Gemini Surfactant with Unsaturated Long Tails for Viscoelastic Surfactant (VES) Fracturing Fluid Used in Tight Reservoirs.

The high dosage of surfactant terribly restrains the extensive application of viscoelastic surfactant (VES) fracturing fluid. In this study, a novel gemini surfactant (GLO) with long hydrophobic tails and double bonds was prepared and a VES fracturing fluid with a low concentration of GLO was developed. Because of the long tails bending near the double bonds, there is a significant improvement of the surfactant aggregate architecture, which realized the favorable viscosity of the VES fluid at a more economical concentration than the conventional VES fracturing fluids. Fourier transform infrared spectrometry (FT-IR), nuclear magnetic resonance spectrometry (1H NMR, 13C NMR), and high-resolution mass spectrometry (HRMS) were employed to study the formation of the product and the structure of GLO. The designed GLO was produced according to the results of the structure characterizations. The formula of the VES fracturing fluid was optimized to be 2.0 wt % GLO + 0.4 wt % sodium salicylate (NaSal) + 1.0 wt % KCl based on the measurements of the viscosity. The viscosity of the VES fluid decreased from 405.5 to 98.7 mPa·s as the temperature increased from 18 to 80 °C and reached equilibrium at about 70.2 mPa·s. The VES fluid showed a typical elastic pseudoplastic fluid with a yield stress of 0.5 Pa in the rheological tests. It realized a proppant setting velocity as low as 0.08 g/min in the dynamic proppant transport test carried by GLO-based VES fracturing fluid. Compared to the formation water, the filtrate of the VES fracturing fluid decreased the water contact angle (CA) from 56.2 to 45.4° and decreased the water/oil interfacial tension (IFT) from 19.5 to 1.6 mN/m. Finally, the VES fracturing fluid induced a low permeability loss rate of 10.4% and a low conductivity loss rate of 5.4% for the oil phase in the experiments of formation damage evaluation.

Introduction

Tight oil reservoirs receive more and more attention for their enormous reserves and great exploitation potential.[1−3] Large-scale hydraulic fracturing is indispensable to improve the poor permeability and obtain economical production from these reservoirs.[4−8] Fracturing fluid is critical for the fracture generation and the proppant transportation during hydraulic fracturing.[9,10] However, the conventional fracturing fluids damage the permeability of the reservoir matrix and restrain the conductivity of the fracture seriously because of the presence of the considerable insoluble residue and residual gel.[3,11−14] In recent years, viscoelastic surfactant (VES) fracturing fluid based on entangled micelles attracted huge attention due to its low-damage property (i.e., no insoluble residue and little residual gel).[15−18] Nevertheless, the poor thermal stability of VES fluid leads to a significant viscosity reduction in high-temperature environments,[19,20] which limits the proppant transportation capability severely.[21,22] To obtain the effective transport of proppant at high temperatures, a high dosage (usually 3–5 wt %) of surfactant is generally required in VES,[23−26] which results in the high cost for the immense surfactant consumption and restricts the extensive application of VES fracturing fluids.[6] Therefore, it is necessary to develop novel VES fracturing fluids with low surfactant concentrations that have the favorable performance at high temperatures.

Since the first application of VES fracturing fluid by Schlumberger with cationic single-chain surfactant in 1997,[27] most of the VES fluids were prepared with single-chain surfactants.[28,29] Because there is a sole hydrophobic tail linked to a hydrophilic head, the single-chain surfactants could not associate into micelles readily for the electrostatic repulsion among the charged hydrophilic groups.[30] Compared to the conventional single-chain surfactants, gemini surfactants,[31,32] a class of superior surfactants comprising two amphiphilic moieties linked by a spacer group, could satisfy the request of proppant transportation for fracturing fluid more easily.[15,26] There are better reductions of flow resistance[33] and heat/shear resistance[34,35] in the VES fracturing fluid prepared with gemini cationic surfactants. The contour length of micelles proposed by Magid[36] is a crucial parameter positive to the micellar morphology[36] and the capability of proppant transportation, following eq :[37]where L is the contour length of micelles, ϕ is the volume fraction of surfactant, Ec is the end-cap energy of micelles, k is the rate constant, and T is the absolute temperature. Current research indicated that the long, unsaturated hydrophobic tails led to the increase of end-cap energy,[38] resulting in the enhancement of the contour length of micelles. Mao et al.[39] proposed that the end cap could increase as the volume occupied by the hydrophobic tail increases due to the kink of the cis double bond in the tails. The researches of Zhang et al.[24] and Yang et al.[40] reported that the VESs exhibited good properties with the hydrophobic chains about C22. Rose and Foster[41] stated that the presence of the surfactant with unsaturated tails resulted in the reduction of drag. Consequently, the load of the pumps during fracturing could be decreased.[42] In addition, the length of the spacer group affects the critical micelle concentration (CMC), critical surface tension, and salt resistance significantly. Mao et al.[39] indicated that the wormlike micelles could be generated easily by the gemini surfactants with a spacer of C3, resulting in excellent viscoelasticity. Wei et al.[43] claimed that the hydroxyl group on the spacer group could generate hydrogen bonds with water molecules in the aqueous solution and reduce the electrostatic repulsion between the ionic head groups.[43] Moreover, the amide-containing surfactant is biodegradable due to the enzymatic hydrolysis through environmental microorganism actions.[44−46]

Inspired by the above recognition, cationic gemini surfactants with the abovementioned advantages show a favorable potential to be used for VES fracturing fluid at low concentration theoretically. To the best of our knowledge, there is very little research on this topic. Herein, a novel cationic gemini surfactant (GLO) was prepared, which consists of two long tails of C21 containing olefinic and amide, and a spacer of C3 containing a hydroxyl group. Furthermore, the behaviors of the VES fracturing fluid containing 2.0 wt % of GLO was characterized by different experiments. It was demonstrated that the novel VES fluid offers a promising material to satisfy the fracturing process.

Experiments

Materials

Tallow alkyl (99%) and 1,3-bis (dimethylamino) propan-2-ol (99%) were supported by Hubei Xinkang Pharmaceutical Chemical Co., Ltd. 1,2-Dichloroethane (EDC, 99,9%) and 4-dimethylaminopyridine (DMAP, 99%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Isopropanol (IPA), ethyl alcohol (EtOH), ethyl acetate (EAC), potassium chloride (KCl), and sodium salicylate (NaSal), AR, were obtained from Sinopharm Chemical Reagent Co., Ltd. 3-Bromopropanoyl chloride (95.0%) was supplied from Bailingwei Science and Technology Co., Ltd. LWP3050 proppant and aviation kerosene were purchased from Tianhong proppants Co., Ltd. and Jinan Xinquan Chemical Technology Co., Ltd. respectively. The deionized water (DI water) was prepared in the lab, while natural core samples, formation water (CaCl2, salinity 17,000 mg/L, density 1.01 g/cm3, viscosity 1.38 mPa·s at 18 °C), and crude oil were acquired from Huangling Chang 6 tight oil formation, Ordos Basin, China. The simulated crude oil (density 0.83 g/cm3, viscosity 2.88 mPa·s at 18 °C) was prepared by mixing the kerosene and the crude oil at a volume ratio of 3:1.

Synthesis of GLO

The steps of the synthesis were as follows.

Synthesis of 3-bromo-N-(octadec-9-en-1-yl) propenamide

First, tallow alkyl (0.2 mol) was dissolved with EDC (300 mL). Then, 3-bromopropanoyl chloride (0.25 mol) was diluted with EDC (100 mL) and dripped into the three-necked flask containing the solution of tallow alkyl slowly under stirring. The speed of dripping should be low enough to control the temperature of the solution in the flask lower than 40 °C. DMAP with a concentration of 0.4 wt % was used as the catalyst. The mixture was kept under stirring (150 rpm) for 2 h in a magnetic stirring water bath (HWX-15A, Shanghai Jinwen Instrument Equipment Co., Ltd.). Finally, the solvent and the excess 3-bromopropanoyl chloride were distilled at 90 °C with a rotary evaporator (RE-52AA, Shanghai Yarong Biochemical Instrument Factory).

Synthesis of GLO

Initially, 3-bromo-N-(octadec-9-en-1-yl) propanamide (0.25 mol) and 3-bis (dimethylamino) propan-2-ol (0.1 mol) were added into the flask containing IPA (400 mL). The mixture was then stirred for 6 h at 60 °C followed by the removal of the solvent with the rotary evaporator at 60 °C. The product was recrystallized twice with the mixed solvent of EAC and EtOH (volume ratio 20:1). It was then dried at 80 °C for 24 h in a vacuum drying oven (Shanghai Lichenbangxi Instrument Technology Co., Ltd.). The structure of the novel gemini surfactant and its synthetic route were designed as shown in Figure .

Figure 1

Structure and synthetic route of GLO.

Molecular Structure Characterizations of GLO

The molecular structure of GLO was obtained with Fourier infrared spectrometry (FT-IR, VERTEX 70, Brukeroptics, Germany), nuclear magnetic resonance spectrometry (1H NMR, 13C NMR, DD2-500MH, Agilent Technologies Inc., USA), and high-resolution mass spectrometry (HRMS, LTQ Orbitrap XL, Thermo Fisher, USA).

Test of GLO-based VES Fracturing Fluid

Formation of the GLO-based VES Fracturing Fluid

KCl was used as the clay stabilizer with a dosage of 1.0 wt % here. There are few Cl– counterions ionized from KCl that could penetrate the aggregate of surfactant, while the counterions ionized from NaSal could insert between the charged headgroups of the surfactant to screen the electrostatic repulsions and promote the growth of micelles.[47] Therefore, NaSal was used as the counterion salt here. The concentrations of GLO and NaSal were optimized based on the viscosity measured with a rheometer (DV3T, Brookfield, USA) according to the Chinese industrial standard SY/T 5107-2016. Unless otherwise specified, all experiments were conducted under a room temperature of 18 °C, and the same applies below.

Test of Heat/Shear Resistance

The heat/shear resistance measurements were carried out by a rheometer(MCR 302, Anton Paar, USA) with the measuring system of CC27 (coaxial cylinder) according to the Chinese industrial standard SY/T 5107-2016. The temperature ranged from 18 to 80 °C (the original temperature of the tight reservoir where the core samples were collected from) with a heating rate of 3 °C /min. The shear rate during the experiment was kept at 170 s–1 for 2 h.

Test of Rheological Properties

The rheological properties of the VES fracturing fluid were also tested on the rheometer (MCR 302, Anton Paar, USA) according to the Chinese industrial standards SY/T 5107-2016 and SY/T 6296-2013. The stress–shear rate curve was measured in a shear rate range of 0.01 ∼1000 s–1 with the measuring system of CC27 on the rheometer. The modulus was measured with the measuring system of PP50-1 (plane plate) under the frequency sweep mode ranging from 0.1 to 100 rad/s with constant stress in the linear viscoelastic region.

Test of Proppant Transportation Carried by GLO-based VES Fracturing Fluid

Proppant distribution is a curial criterion to evaluate the fracturing operations.[48,49] Herein, dynamic pan class="Chemical">proppant transportation carried by GLO-based VES fracturing fluid was evaluated with the simulated vertical crack shown in Figure . The length, width, and height of the simulated crack were 100, 0.15, and 20 cm respectively. There were four viewing windows arranged along the flow direction in the crack. The proppant (mesh size 30–50; apparent density 3.0 g/cm3) was used with a concentration of 30 wt %. The proppant carried VES fracturing fluid was injected from the inlet at a constant velocity of 5 mL/min for 2 h and overflown into the beaker from the outlet when the crack was stuffed.

Figure 2

Schematic of the dynamic proppant transportation test.

Evaluation of Formation Damage Caused by GLO-based VES Fracturing Fluid

Matrix permeability damage: The measurements of the matrix permeability damage were conducted according to the Chinese industrial standard of SY/T 5107-2016 with the fracturing fluid filtrate on the core displacement device (Haian Petroleum Scientific Research Instrument Co., Ltd.). First, the core was saturated with formation water and then displaced with simulated crude oil from the inlet of the core container to get the oil-saturated cores containing irreducible water and the initial oil permeability (K1) of the cores. Afterward, the fracturing fluid filtrate was injected into the core container for 36 min from the other end to simulate the procedure of matrix damage. The core container was shut in for 2 h after injecting the fracturing fluid filtrate. The simulated crude oil was then re-injected into the core from the inlet to get the matrix permeability after damage (K2). The injection velocity during the test was invariably kept at 0.2 mL/min. The permeability loss rate (η) is defined as eq . The parameters of the core samples used in the test are listed in Table S1.

Crack conductivity damage: The cores were cracked, propped up with Φ0.5 mm copper, and sealed (except the surface of cracks, shown in Figure S1) in turn to simulate the cracks used in the experiments. The simulated crude oil was injected into the core container first to obtain the initial conductivity of the cracks. After that, 100 mL of the fracturing fluid was injected into the core container with a back pressure of 5 MPa. The container was then shut in for 2 h to simulate the leak off of the fracturing fluid and the generation of the filter cake. Finally, simulated crude oil was re-injected to obtain the conductivity after damage. The velocity of injection during the whole test was invariably kept at 5 mL/min. The parameters of the simulated cracks are listed in Table S2.

Result and Discussion

Molecular Structure

The 1H NMR (pan class="Chemical">CD4O, 499 Hz, δ) spectra were studied to confirm the formation of the product (Figure ). The peaks at δ 0.89 (6H, a-H) for H corresponded to the −CH3 at the end of the hydrophobic chains. The peaks at δ 1.30 (48H, b-H) for H lay in methylene −(CH2)6– of the tallow alkyl. The peaks at δ 2.04 (8H, c-H) for H corresponded to the methylene linked to the olefinic bonds −CH2–CH=CH–CH2–. The peaks around δ 2.56 (4H, d-H) and δ 3.52 (4H, h-H) were generated by the H in the methylene linked to the carbon of acylamino −CH–CH2–CO–NH–CH2– and the H in the methylene linked to the nitrogen of quaternary ammonium −N(CH3)2–CH–CH2–CO–NH–, respectively. The peaks of δ 3.01 (4H, e-H) and δ 7.75 (2H, l-H) were the H on the acylamino −CO–NH–CH2– and methylene linked to the acylamino −CO–NH–CH2–CH2–, respectively. The peaks of δ 3.21 (4H, f-H) and δ 3.29 (12H, g-H) occurred in the methylene of the spacer group −N(CH3)2–CH–CHOH– and the methyl −CH2–N(CH)–CH2– of the quaternary ammonium, respectively. The peaks around δ 4.67 (1H, i-H) and δ 4.81 (1H, j-H) were generated in the hydroxyl −CH2–CHOH–CH2– and methine −CH2–CHOH–CH2– of the spacer group, respectively. The peaks around δ 5.33 (4H, k-H) corresponded to the H in the olefinic bonds −CH2–CH=CH–CH2–.

Figure 3

1H NMR spectra of the production.

The 13C NMR (pan class="Chemical">CD4O, 126 Hz, δ) spectra of the product are shown in Figure . The chemical shift of 13C NMR corresponded with 1H NMR well. Based on the information obtained from the spectra of 1H NMR, we further studied the key data of 13C NMR during the synthesis of GLO. The peak of δ 35.92 (i-C) was the C in methylene linked to the acylamino −CH2–CO–NH–CH2–CH2–, while the peak of δ 175.24 (o-C) occurred in the carbonyl group of the acylamino −CH2–CO–NH–CH2–. The peaks at δ 63.86 (l-C) and δ 65.33 (m-C) corresponded to the methylene linked to the quaternary ammonium from the spacer group (−CHOH–CH2–N(CH3)2–CH2–CH2−) and the tail chain (−CHOH–CH2–N(CH3)2–CH2–CH2−), respectively.

Figure 4

13C NMR spectra of the production.

The spectra of FT-IR are shown in Figure . The adsorption peaks at 3280 and 1036 cm–1 corresponded to the stretching vibration of O–H and C–O on the hydroxy group, respectively. The adsorption peak at 3032 cm–1 indicated the stretching vibration of =CH–. The adsorption peaks at 2926, 2851, and 1460 cm–1 represented the asymmetric stretching vibration, the symmetric vibration, and the bending vibration of −CH2–, respectively. The adsorption peaks at 1649, 1547, 1253, and 1092 cm–1 showed the stretching vibration of C=O, the bending vibration of N–H, the bending vibration of −CONH–, and the stretching vibration of C–N on the secondary amide group, respectively. The adsorption peak at 1373 cm–1 expressed the bending vibration of −CH3. The quaternization could be deduced from the weakened adsorption peak at 613 cm–1, which inferred the decrease of the C–Br bond.

Figure 5

Infrared spectra of the intermediate and the production.

Figure demonstrates the HRMS (Fourier transform ion cyclotron resonance MS, FT-MS; electrospray ionization, ESI) spectra of the product. [M]2+ calculated for [C49H98N4O3]2+ = 395.38 was found at the highest peak. As supported by the molecular structure analysis of FT-IR, 1H NMR, 13C NMR, and HRMS, it could be confirmed that the designed gemini surfactant GLO was produced.

Figure 6

HRMS spectra of the production.

VES Fracturing Fluid Formulation

The viscosities of the VES fluid prepared with different concentrations of GLO were studied. Due to the phase separation, the records at the points such as the concentration of NaSal higher than 0.3 wt % under the condition of 0.5 wt % GLO were sifted out, which were removed from the results. As shown in Figure , when the concentration of GLO was lower than 2.0 wt %, the viscosity increased dramatically with the increase of GLO concentration. However, the increase became slightly equilibrium when the dosage of GLO concentration was higher than 2.0 wt %. Because NaSal provides the counterions for the surfactant aggregate, the viscosity of the VES fluid was greatly affected by the concentration of NaSal.[38] The viscosity increased significantly in the region of low NaSal concentration before reaching the threshold points, while decreased rapidly under the high concentration of NaSal.

Figure 7

Viscosity under different formulations (1.0 wt % KCl was used here as the clay stabilizer). Error bar = RSD (n = 3).

According to the theory proposed by Israelachvili,[50] the viscosity of the VES fluid depends on the architecture of the surfactant aggregate, which could be predicted by the packing parameter p defined as:where V is the volume of the surfactant tail, A is the effective area of per surfactant hydrophilic head at the aggregate surface, l is the length of the surfactant tail in the solvent. When p increased from less than 1/3 to about 1/2, the surfactant molecules assemble in different shapes from sphere to wormlike micelle.[51] Keeping p increasing led to the aggregate of surfactant and the formation of vesicles (1/2 < p < 1) which finally separated from the rest of the solution (p > 1). As shown in Figure , a large volume and short length of the tails were obtained in GLO by the long hydrophobic chains bending adjacent to the double bonds (the green segment in Figure ). On the other hand, the counterions from NaSal reduced the electrostatic repulsion between the charged hydrophilic head, which compressed the effective area of the hydrophilic head and led to a tighter network based on increased wormlike micellar aggregations. With a combination of the above effects, GLO realized a high viscosity even at a low concentration with a proper dosage of NaSal. With the continual increase of counterion concentration, the short distance between surfactant molecules resulting from the exceeded counterions led to precipitates of the surfactant from the solution as a nonionic molecule.[52] As a result, the phase separation occurred inevitably, and the viscosity of the fluid drops.

Figure 8

3D structure of GLO.

With a view to the combination of viscosity and the cost, the formula of 2.0 wt % GLO + 0.4 wt % NaSal +1.0% KCl was most promising (the viscosity corresponding to this formula was 392.4 mPa·s at 18 °C) for the VES fracturing fluid, which was employed in the following studies.

Heat/Shear Resistance

The result of the heat/shear resistance measurement for the VES fracturing fluid is presented in Figure . The viscosity of the VES fracturing fluid decreased from 405.5 to 98.7 mPa·s as the temperature increased from 18 to 80 °C and became equilibrium at about 70.2 mPa·s. To reveal this phenomenon, the microstructures of the VES fracturing fluid before and after the temperature change were characterized by the high-resolution mass spectrometer scanning electron microscopy (SEM, S4800, Hitachi, Japan). The samples were prepared by coating the quartz slice with the fluid droplets and then drying it at 18 and 80 °C, respectively. The results of SEM are shown in Figure . It was found that the microstructure of the VES at 18 °C was very tight and homogenously distributed, which got extruded and re-assembled owing to the fierce molecule thermal motion at 80 °C. Thanks to the special structure of GLO, there was an evident packing in the VES fracturing fluid even at 80 °C.[36,38]

Figure 9

Curves of heat/shear resistance.

Figure 10

SEM photos of VES fracturing fluid (a) before the measurement and (b) after the measurement.

Rheological Property

The stress–shear rate curve of the VES fracturing fluid is shown in Figure a. The rheological parameters of non-Newtonian fluid are usually studied with the Herschel–Bulkley equation:[53]where τ is the shear pan class="Disease">stress, Pa; τ0 is the yield stress, Pa; K is the consistency coefficient, mPa·s; γ is the shear rate, s–1; and n is the flow behavior index. The fluid type of the VES fracturing fluid was obtained by fitting its stress–shear rate curve with eq , and the result is presented in eq . The fracturing was a typical pseudoplastic fluid for the flow behavior index was lower than 1.

Figure 11

Rheological curves of the VES fracturing fluid. (a) Stress–shear rate curve and (b) the measurement of the viscoelasticity.

The viscoelasticity was measured under the mode of frequency scanning with a constant shear stress of 1.0 Pa in the linear viscoelastic region (the region was determined by Figure S3). As shown in Figure b, the elastic modulus G′ (storage modulus) was always higher than the viscous modulus G″ (loss modulus) during the whole test. It indicated that the fracturing fluid exhibited typical elastic material characteristics. To conclude, the VES fracturing fluid was a typical elastic pseudoplastic fluid with a yield stress of 0.5 Pa.

Proppant Transportation Carried by GLO-based VES Fracturing Fluid

The proppant transportation capability was demonstrated by studying the distribution of the pan class="Chemical">proppant in the crack after the injection (Figure ). A uniform proppant distribution was observed here. The statistics of the proppant mass collected from the different segments of the crack are listed in Table . The difference of the proppant mass proportion among the four segments was no more than 4.3%, indicating that the proppant was homogenously carried by the VES fracturing fluid during the injection.

Figure 12

Dynamic proppant distribution.

Table 1

Statistics of the Proppant Collected from Different Segments

segments	window 1 (inlet)	window 2	window 3	window 4 (outlet)
proppant mass (g)	41.8	42.5	45.2	49.5
mass Proportion (%)	23.4	23.7	25.3	27.7
setting velocity (g/min)	0.06	0.06	0.08	0.12

The dynamic setting velocity of proppant is defined in eq :where v is the pan class="Chemical">proppant setting velocity, g/min; ΔM is the mass of the proppant setting in each segment during the proppant transport, g; T is the time of setting/transport, which was 120 min; Mc is the mass of the proppant collected from each segment, g; M is the mass of the proppant suspended in the fluid filling each segment of the crack, g, which could be obtained from the following eq :where V is the volume of each segment, cm3; ρ is the density of the sand-carrying fluid, g/cm3; C is the mass concentration of the proppant suspended in the fluid, wt %; W, L, and H are the width, length, and height of the simulated crack, respectively, in cm. The proppant settling velocities in different segments are also listed in Table . The proppant was setting at a very low velocity ranging from 0.06 g/min (in segment 1) to 0.12 g/min (in segment 4) with an average value of 0.08 g/min along the simulated crack. The difference of the proppant setting velocity among these segments resulted from the difference of the fluid flow velocity along the crack. Because the inlet and outlet were placed at the upper right corner of segment 1 and the upper left corner of segment 4, respectively, there was a low-velocity zone observed at the bottom of segment 3 and segment 4 (shown in the green ellipse of Figure ). As a result, the proppant setting velocity in this zone was higher.

Formation Damage

The results of the matrix damage tests are shown in Figure . The permeability loss rates of the four natural cores lay in 9.0–11.6%. The average value of the permeability loss rates was 10.4%.

Figure 13

Matrix permeability damage.

The VES fracturing fluid comprised no insoluble components that would bring about the blocking in the matrix. Therefore, the damage just depended on the properties of the interface between the liquids and channel surface. The contact angles (CAs) of the formation water and fracturing fluid filtrate on the core slices and the interfacial tensions (IFTs) of simulated crude oil/formation water and simulated crude oil/filtrate were measured. As shown in Figure , affected by the surfactant GLO, the CA of the filtrate decreased from 56.2 to 45.4° and the IFT between oil and the filtrate decreased from 19.5 to 1.6 mN/m compared to the formation water. It means that the drag on the oil flow and the water lock effect, a phenomenon closely related to the interfacial tension (IFT), were reduced. As a result, the VES fluid showed little damage to matrix permeability caused by the invaded filtrate.

Figure 14

Photos of CAs and IFTs. (a) CA of formation water, (b) CA of the filtrate, (c) IFT between simulated crude oil and formation water, and (d) IFT between simulated crude oil and filtrate.

Photos of CAs and IFTs. (a) CA of formation water, (b) CA of the filtrate, (c) IFT between simulated crude pan class="Chemical">oil and formation water, and (d) IFT between simulated crude oil and filtrate.

The results of the conductivity damage measurements in the simulated cracks are shown in Figure . The conductivity loss rates of the four simulated pan class="Chemical">cracks ranged from 4.0 to 6.4% with an average value as low as 5.4%.

Figure 15

Crack conductivity damage.

As the analysis in section , VES fluid showed great differences in viscosity due to the different forms of surfactant aggregate.[39] In the presence of salicylate counterions, the injected VES fracturing fluid was in a surfactant aggregate of entangled wormlike micelles at the beginning. When the simulated crude oil was injected into the cracks, the surfactant molecules would migrate to the interface of the oil droplets/water to solubilize the oil phase. Consequently, the entangled micellar network was destroyed into dispersed small aggregates around the oil droplets. In other words, the VES fluid could be broken by the crude oil. That was why the conductivity of the crack was damaged slightly by the VES fracturing fluid.

Conclusions

A novel gemini surfactant GLO with long tails and double bonds was synthesized as the low-concentration thickener of VES fracturing fluid. A series of experiments were conducted to ascertain the structure of the product and evaluate the properties of the VES fracturing fluid. The following conclusions could be drawn:

The results of molecular structure characterizations confirmed that the designed gemini surfactant (GLO) containing long tail and double bonds was produced. Moreover, the formula of 2.0 wt % GLO + 0.4 wt % NaSal +1.0 wt % KCl for the VES fracturing fluid was optimized. Different from the previous researches,[15,39] the dosage of surfactant decreased significantly.

The VES fracturing fluid showed a high viscosity of 392.4 mPa·s at 18 °C and the desired property of heat/shear resistance. The viscosity kept as high as 70.2 mPa·s, and an evident network structure was observed with SEM in the VES fracturing fluid after the measurement under 170 s–1 and 80 °C for 2 hours.

The VES fracturing fluid showed to be a pseudoplastic fluid with favorable viscoelasticity, which resulted in the effective transport of the proppant. The maximum difference of the proppant mass proportion among the four segments of the simulated crack was 4.3%. The average value of the proppant setting velocity was as low as 0.08 g/min.

Compared to the formation water, the pan class="Chemical">water CA could be decreased from 56.2 to 45.4° and the water/oil IFT could be decreased from 19.5 to 1.6 mN/m by the fracturing fluid filtrate. Furthermore, there were no insoluble components in the VES fracturing fluid and the surfactant micelle could be destroyed in the presence of crude oil. In the combination of the above effects, the average loss rates of the matrix permeability and the crack conductivity were only 10.4% and 5.4% respectively.