimization of hydraulically fractured well configuration in anisotropic coal-bed methane reservoirs. Fuel 2013, 107, 859−865. (12) Zou, Q. L.; Lin, B. Q.; Zheng, C. S.; Hao, Z. Y.; Zhai, C.; Liu, T.; Liang, J. Y.; Yan, F. Z.; Yang, W.; Zhu, C. J. Novel integrated techniques of drilling−slotting−separation-sealing for enhanced coal bed methane recovery in underground coal mines. J. Nat. Gas Sci. Eng. 2015, 26, 960−973. (13) Wang, W. C.; Li, X. Z.; Lin, B. Q.; Zhai, C. Pulsating hydraulic fracturing technology in low permeability coal seams. Int. J. Min. Sci. Technol. 2015, 25 (4), 681−685. (14) Puller, J. W.; Mills, K. W.; Jeffrey, R. G.; Walker, R. J. In-situ stress measurements and stress change monitoring to monitor overburden caving behaviour and hydraulic fracture pre-conditioning. Int. J. Min. Sci. Technol. 2016, 26 (1), 103−110. (15) Liu, J. Z.; Zhu, J. F.; Cheng, J.; Zhou, J. H.; Cen, K. F. Pore structure and fractal analysis of Ximeng lignite under microwave irradiation. Fuel 2015, 146, 41−50. (16) Guo, J. Q.; Kang, T. H.; Kang, J. T.; Chai, Z. Y.; Zhao, G. F. Accelerating methane desorption in lump anthracite modified by electrochemical treatment. Int. J. Coal Geol. 2014, 131, 392−399. (17) Ji, H. J.; Li, Z. H.; Peng, Y. J.; Yang, Y. L.; Tang, Y. B.; Liu, Z. Pore structures and methane sorption characteristics of coal after extraction with tetrahydrofuran. J. Nat. Gas Sci. Eng. 2014, 19, 287− 294. (18) Mushtaq, F.; Mat, R.; Ani, F. N. A review on microwave assisted pyrolysis of coal and biomass for fuel production. Renewable Sustainable Energy Rev. 2014, 39, 555−574. (19) Al-Abri, A.; Sidiq, H.; Amin, R. Mobility ratio, relative permeability and sweep efficiency of supercritical CO2 and methane injection to enhance natural gas and condensate recovery: Coreflooding experimentation. J. Nat. Gas Sci. Eng. 2012, 9, 166−171. (20) Ni, X. M.; Li, Q. Z.; Wang, Y. B.; Gao, S. S. Permeability variation characteristics of coal after injecting carbon dioxide into a coal seam. Int. J. Min. Sci. Technol. 2015, 25 (4), 665−670. (21) Arosio, D.; Longoni, L.; Mazza, F.; Papini, M.; Zanzi, L. Freezethaw cycle and rockfall monitoring. In Landslide Science and Practice; Springer: Heidelberg, Germany, 2013; pp 385−390.

Figure 12. Graphs showing the changes in pore and fracture growth rates and P-wave velocities versus LN2 absolute freezing time.

growth−rapid growth with an increasing number of freeze− thaw cycles. The rates of growth for the adsorption pores, full pores, and free water space, however, show a rapid increase followed by stable growth. In addition, the growth rate of the bound water space is negative and is negatively correlated with the number of LN2 freeze−thaw cycles. The incremental rates of change for both the effective and total porosity in the coal are positively correlated with the LN2 freezing time and the number of cycles. The incremental rate of change for the residual porosity of the coal exhibits a negative correlation with the LN2 freezing time and cycle number. For the same absolute LN2 freezing time, cyclical freeze− thaw has a greater influence on the growth rate of seepage pores than a single freeze−thaw period. Multiple freeze−thaw cycles contribute more to the generation of large seepage pores. Efficient fracturing of coal can be realized by controlling the number of freeze−thaw cycles appropriately.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +8613641537958. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51274195, U1361106); the National Major Scientific Instrument and Equipment Development Project (2013YQ17046309); and the Program for New Century Excellent Talents in University (NCET-12-0959), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM14X02).

■

REFERENCES

(1) Vishal, V.; Singh, T. N.; Ranjith, P. G. Influence of sorption time in CO2-ECBM process in Indian coals using coupled numerical simulation. Fuel 2015, 139, 51−58. (2) Sun, W. J.; Feng, Y. Y.; Jiang, C. F.; Chu, W. Fractal characterization and methane adsorption features of coal particles taken from shallow and deep coalmine layers. Fuel 2015, 155, 7−13. K

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels (22) Sandström, T.; Fridh, K.; Emborg, M.; Hassanzadeh, M. The influence of temperature on water absorption in concrete during freezing. Nordic Concrete Research 2012, 45, 45−58. (23) McDaniel, B. W.; Grundmann, S. R.; Kendrick, W. D.; Wilson, D. R.; Jordan, S. W. Field applications of cryogenic nitrogen as a hydraulic fracturing fluid. Proceedings−SPE Annual Technical Conference and Exhibition, San Antonio, TX, October 5−8, 1997; pp 561−572. (24) Ishikawa, M.; Kurashige, Y.; Hirakawa, K. Analysis of crack movements observed in an alpine bedrock cliff. Earth Surf. Processes Landforms 2004, 29 (7), 883−891. (25) Hasler, A.; Gruber, S.; Beutel, J. Kinematics of steep bedrock permafrost. Journal of Geophysical Research: Earth Surface. 2012, 117, F01016. (26) Davidson, G. P.; Nye, J. F. A photoelastic study of ice pressure in rock cracks. Cold Reg. Sci. Technol. 1985, 11 (2), 141−153. (27) Mcgreevy, J. P.; Whalley, W. B. Rock moisture content and frost weathering under natural and experimental conditions: a comparative discussion. Arct. Alp. Res. 1985, 17, 337−346. (28) Matsuoka, N. Mechanisms of rock breakdown by frost action: An experimental approach. Cold Reg. Sci. Technol. 1990, 17 (3), 253− 270. (29) Yao, Y. B.; Liu, D. M.; Cai, Y. D.; Li, J. Q. Advanced characterization of pores and fractures in coals by nuclear magnetic resonance and X-ray computed tomography. Sci. China: Earth Sci. 2010, 53 (6), 854−862. (30) Li, S.; Tang, D. Z.; Pan, Z. J.; Xu, H.; Huang, W. Q. Characterization of the stress sensitivity of pores for different rank coals by nuclear magnetic resonance. Fuel 2013, 111, 746−754. (31) Cai, C. Z.; Li, G. S.; Huang, Z. W.; Shen, Z. H.; Tian, S. C. Rock Pore Structure Damage Due to Freeze During Liquid Nitrogen Fracturing. Arab. J. Sci. Eng. 2014, 39 (12), 9249−9257. (32) Cai, C. Z.; Li, G. S.; Huang, Z. W.; Tian, S. C.; Shen, Z. H.; Fu, X. Experiment of coal damage due to super-cooling with liquid nitrogen. J. Nat. Gas Sci. Eng. 2015, 22, 42−48. (33) Cai, C. Z.; Li, G. S.; Huang, Z. W.; Shen, Z. H.; Tian, S. C.; Wei, J. W. Experimental study of the effect of liquid nitrogen cooling on rock pore structure. J. Nat. Gas Sci. Eng. 2014, 21, 507−517. (34) Mao, L. T.; Hao, N.; An, L. Q.; Chiang, F. P.; Liu, H. B. 3D mapping of carbon dioxide-induced strain in coal using digital volumetric speckle photography technique and X-ray computer tomography. Int. J. Coal Geol. 2015, 147−148, 115−125. (35) Taber, S. The mechanics of frost heaving. J. Geol. 1930, 38, 303−317. (36) Taber, S. Frost heaving. J. Geol. 1929, 37, 428−461. (37) Sass, O. Rock moisture fluctuations during freeze-thaw cycles: Preliminary results from electrical resistivity measurements. Polar Geography. 2004, 28 (1), 13−31. (38) Kenyon, W. E. Petrophysical principles of applications of NMR logging. Log Analyst 1997, 38, 21−43. (39) Kenyon, W. E. Nuclear magnetic resonance as a petrophysical measurement. Nucl. Geophys. 1992, 6, 153−171. (40) Cai, Y. D.; Liu, D. M.; Pan, Z. J.; Yao, Y. B.; Li, J. Q.; Qiu, Y. K. Petrophysical characterization of Chinese coal cores with heat treatment by nuclear magnetic resonance. Fuel 2013, 108, 292−302. (41) Yao, Y. B.; Liu, D. M.; Che, Y.; Tang, D. Z.; Tang, S. H.; Huang, W. H. Petrophysical characterization of coals by low-field nuclear magnetic resonance (NMR). Fuel 2010, 89 (7), 1371−1380. (42) Li, Y.; Tang, D. Z.; Elsworth, D.; Xu, H. Characterization of Coalbed Methane Reservoirs at Multiple Length Scales: A CrossSection from Southeastern Ordos Basin, China. Energy Fuels 2014, 28 (9), 5587−5595. (43) Yao, Y. B.; Liu, D. M.; Tang, D. Z.; Tang, S. H.; Huang, W. H. Fractal characterization of adsorption-pores of coals from North China: An investigation on CH4 adsorption capacity of coals. Int. J. Coal Geol. 2008, 73 (1), 27−42. (44) Khandelwal, M.; Singh, T. N. Correlating static properties of coal measures rocks with P-wave velocity. Int. J. Coal Geol. 2009, 79 (1−2), 55−60.

(45) Song, I.; Suh, M. Effects of foliation and microcracks on ultrasonic anisotropy in retrograde ultramafic and metamorphic rocks at shallow depths. J. Appl. Geophys. 2014, 109, 27−35. (46) Wang, H. C.; Pan, J. N.; Wang, S.; Zhu, H. T. Relationship between macro-fracture density, P-wave velocity, and permeability of coal. J. Appl. Geophys. 2015, 117, 111−117.

L

DOI: 10.1021/acs.energyfuels.6b00920 Energy Fuels XXXX, XXX, XXX−XXX