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The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams
Journal of Natural Gas Science and Engineering xxx (2015) 1e8
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The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams Yiyu Lu a, b, Feng Yang a, b, Zhaolong Ge a, b, *, Shuqi Wang a, b, Qin Wang a a b
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China National and Local Joint Engineering Laboratory of Gas Drainage in Complex Coal Seams, Chongqing University, Chongqing 400044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2015 Received in revised form 22 October 2015 Accepted 23 October 2015 Available online xxx
Hydraulic fracturing is an effective method of increasing the permeability of coal seams, which has gradually come into use in underground coal mines. Fracturing fluid is a key factor affecting the improvement. In this study, the way in which viscoelastic surfactant fracturing fluids affect gas desorption in soft seams was analyzed using the zeta potential and scanning electron microscope (SEM)based methods. The results obtained with the zeta potential method indicated that due to the components of viscoelastic surfactant fracturing fluids, the potential at the coal surface jumped from 16.3 mV to 48 mV, the interaction energy between the fracturing fluid and the coal surface increased, the adsorption potential of the gas decreased and gas desorption increased. The SEM results demonstrated that the number of gas transport channels increased due to the effective dissolution of cement in the acidic viscoelastic surfactant fracturing solution. The cumulative pore volume of the coal samples processed with viscoelastic surfactant fracturing fluids was 0.001 cm3/g, which was 1.8 times the cumulative pore volume of coal samples processed with water. The porosity and connectivity of the coal pores both increased, which was beneficial to gas desorption. Comparative experiments using water and viscoelastic surfactant fracturing fluids were conducted in underground coal mines. The results demonstrated that the amount of gas extracted increased by 26.1% when viscoelastic surfactant fracturing fluids were used. Gas desorption in soft seams was promoted by viscoelastic surfactant fracturing fluids. The new data reported in this study can be used to optimize the use of fluids for underground hydraulic fracturing in Chinese coal mines. © 2015 Elsevier B.V. All rights reserved.
Keywords: Viscoelastic surfactant fracturing fluids Soft seams Gas desorption Porosity
1. Introduction The extraction of coal bed methane (CBM) plays an important role in the development of Chinese energy strategy. Because China is rich in CBM resources with 36 trillion cubic meters available for exploration, exploring and using CBM will alleviate the shortage of natural gas and meet the demand for its safe production in coal mines (Flores, 1998; Zhang et al., 2001). Underground extraction is the main source of CBM in China (Yuan et al., 2013). Various measures have been taken to enhance the amount of gas extracted underground (Zhou et al., 2012; Zhu et al., 2013). Hydraulic fracturing is an effective technology that creates cracks by injecting high-pressure fluids into coal seams. It has gradually come into use
* Corresponding author. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (Z. Ge).
in underground coal mines in China because of its ease of operation and the resulting improvements in seam permeability improvement (Palmer, 2010; Aminto and Olson, 2012). For the low permeability and soft CBM reservoirs in China, the performance of the fracturing fluid is the key factor affecting the improvement of hydraulic fracturing. Guar gum fracturing fluids are difficult to use due to gel breaking and because serious pollution is caused when they remain in a reservoir. These issues counteract the fracturing effect (Wang et al., 2012). Due to the lack of efficient and economical alternatives, water is still widely used in underground fracturing. The problems with water are the high filtration requirements and the poor results (Zhang and Bian, 2014; Dag, 2009). Scholars (Khair et al., 2011; Zhang, 2012) determined that viscoelastic surfactant fracturing fluids enhance the amount of gas extracted well. Using fracturing fluids for coal gas extraction produces fractured channels and affects the adsorption characteristics of coal seams (Barati and Liang, 2014; Zhang et al., 2012). Because of the mechanical properties of soft seams and the lack of proppants,
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Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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hydraulic fracturing cannot create crack channels for gas migration in underground coal mines (Lu et al., 2014; Tian and Zhang, 2015). Affecting the gas desorption using viscoelastic surfactant fracturing fluids is a key factor in gas extraction in soft seams, but has not been studied extensively. Therefore, the widespread use of viscoelastic surfactant fracturing fluids in soft seams is limited. The interactions between fracturing fluids and coal seams have been studied in recent years. Mather et al. found that viscoelastic surfactant fracturing fluids can improve coal seam permeability based on tests such fluids in low-permeability coal seams in comparison with conventional fracturing fluids (Mather, 2000). Li et al. conducted a field test using viscoelastic surfactant fracturing fluids in the coal-bed gas wells in the Qinshui Basin in China (Li et al., 2012). They found that viscoelastic surfactant fracturing fluids performed well at creating cracks and preventing the expansion of clay in coal seams. Wang et al. investigated the influence of fluids on the seam permeability and found that permeability of coal samples containing cracks was more strongly influenced by the performance of fracturing fluids (Wang et al., 2011). Lu et al. determined viscoelastic surfactant fracturing fluids could increase the size of gas migration channels and improve the permeability of coal seams because their surface tension was lower than that of water (Lu et al., 2015). Roman reported that the surfactant in viscoelastic surfactant fracturing fluids was able to adsorb onto coal surfaces and, therefore, affected the adsorption characteristics of coal (Roman and Boleslav, 2010). Chen et al. performed adsorption experiments and concluded that viscoelastic surfactant fracturing fluids restrained gas adsorption (Chen et al., 2009). At present, hydraulic fracturing is primarily used to create cracks in coal seams. Most of the theoretical studies on the improvement in permeability have focused on how the fracturing fluid affected the generation and propagation of cracks in coal seams; there have been few studies of the enhancement of gas desorption in soft coal seams (Lzadi et al., 2011). Because gas desorption is a key factor in the amount of gas extracted, the theory of how gas desorption in soft seams is affected by viscoelastic surfactant fracturing fluids should be studied. There are many approaches that can be used to study the influence of fracturing fluids on soft seams. Currently, the zeta potential method is widely used in coal floatation (Jiang et al., 2011). It has been proven to effectively reflect changes in the adsorption characteristics of coal surfaces. Scanning electron microscopes (SEMs) are also commonly used because they allow visual observations at the nanometer scale (Gosiewska et al., 2002). As a result, pore structure changes can be studied intuitively using SEM-based methods. In this study, the viscoelastic surfactant fracturing fluid used was a mixture of CTAC, NaSal and KCl that has been used for hydraulic fracturing in coal mines. To investigate the influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, we determined the changes in the adsorption characteristics of coal surfaces using the zeta potential method and studied changes in the pore structures of coal samples using a SEM-based method and a porosity test. Then, we performed underground hydraulic fracturing experiments with water and viscoelastic surfactant fracturing fluids at the Yuyang coal mine in Chongqing, China to verify the results. The mechanism of gas desorption enhancement in soft seams using viscoelastic surfactant fracturing fluids was discussed, and the results provide fundamental data to promote field use of viscoelastic surfactant fracturing fluids. 2. Methodology The zeta potential is the potential of the shear plane of dispersed particles. A change in its value directly reflects a change in the
adsorption characteristics of the particle surfaces (Ren et al., 2015; Idrissa et al., 2015). Because coal adsorbs well, coal surfaces and their surrounding media have zeta potentials. To determine the effects of water and viscoelastic surfactant fracturing fluids on coal adsorption characteristics, we measure the zeta potentials of coal particles and compare them to determine the effects of the two liquids on coal surfaces. Therefore, the effects of viscoelastic surfactant fracturing fluids on the adsorption characteristics of coal samples are reflected by changes in the surface potential. The gas desorption of coal seams are affected by pore structure and porosity. As the coal porosity increased, the gas migration rate accelerated and the free gas content increased. Gas desorption was promoted. This can be described using Equation (1) (Zhou and Lin, 1997):
X¼
VpT0 Tp0 ε
(1)
where X [m3/t] is the free gas content, V [m3/t] is the pore volume per unit mass of coal, p [MPa] is the gas pressure, T0 [K] and p0 [MPa] are the temperature and pressure under standard conditions, respectively, T [K] is the gas temperature, and ε is the gas compression factor. So we studied changes in the pore structures of coal samples using a SEM-based method and a porosity test. In the field procedure, experiments with water and viscoelastic surfactant fracturing fluids were performed underground a coal mine with soft seams. We recorded the volume of gas extracted after the experiments to verify the results for the gas desorption. 3. Experiments 3.1. The zeta potential test The materials for the zeta potential test were N-Hexadecyltrimethylammonium Chloride (C19H42ClN, CTAC), sodium salicylate (C7H5NaO3, Nasal), potassium chloride (KCl), hydrochloric acid (HCl), sodium hydroxide (NaOH), pure water, and Yuyang coal samples. Fig. 1 shows the zeta test instrument (ZetasizerNano ZS90, British Malvern Instrument Ltd) and its principles of operation. The coal samples should be pulverized down below 200 mesh and dried for 24 h at 100 C for measurement using the electrophoresis method (Milad et al., 2015). Then, the container was subjected to a vacuum to remove the gas adsorbed by the coal samples. Subsequently, the pure water and viscoelastic surfactant fracturing fluids were injected into the container. The PH was found to be 7 and regulated by the hydrochloric acid and sodium hydroxide in the presence of viscoelastic surfactant fracturing fluids. When the solution had been stirred for 48 h, the zeta potential analyzer was used to measure the zeta potential of the supernatant removed from the container. To ensure the accuracy of data, the tests were conducted repeatedly and questionable points were eliminated from the average value. 3.2. SEM observations and porosity tests of the coal samples To compare the differences between the effects of the two fracturing fluids on the porosity of coal samples, coal samples were separated from a lump of fresh coal and added to the distilled water and viscoelastic surfactant fracturing fluids respectively. Then, the coal samples were soaked for 48 h and dried for 24 h in the oven. Finally, the porosity of these samples was observed and analyzed using a field emission scanning electron microscope (Field Emission Gun Scanning Election Microscope Nova400, German Zeiss Company (resolution: 1.0 nm)). The cumulative pore volumes of the
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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Fig. 1. The potential test instrument and its principles of operation.
coal samples processed with water and viscoelastic surfactant fracturing fluids were determined using an ASAP2020 micropore analyzer (Micromeritics Instrument Company, USA) with a pore diameter range from 1.5 nm to 500 nm. 3.3. A comparison of water and viscoelastic surfactant fracturing fluids for use in underground coal mines The experiment was conducted on the N3702 coal face of the M7 coal seam in mining area N3 of the Yuyang coal mine, as shown in Fig. 2. The gas content and pressure in the coal seam were 21.2 m3/t and 1.99 MPa, respectively. The permeability and tightness coefficients were 0.0025 m2/(MPa2$d) and 0.3, respectively (Huang and Yang, 2009). The Yuyang coal mine is a soft coal mine with outbursts, and it is difficult to drain gas from it. The experimental device consisted of a coal seam infusion pump, a water tank, a console, a set of control systems, a highpressure hose, a high-pressure connection, and related components such as device connectors. The infusion pump was a BRW 200/31.5 emulsification pump, and the water tank was an SX-1600 water tank. The high-pressure hose had a pressure of 40 MPa and the amount of water in the water tank was ensured by a 50 mm supply tube. Fig. 3 shows all of the connections between the components and the connection methods. The experiment was conducted at a location close to the railway lane of the N3702 coal face. To avoid contingency emergencies during the experiment, 12 experimental holes were drilled on the eastern and western sides of the railway lane. Between 1 and 6 hydraulic fracturing holes were drilled on the eastern side for the hydraulic fracturing tests using water, and 7e12 holes were drilled on the western side for the hydraulic fracturing tests using
viscoelastic surfactant fracturing fluids. As shown in Fig. 4, the distance between two holes was 20 m and the holes were 60 m deep. After the holes were drilled, the devices and the equipment were connected and tested, as shown in Fig. 3. The tests were conducted once the air tightness of each device had been confirmed. The test pressure was set to 6 MPa and the volume injected into each hole was 2 tons. Because the formation properties and geological structure could lead to the accidental comparison of extractions from single holes, we used a statistical method and increased the number of extraction holes to ensure the accuracy of the data. Therefore, gas drainage holes were arranged at 5 m intervals between the fracturing holes until the fracturing experiment was completed. There were 23 gas drainage holes (including 6 fracturing holes) on each side of the arrangement; these were marked as drainage holes for water and viscoelastic surfactant fracturing fluids, as shown in Fig. 5. The negative drainage pressure was 13 KPa, a value commonly used in underground coal mines. Gas flowmeters were installed and used to calculate the volume of gas extracted. 4. Results and discussion 4.1. The influence of viscoelastic surfactant fracturing fluids on the gas desorption characteristics of soft seams Fig. 6 presents the zeta potential values of coal particles obtained through the use of viscoelastic surfactant fracturing fluids and water. As seen from the results, the average zeta potential of coal particles in water is 16.3 mV, but for viscoelastic surfactant
Fig. 2. The site of the experiment.
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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Fig. 3. The experimental system.
Fig. 4. A schematic of the experimental drilling.
Fig. 5. Arrangement of the gas drainage boreholes.
fracturing fluids, the value is 48 mV. This means that the surfaces of coal particles in water carry negative charges, which is consistent with the results reported in the literature (Cai et al., 2010). Li and Nie (2006) reported that the adsorption of water onto coal is primarily caused by the interaction between the surface of the coal and hydromel. For viscoelastic surfactant fracturing fluids, the electrostatic forces between the solution and the coal particles, which contain negative charges, increases obviously due to the presence of an active surfactant; this increase makes it easier for this solution to adsorb onto the surface of a coal sample than it is for water. However, zeta potential of water and coal particles was
negative, while that of the viscoelastic surfactant fracturing fluids was positive, which means that the coal sample surface adsorbed the cationic surfactant. Due to the adsorption characteristics of the coal sample surface, the effective adsorption potential of methane molecules reduced the competitive adsorption, resulting in a reduced adsorption capacity of the coal seam for gas, which improves the desorption for gas drainage. The experiments on gas adsorption were conducted using original coal samples, coal samples processed with water and coal samples processed with viscoelastic surfactant fracturing fluids described in the literature (Chen et al., 2009). The adsorption curves are shown in Fig. 7, under
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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organic hydrochloric acid that contains both a hydroxyl group and a carboxyl group. The alkaline hydrolysis activity of the solution is weaker than its acidity, which results from the ionization of its hydroxyl group. The ionization mode is shown in Equation (2).
(2)
Fig. 6. The zeta potential of coal particles in viscoelastic surfactant fracturing fluids and water.
Fig. 7. A comparison of gas adsorbed by original coal samples and coal samples processed with water and viscoelastic surfactant fracturing fluids.
identical conditions, the gas adsorption capacity of the original coal samples is greater than that of the samples processed with water and with viscoelastic surfactant fracturing fluids, which means that the adsorption sites of the coal samples are occupied by the viscoelastic surfactant fracturing fluids due to competition. The viscoelastic surfactant fracturing fluids reduce the adsorptivity of the coal and promote gas desorption and extraction in coal seams. These results agree well with the observations reported in this paper. 4.2. The influence of viscoelastic surfactant fracturing fluids on coal porosity Coal is a complex mixture resulting from a long-term geological formation process. Chen and Zhang (2010) studied components of soft coal seams in China and found that, in addition to carboncontaining organic matter, the primary components of coal are clay and carbonate rocks containing calcium oxide, aluminum oxide and other components. These substances not only reduce the quality and value of coal but also clog the pores, hindering gas migration and reducing the amount of CBM extracted (Liu et al., 2014). The primary components of viscoelastic surfactant fracturing fluids are CTAC, NaSal, and KCl. Sodium salicylate is a special
In an acidic environment, oxides and carbonate rocks of clay minerals react to generate water-soluble compounds (see Equations (3)e(5)) and remove the impurities clogging the pores. At the same time, because the surface-active agent gathers at the surface of the solution, the effect of contact between the solution and the clogging substance increases, which makes it easier to expel clogging impurities from pores and improves the porosity and pore connectivity, increasing the drainage of coal seam gas and the gas desorption.
Fe2 O3 þ H þ /Fe3þ þ H2 O
(3)
CaO þ H þ /Ca2þ þ H2 O
(4)
þ CO2 3 þ H /CO2 þ H2 O
(5)
As shown in Fig. 8, comparing the SEM pictures of coal samples processed with water and viscoelastic surfactant fracturing fluids, the images of the surfaces of the coal samples under different magnifications show that the surfaces of coal samples processed with water are less porous and subject to pore clogging; in contrast, the surfaces of coal samples processed with viscoelastic surfactant fracturing fluids obviously have more pores, which are not subject to clogging; this implies that clean coal fracturing fluids improve the porosity and connectivity. Fig. 9 shows the pore volume of coal samples processed with water and viscoelastic surfactant fracturing fluids. From this figure, we determine that the cumulative pore volume of the coal samples processed with viscoelastic surfactant fracturing fluids is 0.001 cm3/g, which is 1.8 times of the cumulative pore volume of the coal samples processed with water. A pore diameter that is greater than 50 nm results in a greater pore volume in coal processed with viscoelastic surfactant fracturing fluids; this could ease gas migration and improve gas desorption. These results are consistent with those of the theoretical analysis. 4.3. A comparison of the amount of gas extracted using water and viscoelastic surfactant fracturing fluids As shown in Table 1, data on the amount of gas extracted from both sides of the drainage boreholes are obtained statistically every ten days and marked as Q10 to Q60. The average amount of gas extracted from a single hole can be calculated by Equation (6).
Xn ¼
n X Qi ðn ¼ 10; 20…60Þ 23 i¼10
(6)
where Xn [m3] is the average amount of gas extracted from a single hole, n [d] is the extraction time. The average amount of gas extracted from a single hole in 60 days is determined and shown in Fig. 10. Compared to the results of the water-based test, the cumulative quantity extracted is increased by 26.1%, indicating that viscoelastic surfactant fracturing fluids effectively improve gas desorption and extraction. Under average single-hole gas drainage conditions, an
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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Fig. 8. Electron microscope images of coal samples tested with water and viscoelastic surfactant fracturing fluids: (a1) 1000 times magnification of coal samples processed with water; (a2) 1000 times magnification of coal samples processed with viscoelastic surfactant fracturing fluids; (b1) 5000 times magnification of coal samples processed with water; (b2) 5000 times magnification of coal samples processed with viscoelastic surfactant fracturing fluids.
affected differently by the fracturing fluids. The results of this comparison showed that viscoelastic surfactant fracturing fluids effectively enhanced gas desorption and increased gas migration in soft seams. In relevant laboratory experiments, viscoelastic surfactant fracturing fluids dissolved and removed substances that clogged coal pores, such as clay and carbonates, improving the porosity and connectivity. At the same time, viscoelastic surfactant fracturing fluids increased the amount of interaction with the coal surfaces and produced the adsorption characteristics of the coal surface. These changes promote the displacement of gas in coal pores and gas desorption in coal seam surfaces. Overall, the experiment verified that viscoelastic surfactant fracturing fluids enhanced gas desorption in soft seams due to the interactions of several of their features. Fig. 9. The pore volume of coal samples tested with water and viscoelastic surfactant fracturing fluids.
increase in the extraction time corresponds to a decrease in the rate of gas extracted using water that is larger than that for gas extracted using viscoelastic surfactant fracturing fluids. Because no proppants are used in the hydraulic fracturing of soft seams in underground coal mines, it is difficult to form cracks for gas migration. The positive effect of viscoelastic surfactant fracturing fluids on coal gas extraction observed in this study is because the seam adsorption characteristics and pore structures were
5. Conclusions The abilities of water and viscoelastic surfactant fracturing fluids to enhance gas desorption in soft coal seams were compared in this study. With viscoelastic surfactant fracturing fluids, the interaction energy between the fracturing fluid and the coal surface was greater, the validity adsorption potential of the gas decreased due to competitive adsorption and gas desorption was promoted. Due to the dissolution of cements and capillary forces, viscoelastic surfactant fracturing fluids also increased the porosity and connectivity. The cumulative pore volume of the coal samples
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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Table 1 Gas drainage statistics. Drainage time (d)
10 20 30 40 50 60
Drainage scalar (m3) Drainage holes for water
Drainage holes for viscoelastic surfactant fracturing fluids
15,870 8970 6210 4140 3450 2760
19,320 10,120 7130 6210 5060 4370
Fig. 10. The average amount of gas extracted from a single drainage borehole.
processed with viscoelastic surfactant fracturing fluids was 0.001 cm3/g, which was 1.8 times the cumulative pore volume of coal samples processed with water. This improvement in the cumulative pore volume was beneficial to gas desorption. Experiments conducted under the coal mine showed that using viscoelastic surfactant fracturing fluids increased the amount of gas extracted by 26.1%. This verified the theoretical results and proved that viscoelastic surfactant fracturing fluids are more effective at enhancing gas extraction in soft seams than water. Acknowledgments This study was funded by the National Science and Technology Major Projects of China (No. 2011ZX05065), the National Natural Science Foundation of China (No. 51374258), and the Program for Changjiang Scholars and Innovative Research Team in University of China (No. IRT13043). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jngse.2015.10.031. References Aminto, A., Olson, M.S., 2012. Four-compartment partition model of hazardous components in hydraulic fracturing fluid additives. J. Nat. Gas Sci. Eng. 7, 16e21. Barati, R., Liang, J.T., 2014. A review of fracturing fluid systems used for hydraulic fracturing of oil and gas wells. J. Appl. Polymer Sci. 131 (16), 1002e1013. Cai, C.F., Zheng, X.Q., Tang, C.G., Gao, Q., 2010. Effect of major pollutants on floatability of coal in coking wastewater and its mechanism analysis. J. China Coal Soc. 6, 1002e1008. Chen, F., Zhang, J., 2010. Study of clay minerals in coal in Guizhou Xingren area. Modern Min. 5, 51e54. Chen, S.B., Zhu, Y.M., Liu, T.Y., Zhang, C., Yang, H., 2009. Impact of the clear fracturing fluid on the adsorption properties of CBM. J. China Coal Soc. 1, 89e94.
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Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams Yiyu Lu a, b, Feng Yang a, b, Zhaolong Ge a, b, *, Shuqi Wang a, b, Qin Wang a a b
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China National and Local Joint Engineering Laboratory of Gas Drainage in Complex Coal Seams, Chongqing University, Chongqing 400044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2015 Received in revised form 22 October 2015 Accepted 23 October 2015 Available online xxx
Hydraulic fracturing is an effective method of increasing the permeability of coal seams, which has gradually come into use in underground coal mines. Fracturing fluid is a key factor affecting the improvement. In this study, the way in which viscoelastic surfactant fracturing fluids affect gas desorption in soft seams was analyzed using the zeta potential and scanning electron microscope (SEM)based methods. The results obtained with the zeta potential method indicated that due to the components of viscoelastic surfactant fracturing fluids, the potential at the coal surface jumped from 16.3 mV to 48 mV, the interaction energy between the fracturing fluid and the coal surface increased, the adsorption potential of the gas decreased and gas desorption increased. The SEM results demonstrated that the number of gas transport channels increased due to the effective dissolution of cement in the acidic viscoelastic surfactant fracturing solution. The cumulative pore volume of the coal samples processed with viscoelastic surfactant fracturing fluids was 0.001 cm3/g, which was 1.8 times the cumulative pore volume of coal samples processed with water. The porosity and connectivity of the coal pores both increased, which was beneficial to gas desorption. Comparative experiments using water and viscoelastic surfactant fracturing fluids were conducted in underground coal mines. The results demonstrated that the amount of gas extracted increased by 26.1% when viscoelastic surfactant fracturing fluids were used. Gas desorption in soft seams was promoted by viscoelastic surfactant fracturing fluids. The new data reported in this study can be used to optimize the use of fluids for underground hydraulic fracturing in Chinese coal mines. © 2015 Elsevier B.V. All rights reserved.
Keywords: Viscoelastic surfactant fracturing fluids Soft seams Gas desorption Porosity
1. Introduction The extraction of coal bed methane (CBM) plays an important role in the development of Chinese energy strategy. Because China is rich in CBM resources with 36 trillion cubic meters available for exploration, exploring and using CBM will alleviate the shortage of natural gas and meet the demand for its safe production in coal mines (Flores, 1998; Zhang et al., 2001). Underground extraction is the main source of CBM in China (Yuan et al., 2013). Various measures have been taken to enhance the amount of gas extracted underground (Zhou et al., 2012; Zhu et al., 2013). Hydraulic fracturing is an effective technology that creates cracks by injecting high-pressure fluids into coal seams. It has gradually come into use
* Corresponding author. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (Z. Ge).
in underground coal mines in China because of its ease of operation and the resulting improvements in seam permeability improvement (Palmer, 2010; Aminto and Olson, 2012). For the low permeability and soft CBM reservoirs in China, the performance of the fracturing fluid is the key factor affecting the improvement of hydraulic fracturing. Guar gum fracturing fluids are difficult to use due to gel breaking and because serious pollution is caused when they remain in a reservoir. These issues counteract the fracturing effect (Wang et al., 2012). Due to the lack of efficient and economical alternatives, water is still widely used in underground fracturing. The problems with water are the high filtration requirements and the poor results (Zhang and Bian, 2014; Dag, 2009). Scholars (Khair et al., 2011; Zhang, 2012) determined that viscoelastic surfactant fracturing fluids enhance the amount of gas extracted well. Using fracturing fluids for coal gas extraction produces fractured channels and affects the adsorption characteristics of coal seams (Barati and Liang, 2014; Zhang et al., 2012). Because of the mechanical properties of soft seams and the lack of proppants,
http://dx.doi.org/10.1016/j.jngse.2015.10.031 1875-5100/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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hydraulic fracturing cannot create crack channels for gas migration in underground coal mines (Lu et al., 2014; Tian and Zhang, 2015). Affecting the gas desorption using viscoelastic surfactant fracturing fluids is a key factor in gas extraction in soft seams, but has not been studied extensively. Therefore, the widespread use of viscoelastic surfactant fracturing fluids in soft seams is limited. The interactions between fracturing fluids and coal seams have been studied in recent years. Mather et al. found that viscoelastic surfactant fracturing fluids can improve coal seam permeability based on tests such fluids in low-permeability coal seams in comparison with conventional fracturing fluids (Mather, 2000). Li et al. conducted a field test using viscoelastic surfactant fracturing fluids in the coal-bed gas wells in the Qinshui Basin in China (Li et al., 2012). They found that viscoelastic surfactant fracturing fluids performed well at creating cracks and preventing the expansion of clay in coal seams. Wang et al. investigated the influence of fluids on the seam permeability and found that permeability of coal samples containing cracks was more strongly influenced by the performance of fracturing fluids (Wang et al., 2011). Lu et al. determined viscoelastic surfactant fracturing fluids could increase the size of gas migration channels and improve the permeability of coal seams because their surface tension was lower than that of water (Lu et al., 2015). Roman reported that the surfactant in viscoelastic surfactant fracturing fluids was able to adsorb onto coal surfaces and, therefore, affected the adsorption characteristics of coal (Roman and Boleslav, 2010). Chen et al. performed adsorption experiments and concluded that viscoelastic surfactant fracturing fluids restrained gas adsorption (Chen et al., 2009). At present, hydraulic fracturing is primarily used to create cracks in coal seams. Most of the theoretical studies on the improvement in permeability have focused on how the fracturing fluid affected the generation and propagation of cracks in coal seams; there have been few studies of the enhancement of gas desorption in soft coal seams (Lzadi et al., 2011). Because gas desorption is a key factor in the amount of gas extracted, the theory of how gas desorption in soft seams is affected by viscoelastic surfactant fracturing fluids should be studied. There are many approaches that can be used to study the influence of fracturing fluids on soft seams. Currently, the zeta potential method is widely used in coal floatation (Jiang et al., 2011). It has been proven to effectively reflect changes in the adsorption characteristics of coal surfaces. Scanning electron microscopes (SEMs) are also commonly used because they allow visual observations at the nanometer scale (Gosiewska et al., 2002). As a result, pore structure changes can be studied intuitively using SEM-based methods. In this study, the viscoelastic surfactant fracturing fluid used was a mixture of CTAC, NaSal and KCl that has been used for hydraulic fracturing in coal mines. To investigate the influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, we determined the changes in the adsorption characteristics of coal surfaces using the zeta potential method and studied changes in the pore structures of coal samples using a SEM-based method and a porosity test. Then, we performed underground hydraulic fracturing experiments with water and viscoelastic surfactant fracturing fluids at the Yuyang coal mine in Chongqing, China to verify the results. The mechanism of gas desorption enhancement in soft seams using viscoelastic surfactant fracturing fluids was discussed, and the results provide fundamental data to promote field use of viscoelastic surfactant fracturing fluids. 2. Methodology The zeta potential is the potential of the shear plane of dispersed particles. A change in its value directly reflects a change in the
adsorption characteristics of the particle surfaces (Ren et al., 2015; Idrissa et al., 2015). Because coal adsorbs well, coal surfaces and their surrounding media have zeta potentials. To determine the effects of water and viscoelastic surfactant fracturing fluids on coal adsorption characteristics, we measure the zeta potentials of coal particles and compare them to determine the effects of the two liquids on coal surfaces. Therefore, the effects of viscoelastic surfactant fracturing fluids on the adsorption characteristics of coal samples are reflected by changes in the surface potential. The gas desorption of coal seams are affected by pore structure and porosity. As the coal porosity increased, the gas migration rate accelerated and the free gas content increased. Gas desorption was promoted. This can be described using Equation (1) (Zhou and Lin, 1997):
X¼
VpT0 Tp0 ε
(1)
where X [m3/t] is the free gas content, V [m3/t] is the pore volume per unit mass of coal, p [MPa] is the gas pressure, T0 [K] and p0 [MPa] are the temperature and pressure under standard conditions, respectively, T [K] is the gas temperature, and ε is the gas compression factor. So we studied changes in the pore structures of coal samples using a SEM-based method and a porosity test. In the field procedure, experiments with water and viscoelastic surfactant fracturing fluids were performed underground a coal mine with soft seams. We recorded the volume of gas extracted after the experiments to verify the results for the gas desorption. 3. Experiments 3.1. The zeta potential test The materials for the zeta potential test were N-Hexadecyltrimethylammonium Chloride (C19H42ClN, CTAC), sodium salicylate (C7H5NaO3, Nasal), potassium chloride (KCl), hydrochloric acid (HCl), sodium hydroxide (NaOH), pure water, and Yuyang coal samples. Fig. 1 shows the zeta test instrument (ZetasizerNano ZS90, British Malvern Instrument Ltd) and its principles of operation. The coal samples should be pulverized down below 200 mesh and dried for 24 h at 100 C for measurement using the electrophoresis method (Milad et al., 2015). Then, the container was subjected to a vacuum to remove the gas adsorbed by the coal samples. Subsequently, the pure water and viscoelastic surfactant fracturing fluids were injected into the container. The PH was found to be 7 and regulated by the hydrochloric acid and sodium hydroxide in the presence of viscoelastic surfactant fracturing fluids. When the solution had been stirred for 48 h, the zeta potential analyzer was used to measure the zeta potential of the supernatant removed from the container. To ensure the accuracy of data, the tests were conducted repeatedly and questionable points were eliminated from the average value. 3.2. SEM observations and porosity tests of the coal samples To compare the differences between the effects of the two fracturing fluids on the porosity of coal samples, coal samples were separated from a lump of fresh coal and added to the distilled water and viscoelastic surfactant fracturing fluids respectively. Then, the coal samples were soaked for 48 h and dried for 24 h in the oven. Finally, the porosity of these samples was observed and analyzed using a field emission scanning electron microscope (Field Emission Gun Scanning Election Microscope Nova400, German Zeiss Company (resolution: 1.0 nm)). The cumulative pore volumes of the
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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Fig. 1. The potential test instrument and its principles of operation.
coal samples processed with water and viscoelastic surfactant fracturing fluids were determined using an ASAP2020 micropore analyzer (Micromeritics Instrument Company, USA) with a pore diameter range from 1.5 nm to 500 nm. 3.3. A comparison of water and viscoelastic surfactant fracturing fluids for use in underground coal mines The experiment was conducted on the N3702 coal face of the M7 coal seam in mining area N3 of the Yuyang coal mine, as shown in Fig. 2. The gas content and pressure in the coal seam were 21.2 m3/t and 1.99 MPa, respectively. The permeability and tightness coefficients were 0.0025 m2/(MPa2$d) and 0.3, respectively (Huang and Yang, 2009). The Yuyang coal mine is a soft coal mine with outbursts, and it is difficult to drain gas from it. The experimental device consisted of a coal seam infusion pump, a water tank, a console, a set of control systems, a highpressure hose, a high-pressure connection, and related components such as device connectors. The infusion pump was a BRW 200/31.5 emulsification pump, and the water tank was an SX-1600 water tank. The high-pressure hose had a pressure of 40 MPa and the amount of water in the water tank was ensured by a 50 mm supply tube. Fig. 3 shows all of the connections between the components and the connection methods. The experiment was conducted at a location close to the railway lane of the N3702 coal face. To avoid contingency emergencies during the experiment, 12 experimental holes were drilled on the eastern and western sides of the railway lane. Between 1 and 6 hydraulic fracturing holes were drilled on the eastern side for the hydraulic fracturing tests using water, and 7e12 holes were drilled on the western side for the hydraulic fracturing tests using
viscoelastic surfactant fracturing fluids. As shown in Fig. 4, the distance between two holes was 20 m and the holes were 60 m deep. After the holes were drilled, the devices and the equipment were connected and tested, as shown in Fig. 3. The tests were conducted once the air tightness of each device had been confirmed. The test pressure was set to 6 MPa and the volume injected into each hole was 2 tons. Because the formation properties and geological structure could lead to the accidental comparison of extractions from single holes, we used a statistical method and increased the number of extraction holes to ensure the accuracy of the data. Therefore, gas drainage holes were arranged at 5 m intervals between the fracturing holes until the fracturing experiment was completed. There were 23 gas drainage holes (including 6 fracturing holes) on each side of the arrangement; these were marked as drainage holes for water and viscoelastic surfactant fracturing fluids, as shown in Fig. 5. The negative drainage pressure was 13 KPa, a value commonly used in underground coal mines. Gas flowmeters were installed and used to calculate the volume of gas extracted. 4. Results and discussion 4.1. The influence of viscoelastic surfactant fracturing fluids on the gas desorption characteristics of soft seams Fig. 6 presents the zeta potential values of coal particles obtained through the use of viscoelastic surfactant fracturing fluids and water. As seen from the results, the average zeta potential of coal particles in water is 16.3 mV, but for viscoelastic surfactant
Fig. 2. The site of the experiment.
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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Fig. 3. The experimental system.
Fig. 4. A schematic of the experimental drilling.
Fig. 5. Arrangement of the gas drainage boreholes.
fracturing fluids, the value is 48 mV. This means that the surfaces of coal particles in water carry negative charges, which is consistent with the results reported in the literature (Cai et al., 2010). Li and Nie (2006) reported that the adsorption of water onto coal is primarily caused by the interaction between the surface of the coal and hydromel. For viscoelastic surfactant fracturing fluids, the electrostatic forces between the solution and the coal particles, which contain negative charges, increases obviously due to the presence of an active surfactant; this increase makes it easier for this solution to adsorb onto the surface of a coal sample than it is for water. However, zeta potential of water and coal particles was
negative, while that of the viscoelastic surfactant fracturing fluids was positive, which means that the coal sample surface adsorbed the cationic surfactant. Due to the adsorption characteristics of the coal sample surface, the effective adsorption potential of methane molecules reduced the competitive adsorption, resulting in a reduced adsorption capacity of the coal seam for gas, which improves the desorption for gas drainage. The experiments on gas adsorption were conducted using original coal samples, coal samples processed with water and coal samples processed with viscoelastic surfactant fracturing fluids described in the literature (Chen et al., 2009). The adsorption curves are shown in Fig. 7, under
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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organic hydrochloric acid that contains both a hydroxyl group and a carboxyl group. The alkaline hydrolysis activity of the solution is weaker than its acidity, which results from the ionization of its hydroxyl group. The ionization mode is shown in Equation (2).
(2)
Fig. 6. The zeta potential of coal particles in viscoelastic surfactant fracturing fluids and water.
Fig. 7. A comparison of gas adsorbed by original coal samples and coal samples processed with water and viscoelastic surfactant fracturing fluids.
identical conditions, the gas adsorption capacity of the original coal samples is greater than that of the samples processed with water and with viscoelastic surfactant fracturing fluids, which means that the adsorption sites of the coal samples are occupied by the viscoelastic surfactant fracturing fluids due to competition. The viscoelastic surfactant fracturing fluids reduce the adsorptivity of the coal and promote gas desorption and extraction in coal seams. These results agree well with the observations reported in this paper. 4.2. The influence of viscoelastic surfactant fracturing fluids on coal porosity Coal is a complex mixture resulting from a long-term geological formation process. Chen and Zhang (2010) studied components of soft coal seams in China and found that, in addition to carboncontaining organic matter, the primary components of coal are clay and carbonate rocks containing calcium oxide, aluminum oxide and other components. These substances not only reduce the quality and value of coal but also clog the pores, hindering gas migration and reducing the amount of CBM extracted (Liu et al., 2014). The primary components of viscoelastic surfactant fracturing fluids are CTAC, NaSal, and KCl. Sodium salicylate is a special
In an acidic environment, oxides and carbonate rocks of clay minerals react to generate water-soluble compounds (see Equations (3)e(5)) and remove the impurities clogging the pores. At the same time, because the surface-active agent gathers at the surface of the solution, the effect of contact between the solution and the clogging substance increases, which makes it easier to expel clogging impurities from pores and improves the porosity and pore connectivity, increasing the drainage of coal seam gas and the gas desorption.
Fe2 O3 þ H þ /Fe3þ þ H2 O
(3)
CaO þ H þ /Ca2þ þ H2 O
(4)
þ CO2 3 þ H /CO2 þ H2 O
(5)
As shown in Fig. 8, comparing the SEM pictures of coal samples processed with water and viscoelastic surfactant fracturing fluids, the images of the surfaces of the coal samples under different magnifications show that the surfaces of coal samples processed with water are less porous and subject to pore clogging; in contrast, the surfaces of coal samples processed with viscoelastic surfactant fracturing fluids obviously have more pores, which are not subject to clogging; this implies that clean coal fracturing fluids improve the porosity and connectivity. Fig. 9 shows the pore volume of coal samples processed with water and viscoelastic surfactant fracturing fluids. From this figure, we determine that the cumulative pore volume of the coal samples processed with viscoelastic surfactant fracturing fluids is 0.001 cm3/g, which is 1.8 times of the cumulative pore volume of the coal samples processed with water. A pore diameter that is greater than 50 nm results in a greater pore volume in coal processed with viscoelastic surfactant fracturing fluids; this could ease gas migration and improve gas desorption. These results are consistent with those of the theoretical analysis. 4.3. A comparison of the amount of gas extracted using water and viscoelastic surfactant fracturing fluids As shown in Table 1, data on the amount of gas extracted from both sides of the drainage boreholes are obtained statistically every ten days and marked as Q10 to Q60. The average amount of gas extracted from a single hole can be calculated by Equation (6).
Xn ¼
n X Qi ðn ¼ 10; 20…60Þ 23 i¼10
(6)
where Xn [m3] is the average amount of gas extracted from a single hole, n [d] is the extraction time. The average amount of gas extracted from a single hole in 60 days is determined and shown in Fig. 10. Compared to the results of the water-based test, the cumulative quantity extracted is increased by 26.1%, indicating that viscoelastic surfactant fracturing fluids effectively improve gas desorption and extraction. Under average single-hole gas drainage conditions, an
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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Fig. 8. Electron microscope images of coal samples tested with water and viscoelastic surfactant fracturing fluids: (a1) 1000 times magnification of coal samples processed with water; (a2) 1000 times magnification of coal samples processed with viscoelastic surfactant fracturing fluids; (b1) 5000 times magnification of coal samples processed with water; (b2) 5000 times magnification of coal samples processed with viscoelastic surfactant fracturing fluids.
affected differently by the fracturing fluids. The results of this comparison showed that viscoelastic surfactant fracturing fluids effectively enhanced gas desorption and increased gas migration in soft seams. In relevant laboratory experiments, viscoelastic surfactant fracturing fluids dissolved and removed substances that clogged coal pores, such as clay and carbonates, improving the porosity and connectivity. At the same time, viscoelastic surfactant fracturing fluids increased the amount of interaction with the coal surfaces and produced the adsorption characteristics of the coal surface. These changes promote the displacement of gas in coal pores and gas desorption in coal seam surfaces. Overall, the experiment verified that viscoelastic surfactant fracturing fluids enhanced gas desorption in soft seams due to the interactions of several of their features. Fig. 9. The pore volume of coal samples tested with water and viscoelastic surfactant fracturing fluids.
increase in the extraction time corresponds to a decrease in the rate of gas extracted using water that is larger than that for gas extracted using viscoelastic surfactant fracturing fluids. Because no proppants are used in the hydraulic fracturing of soft seams in underground coal mines, it is difficult to form cracks for gas migration. The positive effect of viscoelastic surfactant fracturing fluids on coal gas extraction observed in this study is because the seam adsorption characteristics and pore structures were
5. Conclusions The abilities of water and viscoelastic surfactant fracturing fluids to enhance gas desorption in soft coal seams were compared in this study. With viscoelastic surfactant fracturing fluids, the interaction energy between the fracturing fluid and the coal surface was greater, the validity adsorption potential of the gas decreased due to competitive adsorption and gas desorption was promoted. Due to the dissolution of cements and capillary forces, viscoelastic surfactant fracturing fluids also increased the porosity and connectivity. The cumulative pore volume of the coal samples
Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031
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Table 1 Gas drainage statistics. Drainage time (d)
10 20 30 40 50 60
Drainage scalar (m3) Drainage holes for water
Drainage holes for viscoelastic surfactant fracturing fluids
15,870 8970 6210 4140 3450 2760
19,320 10,120 7130 6210 5060 4370
Fig. 10. The average amount of gas extracted from a single drainage borehole.
processed with viscoelastic surfactant fracturing fluids was 0.001 cm3/g, which was 1.8 times the cumulative pore volume of coal samples processed with water. This improvement in the cumulative pore volume was beneficial to gas desorption. Experiments conducted under the coal mine showed that using viscoelastic surfactant fracturing fluids increased the amount of gas extracted by 26.1%. This verified the theoretical results and proved that viscoelastic surfactant fracturing fluids are more effective at enhancing gas extraction in soft seams than water. Acknowledgments This study was funded by the National Science and Technology Major Projects of China (No. 2011ZX05065), the National Natural Science Foundation of China (No. 51374258), and the Program for Changjiang Scholars and Innovative Research Team in University of China (No. IRT13043). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jngse.2015.10.031. References Aminto, A., Olson, M.S., 2012. Four-compartment partition model of hazardous components in hydraulic fracturing fluid additives. J. Nat. Gas Sci. Eng. 7, 16e21. Barati, R., Liang, J.T., 2014. A review of fracturing fluid systems used for hydraulic fracturing of oil and gas wells. J. Appl. Polymer Sci. 131 (16), 1002e1013. Cai, C.F., Zheng, X.Q., Tang, C.G., Gao, Q., 2010. Effect of major pollutants on floatability of coal in coking wastewater and its mechanism analysis. J. China Coal Soc. 6, 1002e1008. Chen, F., Zhang, J., 2010. Study of clay minerals in coal in Guizhou Xingren area. Modern Min. 5, 51e54. Chen, S.B., Zhu, Y.M., Liu, T.Y., Zhang, C., Yang, H., 2009. Impact of the clear fracturing fluid on the adsorption properties of CBM. J. China Coal Soc. 1, 89e94.
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Please cite this article in press as: Lu, Y., et al., The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/j.jngse.2015.10.031