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SPE-175154-MS Advances in Flowback Chemical Analysis of Gas Shales Ashkan Zolfaghari, Yingzhe Tang, Jordan Holyk, Mojtaba Binazadeh, and Hassan Dehghanpour, University of Alberta; Doug Bearinger, Nexen Energy ULC
Copyright 2015, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Houston, Texas, USA, 28 –30 September 2015. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.
Abstract Recently, flowback chemical analysis has been considered as a complementary approach for evaluating fracturing operations and characterizing reservoir properties. Understanding the source of flowback salts and the mechanisms controlling the water chemistry is essential but also challenging due to the complexity of shale-water interactions. In this study, samples of flowback water and downhole shales are analyzed to investigate the mechanisms controlling the chemistry of flowback water. The water samples at different flowback times and the shale samples are collected from three wells completed in the Muskwa, Otter-Park, and Evie members of the Horn River Basin. The water samples consist of aqueous solution and precipitated salts. The water samples are digested in nitric acid to dissolve the precipitated salts, and are analyzed at both intact and acid-digested conditions using ICP-MS. The flowback salts are weighted and analyzed using XRD and SEM-EDXS. A sequential ion-extraction is performed on the shale samples; and the extracted ions are categorized into three tiers of loosely-, moderately-, and strongly-attached ions. The concentration of monovalent cations in both intact and acid-digested samples is higher than that of divalent cations. Also, the concentration of all cations is higher in the acid-digested samples compared with that in the intact samples. The ratio of divalent cations concentration in the acid-digested samples to that in the intact samples is higher than that for the monovalent cations. This ratio increases for the divalent cations over time, while it remains constant for the monovalent cations. Additionally, for the acid-digested samples the monovalent cations concentration has an initial sharp increase followed by a slower increase at later flowback stages; while the divalent cations concentration increases continuously over time. These results suggest that the majority of the ions in the early flowback water are looselyattached monovalent ions. These ions can be originated from the mixing with in-situ formation brine, dissolution of soluble precipitated salts, or leaching of exchangeable cations from the clay minerals. Similarly, the role of relatively slow water-rock interactions (such as leaching of divalent exchangeable cations, e.g. Ca2⫹,) increases at the later flowback stages. XRD and SEM-EDXS analyzes of the flowback salts indicate that sodium chloride, potassium chloride, and calcium carbonate are the major salts. The sequential ion-extraction reveals that the majority of the monovalent cations are in the loosely-attached tier. However, majority of the divalent cations are moderately- /strongly-attached to the rock. The strongly-attached portion of the ions is determined by acid digestion of the rock sample at the final stage
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of sequential extraction process. These strongly-attached ions cannot be easily released by hydraulic fracturing and therefore, has small effect on the flowback water chemistry.
Introduction Unconventional shale gas resources characterized by their low permeability (Frantz and Jochen; 2005), has emerged as a reliable source of energy in North America. Recent advances in multi-lateral horizontal drilling and multi-stage hydraulic fracturing techniques have made the exploitation of the shale resources feasible and economical (IEA, 2012). During a hydraulic fracturing, up to 20 metric tons of aqueous fluid is injected into a typical horizontal well to induce fracture networks (Engelder et al., 2014). The well is then sometimes shut-in (soaking period) to improve the hydrocarbon production (King, 2012; Lan et al., 2014-a; Makhanov et al., 2014). Later on, the wells are put on flowback to recover the injected fracturing fluid at the surface facilities (Abbasi et al., 2014). Depending on the reservoir properties (such as rock mineralogy) and the operational conditions (such as fracturing fluid type), about 10% to 70% of the injected fluid is recovered during the flowback process (Huntington, 2013). Field data show that chemistry of flowback water is substantially different than that of the injected water (Rimassa et al., 2009; Haluszczaket al., 2012; Zolfaghari et al., 2015). For instance, in the Horn River Basin (HRB), slick water (which has similar salinity levels as fresh water) is injected into the reservoir to create fractures (Johnson and Jonson, 2012), while the recovered flowback water is highly saline (40,000 –70,000 ppm) (Bearinger, 2013; Zolfaghari et al., 2014, Engle and Rowan, 2014; Capo et al., 2014). Several past studies focused on the flowback chemical analysis to evaluate the hydraulic fracturing operations and to forecast reservoir performance. For instance, the flowback chemical data have been as an alternative approach for a quick characterization of the induced fracture network (Gdanski, 2010, Ghanbari et al. 2013, Bearinger 2013). Analysis of salt concentration in the flowback water from hydraulically fractured wells in the HRB indicated that the shape of the salt concentration profiles could be used to evaluate the fracture network complexity (Ghanbari et al. (2013 and 2015). Bearinger (2013) qualitatively correlated the complexity of the fracture network and the salt concentration profiles measured during the flowback process. A mathematical model was recently developed to characterize the complexity of fracture network using flowback salinity profiles (Zolfaghari et al., 2015). Despite the economic justification of hydraulic fracturing, its adverse environmental impacts has raised serious concerns about the contamination of ground and surface water as well as the consequential hazards to environment and public health (Howarth et al., 2011; Jackson et al., 2013; Vengosh, 2014). Osborn et al. (2011) analyzed samples from water wells near hydraulic fracking area. They found that the water was contaminated with methane from the deep shale formations due to fracking operations. Furthermore, flowback water is highly saline and contains toxic elements such as strontium and barium (Warner et al., 2012; Haluszczak et al., 2013). Knowledge of flowback water composition is also required for water environmental assessment and selection of appropriate remediation strategy. Although flowback chemical analysis has been widely used to assess the fracturing operations, the source of the ions in the flowback water is still a matter of debate. For instance, Blaunch et al. (2009) believe that dissolution of rock constituents is the major reason for altered chemical composition in the flowback water. However, Haluszczak et al. (2013) believe that mixing of the injected frack fluid with in situ formation brine is responsible for highly saline flowback water. Moreover, shales have clay minerals (Carman and Lant, 2010) which may undergo ion exchange reactions in the presence of water. Such reactions can further impact chemical composition of the flowback water. Furthermore, mineral-filled natural fractures and local precipitated salts (Gale et al., 2014; Zolfaghari et al., 2014) can react with water and impact the flowback water chemistry during the hydraulic fracturing operations. Overall, understanding the source of produced ions and the factors controlling the flowback water chemistry is essential for interpreting the flowback chemical data.
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In this study, we analyze the flowback water and the shale samples to investigate the source of the ions and factors controlling the flowback water chemistry. The intact flowback water samples are digested in acid to dissolve the precipitated salts and possible colloidal particles. A comparative analysis of the intact and acid-digested flowback water samples is performed to better understand the mechanisms impacting the flowback water chemistry. The intact flowback water samples are evaporated and the remaining salts are investigated using XRD and SEM-EDXS analyzes. Furthermore, a sequential ion-extraction method is developed to identify the loosely-, moderately-, and strongly-attached ions.
Experiments The flowback water and shale samples are analyzed to investigate the source of the ions and also the mechanisms controlling the chemistry of the flowback water. Materials The flowback water samples are collected from wells completed in the Muskwa (Mu), Otter-Park (OP), and Evie (Ev) shale members of the HRB which is a stratigraphic unit of Devonian age in the Western Canadian Sedimentary Basin, located in northeastern British Columbia and extends into the Northwest Territories (Figure 1). The flowback water samples are collected at different times during the flowback process. The shale samples from the same formations are crushed into powders for the sequential ion-extraction experiment. Magnesium chloride, hydroxylamine chloride, 30% hydrogen peroxide, hydrochloride, sodium acetate, and nitric acid were used as extractants in the sequential ion-extraction experiments.
Figure 1—Location and depositional stratigraphy of the HR Basin (the star sign highlights the location of Mu, OP and Ev members)
Experimental Procedure Electrical conductivity and total salts concentration The electrical conductivity (EC) is measured for flowback water samples collected at different times. The total mass of the salts are measured after drying the intact water samples. For this purpose, the samples were shaken for 5 minutes. Then 1 ml sample was taken from the homogenous suspension and dried overnight at 200°F. The mass of the remaining salts is reported for different times over the course of the flowback process. Ion analysis Figure 2 illustrates the workflow for analysis of the flowback water samples. The concentration of individual ions is measured for both the intact and acid-digested flowback samples. For the
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intact samples, ICP-MS and IC techniques are used to determine the cations and anions, respectively. The intact water contains visible precipitated salts. Nitric acid is used to digest (dissolve all salts in) the intact water samples (including the precipitated salts). It is not possible to utilize IC for the acid-digested samples. Therefore, ICP-MS analysis is used for determination of the cations in the acid-digested water samples.
Figure 2—The workflow for flowback water analysis
XRD and EDXS analyzes The intact flowback water samples are dried out overnight at 200°F. The precipitated salts are analyzed using SEM-EDXS and qualitative XRD analyzes. Sequential ion-extraction We used the sequential ion-extraction method developed by Tessier et al. (1979) to investigate how easy we can extract the ions from the shale samples. More specifically, the goal is to determine the contribution of each ion source to the total recoverable ions from shales. This will ultimately help in understanding the source of flowback salts. The OP and Ev shale samples are analyzed using the sequential ion-extraction method. The Mu sample was not available for this analysis. We have categorized the extracted ions in three groups of (i) loosely-attached, (ii) moderately-attached, and (iii) strongly-attached ions. In brief, shale rocks were grinded into powder (~ 1 m diameter). Extractants of different strength are used to extract the ions from the powdered shale samples. Figure 3 shows a schematic of the sequential ion-extraction set-up. The following sequences are performed for the sequential ion-extraction experiment:
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Figure 3—Schematic of the sequential ion-extraction set-up
➢ Loosely-attached ions: 1-1) Add 20 mL of 1.0 M MgCl2 solution into powder samples and stir for 1 hr at room temperature. Liquid phase is taken for ion concentration analysis. 1-2) Add 20 mL of 1.0 M sodium acetate solution to the residual rock powders obtained in step 1–1. The solution is stirred for 4 hrs at room temperature. Liquid phase is taken for ion concentration analysis. The extracted solutions are analyzed by ICP-MS. The summation of the ion concentration obtained by ICP-MS for both steps is reported as the loosely attached ion. ➢ Moderately-attached ions: 2-1) Add 50 mL of 0.04 M NH2OH.HCl to the residual rock powder obtained in step 1–2. The solution is stirred at 96⫾3°C for 5.5 hrs. Liquid phase is taken for ion concentration analysis. 2-2) Add 7.5 mL of 0.02 M HNO3 into 12.5 mL of a 30% H2O2 solution and the pH was adjusted to 2.0. The resulting solution is added into the residual rock powder obtained in step 1–3. The solution is stirred at 85oC. After 2 hrs, 7.5 mL of 30 % H2O2 is added to the solution. The solution is further stirred at 85oC for 3 hrs. Liquid phase is taken for ion concentration
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analysis. The extracted solutions are analyzed by ICP-MS. The summation of the ion concentration obtained by ICP-MS for both steps is reported as the loosely attached ion. ➢ Strongly-attached ions: 3-1) Add 7 mL HCl (32% – 35%) and 2.3 mL HNO3 (1 N) to the residual powder obtained in step 2–2. The temperature of the reaction mixture is slowly risen and reflux condition is maintained for 2 hrs. After cooling, the supernatant was taken for ICP-MS analysis.
Results and Discussions EC and flowback salts in the intact flowback water samples In Figure 4, we plot the EC and mass of the flowback salts (after evaporation) for the intact flowback samples collected at different times during the flowback of the Mu, OP, and Ev wells. The EC and flowback salt concentration increase by time for all the samples with similar trends. For the Mu and OP samples, the EC show an initial increase and reach to plateau at later flowback times. However, the Ev sample shows a continues increase over time. Similar observations were also reported previously (Bearinger, 2013; Zolfaghari et al. 2014 and 2015).
Figure 4 —EC (Red) and mass of dissolved salts (Black) of the intact flowback samples versus time for (a) Mu, (b) OP, and (c) Ev.
Ion analysis of the intact and acid-digested flowback water samples The concentration of Na⫹, K⫹, Cl⫺, Ca2⫹, Ba2⫹, and SO42– versus time for both the intact and the acid-digested flowback water samples for the Mu, OP, and Ev samples are shown in Figures 5–7. Generally, the concentration of ions increases by time. The concentration of cations is higher for the acid-digested water samples due to dissolution of the precipitated salts by digestion in acid. It was not possible to measure the anion concentration after acid digestion.
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Figure 5—Individual ion concentration in the intact and acid digested flowback water from Mu versus time: (a) Naⴙ, (b) Kⴙ, (c) Cl–, (d) Ca2ⴙ, (e) Ba2ⴙ, and (f) SO42ⴚ
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Figure 6 —Individual ion concentration in the intact and acid digested flowback water from OP versus time: (a) Naⴙ, (b) Kⴙ, (c) Cl-, (d) Ca2ⴙ, (e) Ba2ⴙ, and (f) SO42ⴚ
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Figure 7—Individual ion concentration in the intact and acid digested flowback water from Ev versus time: (a) Naⴙ, (b) Kⴙ, (c) Cl-, (d) Ca2ⴙ, (e) Ba2ⴙ, and (f) SO42ⴚ
Figure 8 —Schematic of clay (2:1 type) structure showing common exchangeable cations in the interlayer of the clay minerals
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The ratio of cation concentration in the intact samples to that in the acid-digested samples increases over time for Ca2⫹ and Ba2⫹. This ratio remains constant for the monovalent cations (Na⫹ and K⫹). Higher surface charge density (charge per surface area) of the divalent cations compared with the monovalent cations (Essington, 2005) is a possible reason for this behavior. The values for the surface charge density of the focus ions are presented in Table 1. Increasing surface charge density increases the affinity of the divalent cations for counter ions, and therefore, increases the chance to create complex ions and precipitate in the intact flowback water samples. Digesting the water sample in acid facilitates dissolution of the precipitated salts. Thus, the difference between the ion concentration in the intact and digested samples is higher for the divalent cations compared with that for the monovalent cations.
Table 1—The surface charge density of the focus ions (Essington, 2005) Ion
Charge [C]
Ion radius [pm]
Surface Area [m2]
Surface Charge Density [C/m2]
Na⫹ K⫹ Ca2⫹ Ba2⫹ Cl⫺ SO42⫺
1.6E-19 1.6E-19 3.2E-19 3.2E-19 ⫺1.6E-19 ⫺3.2E-19
116 152 114 149 167 290
1.69E-19 2.90E-19 1.63E-19 2.79E-19 3.50E-19 1.06E-18
0.9462 0.5511 1.9594 1.1470 ⫺0.4565 ⫺0.3028
The concentration of the divalent cations in the acid-digested flowback samples continuously increases over time. However, the concentration of the monovalent cations has slower rate of increase at later flowback stages. The results suggest that the majority of the ions in the early flowback water are monovalent ions. The concentration of the divalent cations in the flowback water increases as the flowback process progresses. This interpretation is further analyzed in Section 3.3. For the acid-digested samples, the Ba2⫹ profiles of the Mu and OP samples have slower rate of increase compared with that of the Ev samples. Similar observations were reported previously by Zolfaghari et al. (2014) who concluded that Ba2⫹ is primarily originated from the natural fractures. According to Figures 5–7, Na⫹ and K⫹ have higher concentrations compared with Cl⫺ in both of the intact and acid-digested flowback samples. Similar observations were also reported by other researchers (Zolfaghari et al., 2014; Ghanbari et al., 2014). Clay leaching is a possible reason for this phenomenon (Keller and Da-Costa, 1989). More specifically, the surface of clay minerals is negatively charged (Figure 8). These negative charges are balanced by exchangeable cations (such as Na⫹, K⫹, Ca2⫹, Mg2⫹) residing in the interlayer of the clay minerals (Hensen and Smit, 2002). These exchangeable cations can be leached out during the fracturing operations and impact the water chemistry. We utilized XRD analysis to analyze the clay content of our shale samples. Table 2 summarizes the clay content of the OP and Ev samples from the XRD analysis. The Mu shale sample was not available for this analysis. According to Table 2, illite, mica, and mixed-layer illite/smectite (I/S) are the major minerals in the clay fraction of our shale samples. Sodium is not the major cation in the interlayer of mica and illite (Essington, 2005). Therefore, leaching of the exchangeable sodium in the interlayer of smectite (in the mixed I/S layer) is a possible source for higher concentration of Na⫹ than Cl⫺. Potassium is a common cation in the interlayer of illite and mica (Essington, 2005). Thus, leaching of the exchangeable potassium in the interlayer of illite and smectite can possibly explain why K⫹ concentration is higher than Cl⫺ concentration.
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Table 2—Clay content of the OP and Ev samples from quantitative XRD analysis Sample Depth (m) Clay Content (Wt%) Smectite Illite/Smectite (I/S) Illite⫹Mica Kaolinite Chlorite Total Clay
OP
Ev
2608.75
2682.48
0 5 11 0 0 16
0 14 19 0 0 33
XRD and SEM-EDXS analysis of the flowback salts In order to investigate the major flowback salts, the remaining salts after evaporation of the intact flowback samples are analyzed using XRD and SEM-EDXS. The intact flowback water samples are heated and the remaining salts are analyzed using XRD and SEM-EDXS. The qualitative XRD and SEM-EDXS results are shown in Figures 7 and 8, respectively. According to the XRD and EDXS results it is very possible to have Na/K-bearing salts (such as NaCl and KCl). These results are in line with the ICP-MS analysis of the flowback samples (Figures 5–7) showing that the major ions in all of the samples are Na⫹, K⫹, and Cl⫺. The major salts were similar for flowback samples taken at different times. Therefore, the XRD results are only presented for two flowback times in Figure 9. According to the XRD results, the major salts in the dried flowback water samples from Mu are NaCl and KCl. These results are also in agreement with the SEM-EDXS elemental maps of salts from Mu; as both Na and K are present in similar areas where Cl occurs. The major salts in the flowback water samples from OP are CaCO3 and NaCl (Figure 10). According to the SEM-EDXS results, Ca has a relatively high concentration. Also, Na and Cl occur in the similar locations. KCl is the major salt in the Ev flowback samples, which is in congruent with the SEX-EDXS results presented in Figure 8. Both Na and Cl are observed in the similar locations.
Figure 9 —XRD analysis of the flowback salts at two different flowback times. The results are from days 1 and 8 for the Mu samples, days 1 and 9 for the OP samples, and days 1 and 10 for the Ev samples.
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Figure 10 —Elemental maps of the salts obtained from the flowback water samples. The salts obtained from the flowback water samples are from the 8th, 9th, and 10th days of the flowback process for the wells completed in the Mu, OP, and Ev formations, respectively.
According to the SEM-EDXS results, sulfur is present in all samples. Figures 5–7 also show the presence of SO42- in all flowback water samples. After the fracturing operations, sulfate can be produced
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through reaction of pyrite with water in the presence of calcite and dolomite (Zolfagahri et al., 2014). As a reactive divalent anion, sulfate can react with cations with high surface charge density (such as Ca2⫹ and Ba2⫹) to produce sulfate-bearing salts (such as BaSO4 and CaSO4). These salts are practically insoluble in water at our experimental conditions (Table 3). However, one must note that, the redox reactions impact the mineral solubility after the fracturing operations. For example, sulfate reduction during the flowback process can increase the solubility of the barium-bearing components (Engle and Rowan, 2014). The thermodynamic simulation of shale-water interactions will provide further information about the role of redox reactions and mineral dissolution/precipitation at different conditions.
Table 3—Examples of the soluble, weakly-soluble, and insoluble salts. The solubility values are at 20°C and atmospheric pressure (Green and Perry, 2008). The salts are selected based on the results of XRD and SEM-EDXS analyzes on the flowback salts presented in Section 3.4. Soluble
Solubility (g/100 g H2O)
NaCl 35.89
KCl 34
Weakly-soluble K2SO4 11.11
Insoluble BaSO4 0.00024
BaCO3 0.0022
Sequential ion-extraction Sections 3.1 and 3.2 focused on the chemical analysis of the flowback water samples. Water-shale interactions (such as dissolution of rock constituents and clay leaching) are possible sources for the ions observed in the flowback water (Blaunch et al., 2009). This section focuses on the chemical analysis of the fluid-shale interactions. The goal is to determine the contribution of each ion to the total recoverable ions in shales. We believe that similar interactions occur after fracturing operations when fracturing water is exposed to shale. The sequential ion-extraction method described in Section 2.2 is applied on the shale samples collected from cores dilled in OP and Ev formations. The Mu shale sample was not available for this analysis. The results of sequential ion-extraction experiments are presented in Figure 11.
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Figure 11—The results of the sequential ion-extraction on the OP and Ev shale samples, (a) Naⴙ, (b) Kⴙ, (c) Ca2ⴙ and (d) Mg2ⴙ.
According to Figure 11, the majority of the monovalent cations (Na⫹ and K⫹) belong to the loosely-attached tier. However, divalent cations (Ca2⫹ and Mg2⫹) are mainly associated with the moderately- and strongly-attached tires. Moreover, the portion of the loosely-attached monovalent cations is larger in the OP sample compared with that in the Ev sample. However, the portion of the looselyattached divalent cations is larger in the Ev sample compared with that in the OP sample. We believe that the loosely-attached ions can be produced from (i) the leaching of the exchangeable cations from clay minerals, (ii) mixing with in-situ formation brine, (iii) dissolution of the precipitated soluble salts. Na⫹, K⫹, Ca2⫹, and Mg2⫹ are among the most common exchangeable cations in the interlayer of the clay minerals (Essington, 2005). The surface charge density of the cations impacts their selectivity for the negatively-charged clay surface. The cations selectivity for the clay surface is: Ca2⫹⬎Mg2⫹⬎K⫹⬎Na⫹ (Essington, 2005). Therefore, among these focus cations, sodium and potassium are the most looselyattached ones. This is one of the possible reasons why the majority of the monovalent cations are produced at the initial steps of the ion-extraction experiments. The in-situ formation brine and dissolution of the precipitated soluble salts (Haluszczak et al., 2013; Zolfaghari et al., 2014) are other possible sources for the ions. The shale formations in the HRB are at the state of sub-irreducible water saturation (Dehghanpour et al., 2013). Therefore, there is a possibility for ions to precipitate in the form of salts in the pore structure. According to the XRD and SEM-EDXS results presented in Section 3.4, NaCl and KCl are among the major flowback salts. Both of the NaCl and KCl salts are very soluble in water (Table 3). Therefore, dissolution of these soluble salts can be another
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possible source for the extraction of Na⫹ and K⫹ at the initial steps of the ion-extraction experiments. It must be noted that there are other parameters controlling the ion dissolution/precipitation. For instance, ion activity, ionic strength, and temperature play a key role on the thermodynamic equilibrium of the shale-water. The thermodynamic simulation of the shale-water interactions will be done in the future to provide further information about the nature of the precipitated salts at different conditions. We believe that the moderately-attached ions can be produced from (i) the multivalent exchangeable cations in the clay minerals (e.g. Ca2⫹, and Mg2⫹), (ii) weakly-soluble salts (e.g. K2SO4), and (iii) relatively slow fluid-shale reactions. The multivalent cations have higher selectivity to the clay surface compared with the monovalent cations. Thus, they have less chance to be produced at the initial steps of the sequential ion-extraction experiment. One must note that, although we categorized the weakly-soluble salts and relatively slow fluid-rock reactions in the second group (the moderately-attached ions); these mechanisms might be different at different conditions. For example, redox state and temperature are key parameters controlling the reactions and mineral solubility. These parameters need to be further investigated for the shale-water systems. The strongly-attached portion of the ions is determined by digestion of the shale samples in acid. These ions mainly belong to (i) the structure of the clay/rock and (ii) the insoluble salts (e.g. BaSO4 and BaCO3). These strongly-attached ions are not affected by the interaction between fracturing fluid and rock and therefore, have small effects on the flowback water chemistry. It must be noted that shales are heterogeneous and their properties can significantly vary even in small scales (Diaz et al., 2010). Furthermore, shale-water is a complex system and other parameters might be important during the hydraulic fracturing, soaking, and production phases. For example, although BaSO4 and BaCO3 have very limited solubility at our experimental conditions, sulfate reduction during the soaking and flowback process can increase the solubility of the barium-bearing components (Engle and Rowan, 2014). The future thermodynamic simulation of shale-water interactions will provide further information about the role of redox reactions and mineral dissolution/precipitation at different conditions. The results of the sequential ion-extraction analysis are in agreement with the flowback water analysis. Majority of the monovalent cations (Na⫹ and K⫹) are loosely-attached ions (Figure 11-a, b). The acid digestion of the flowback water shows that the concentration of the divalent cations increases continuously over time due to the low solubility of the salts that they form. However, the concentration of the monovalent cations shows a slower rate of increase at later flowback stages, as majority of the salts that they form has a high solubility in water. These results suggest that the loosely-attached ions are mainly produced at the early stages of the flowback process. The role of the moderately- and strongly-attached ions in the flowback water chemistry increases over flowback time. It must also be noted that, the shut-in time and rock mineralogy impacts the water recovery and flowback water chemistry (Lan et al., 2014-a). Moreover, it has been previously shown that the later flowback water is coming from fractures with smaller aperture size (Zolfaghari et al., 2015). These small fractures have higher surface to volume ratio which can further affect the water-rock interactions. One should note that the chemistry of the water influences the water-rock interactions. For instance, according to Figures 4–7, for most of the ions, the concentration increases during the flowback process. The higher ion concentration at later flowback times increases the ionic strength of the flowback water. Higher ionic strength enhances the mineral dissolution through decreasing activity coefficients of the ions (Essington, 2005). In other words, the role of the dissolution of rock constituents can be improved at later flowback stages. Another example is the dependence of clay leaching on the water chemistry. As we concluded earlier, the exchangeable monovalent cations in interlayer of the clay minerals are mainly produced at early stages of the flowback process. Therefore, the concentration of exchangeable multivalent cations increases at later stages of the flowback. However, the ratio of ions in the solution and on the clay surface (the ratio law) should be the same (Essington, 2005). For instance, let us consider 20 calcium ions attached to the clay surface and 5 sodium ions present in the solution (Figure 12). The ratio law
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distributes the ions in a way that the ratio of calcium to sodium is equal to 4 in both the solution and on the clay surface. These explanations are also in agreement with the flowback chemical analysis presented in Figures 5–7. The elevated concentration of the monovalent cations enhances the leaching of the divalent cations from the interlayer of the clay minerals. All these phenomenon needs to be further investigated for precise evaluation of the flowback chemical data.
Figure 12—Ratio law in the ion exchange reactions. The numbers above the representative ions indicate the number of ions.
Conclusion In this study, the flowback water samples representing different flowback times are collected from wells completed in Muskwa, Otter-Park, and Evie members of the Horn River Basin. The flowback water samples are chemically analyzed at two conditions: (i) intact samples and (ii) acid-digested samples. The intact flowback water samples are heated and the remaining salts are analyzed using XRD and SEMEDXS analyzes. Also, a sequential ion-extraction experiment is performed on the shale samples. The extracted ions are categorized in three groups of loosely-, moderately-, and strongly-attached ions. The primary results of this study are: ➢ Flowback water analysis indicates that the ratio of divalent to monovalent ion concentration in the acid-digested samples is higher than that of the intact samples. This ratio increases by time during the flowback process. ➢ Sequential ion-extraction analysis reveals that the majority of the loosely-attached ions are monovalent cations. These results suggest that the loosely-attached monovalent cations are produced at the early stages of the flowback. ➢ The moderately-attached ions (mainly multivalent) are produce at later flowback stages. The strongly-attached portion of the ions is determined by digestion of the shale samples in acid; and has small impact on the flowback water chemistry. ➢ Qualitative XRD analysis of flowback salts indicates that NaCl, KCl, and CaCO3 are the major flowback salts. The major flowback salts are similar at different times of the flowback process. Further investigations such as thermodynamic analysis of water-shale interactions and quantitative XRD analysis of the flowback salts are required to improve our understanding of the flowback chemical data.
Acknowledgement The authors are grateful to (1) Nexen Energy ULC. and INPEX Gas British Columbia Ltd. (2) National Sciences and Engineering Research Council of Canada (NSERC), FMC Technologies, and Trican Well Service for supporting this study, and (3) Mike Hazelton for the XRD and SEM-EDXS measurements.
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