A Systematic Study of Shear-Tolerant Micro-Emulsion Fracturing Fluid

Lecheng Zhang1,2, Zhengdong Cheng1-4*, Lijuan Han5, Xu Wang6, Fuchen Liu6, Xianwen Li7 and Minxiang Zeng1 1Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USA 2 Mary Kay O’Connor Process Safety Center, Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USA 3Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3003, USA 4Professional Program in Biotechnology, Texas A&M University, College Station, TX 77843-3122, USA 5 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan, P. R. China 6CNPC USA Corporation, Houston, TX 77042, USA 7Changqing Oilfield Company, PetroChina, Xi’an 710018, Shanxi Province, P. R. China


Introduction
Undoubtedly, hydraulic fracturing technique and horizontal drilling techniques have changed the landscape of the world's energy production. Successful applications of hydraulic fracturing technique on the unconventional shale gas formations leveraged world energy production, especially for North America area, meanwhile creating thousands of jobs. However, innovations for fracturing operations are still demanded to reduce cost, increase well life span and secure energy supply for centuries.
Fluid loss and formation damage are two factors limiting fracturing operations since it was developed [1]. The primary concern for a fracturing treatment is fluid loss. Fluid loss issues increase the total water consumption for fracturing treatment, which puts ISSN: 2638-1974 Volume  hydraulic fracturing under controversy and also increases cost of fracturing operation. The second problem, formation damage, reduces formation permeability, which limits well lifetime and undermines productivity.
Several new technologies have been proposed to solve these problems, such as critical state CO 2 or liquefied petroleum gas (LPG) [2] [3]. One drawback of such techniques is that they are utterly different from the conventional fracturing process, thus costing more to modify the current equipments. Visco-Elastic Surfactants (VES) fracture [4] [5] is another new technology developed to solve fluid loss problem. However, under high temperature and high shear rate conditions, fluid viscosity would be compromised, besides the high cost of VES.
Micro-emulsions, which have already been widely applied in upstream, are mixtures of oil and water with surfactants [6] [7] [8]. Applications of micro-emulsions/gel as fracturing fluids were reported by other groups [9] [10] [11]. By altering wettability of pore throat, micro-emulsion can hydrophobize rock surface and reduce capillary pressure. A lower capillary pressure is preferred in many ways, from increasing productivity to reducing trapped fluid phase. In addition, benefited from the fact that micro-emulsion fracturing fluids are still homogeneous liquid phase under ambient temperature and pressure, instruments for conventional fluids can also implement micro-emulsion fracturing fluids with minor changes, which avoids cost for new treatment systems. Last but not least, micro-emulsion has a higher surface volume ratio compared to macro-emulsion system. By virtue of large surface volume ratio, emulsion droplets have higher chance to contact with rock surface hence alter the wettability. Also diesel usage in the fluids is reduced by the large surface volume ratio, which renders the diesel fracturing fluid as an economy method for fracturing.
Here we present a systematic investigation on the application of micro-emulsion based fracturing fluids to reduce fluid loss and formation damage, aiming to provide other researchers a standard protocol to test different microemulsion fracturing fluid systems. To obtain a stable emulsion, we screened different compositions of water, oil and surfactants. With the aid of a ternary phase diagram (Figure 1), we located the stable Winsor IV phase micro-emulsions. Based on that, we developed micro-emulsion fracturing fluids. Fluid rheology is the most crucial parameters for fracturing fluids [12]. In our work, fluid rheological properties were measured and formation damage was evaluated with synthetic core samples. Experiment results showed that the new micro-fracturing fluids can maintain a high viscosity under high shearing rate and high temperature. Further investigations on microemulsion fracturing system also showed a reduced formation damage and fluid loss.

Materials and Methods
Triton-X 100 (4-(1,1,3,3-Tetramethylbutyl)phenylpolyethylene glycol, Surfactant), Ammonium persulfate (Breaker) and n-butanol (Co-surfactant) are purchased from Sigma-Aldrich. Hydroxypropyl Guar Gum (HPGG,Viscosifier), organoboron crosslinker HYJ-2 are purchased from Shuntong Chemical, China. Encapsulated breaker OB-1 is obtained from Southwest Petroleum University, China. Diesel component is directly purchased from market (Shell, United States). We used diesel as a model oil phase to explore the phase behaviors of the micro-emulsion fracturing fluids. Blended surfactant serving as an emulsifier was obtained by mixing primary surfactant Triton-X 100 with co-surfactant n-Butanol under magnetic stirring with 2:1 mass ratio. The mixture was then stocked for later use. A thorough phase behavior study was carried out to locate an optimal composition for the micro-emulsion fracturing fluids. Results are shown in Figure  1 and  A gel based fracturing fluid was formulated according to the formula mentioned as following. The gel based fracturing fluid consists of 2000-6000mg/L Hydroxypropyl Guar Gum (HPGG), 2000-10000 mg/L organo boron crosslinker, 1000-10000 mg/L breaker (ammonium persulfate) and 500-1000 mg/L high temperature stabilizer (alkyl alkanolamine). Within these compositions, the fracturing fluid can quickly form a gel at 40 to 100°C by vigorous mixing. Gel quality was checked visually before next step. Then the single phase micro emulsions were mixed with the gel based fracturing fluid in a 1:9 weight ratio and used for all later tests.
Interfacial tension (IFT) between the diesel oil phase and aqueous phase under influence of blended surfactants was measured with spinning drop method (M6500 Spinning Drop Tensiometer, Grace Instrument ® ). The IFT profile shows in Figure 3 that Triton-X/n-Butanol reaches its critical micelle concentration at 0.2 w/w% surfactant concentration, which was consistent with emulsion phase study results shown in Figure 1 and Figure 2.

Result and Discussion
Rheological Characterization Rheological property is tested with rheometer (HAAKE Rheo Stress 600). To represent the real shearing force induced by underground formation, the shear rate is set at 170s -1 . Meanwhile, we introduced a gradual temperature increase to simulate the temperature increase caused by tube friction. As shown in Figure 4, fracturing fluid was sheared under a 170s -1 shearing rate and temperature was gradually increased to 120 o C at 5 o C /min increasing rate. Due to initial temperature ramping, viscosity decreased, but after temperature reached plateau, fluid system can maintain a viscosity above 100mPa•S for more than 2 hours. This result demonstrate that our microemulsion fracturing fluid exhibits an excellent shear-tolerant property under high temperature.

Fluid Loss Measurement
To evaluate the fluid loss, we compared the microemulsion fracturing and conventional fracturing system and Volume 1 • Issue 1 • 1000106 Int J Petrochem Res. ISSN: 2638-1974 measured leakoffs of different fracturing fluid systems with different micro-emulsion percentages. A lab-built leakoff measuring system is used and experiments are conducted under following conditions. Synthetic 500 PSI (3.5 MPa) differential pressure is applied on the two sides of filtration cell. Then, volume of fluid that flows out is measured with cylinder. Temperature was controlled at 90 o C during the tests. Applied pressure lasted 36 minutes during the test. Experiment results were plotted in Figure 5. The synthetic core samples used in these tests were manufactured according to the composition of Sulige Eastern formation.
Result showed that a concentration of 5% or above of micro-emulsion can significantly reduce fluid loss. Although the change between 5% and 10% was subtle, we decide to use 10% concentration of micro-emulsions in our research for better demonstrations.

Formation Damage Test
The core permeability can be calculated with Darcy-Weisbach equation, Where K is sample permeability, μm 2 , Q is flow rate, ml/s, L is core sample length, cm, A is core sectional area, cm 2 , ∆P is pressure difference, MPa, and Μ is fluid viscosity, mPa•s.
Formation damage can be characterize by the percentage of permeability reduction: Where K 1 is the permeability before fracturing fluid flooding, and K 2 is the permeability after fracturing fluid flooding.
Experiments were conducted under 120 o C with core flooding setup, as shown in Table 1. Data showed that the conventional guar gel fracturing fluid decreases formation permeability by 57.9%, however, our micro-emulsion fracturing fluid only decreases permeability by 16.4%. Hence we conclude that micro-emulsion fracturing fluids only causes marginal changes in permeability compared to the conventional guar gel fracturing fluid.

Tube Friction Test
Tube friction test was carried out by a coiled tubing system with 0.001kPa pressure resolution. The total length of the coil was 3 meters. We used two different inner diameter tubes, 0.46 cm and 1.01 cm, respectively, and each test last 2 hours. The micro-emulsion fracturing fluids formulated according to previous paragraph were then injected. Also we tested the fluids under two different temperatures, 40 o C and 80 o C, respectively. Results showed in Figure 6. The purpose of these experiments is to provide friction reference data for later field applications.

Compatible Breaker Systems
The gel breaking time can be controlled by adding different amount of breakers. Depending on the amount of breaker, it usually takes 60 to 120 minutes for the breaking

Conclusion
To solve the fluid loss and formation damage problems during fracturing operations, we developed a micro-emulsion based fracturing fluids. The ternary phase diagram was used to locate the optimal composition of the fracturing fluids. After optimizing fluid composition, rheological characterization was measured to access the fluid shear tolerance. Fluid loss and formation damage tests were also carried out to evaluate the fracturing fluids. Results showed that our micro-emulsion fracturing fluids have a robust shear resistance, which ensures a high operation fluid viscosity after pumping down fluid into formation. In addition, our micro-emulsion fracturing fluids caused less fluid loss and formation damage comparing to conventional guar gum fracturing fluids. Compatible breaker system was suggested here for consideration in field implementation. Tube friction profile was measured to provide a reference for field applications. The intrinsic properties of micro-emulsions and micro-emulsion fracturing fluids make them promising tools for economy fracturing operation with increased production lifetime. Also, this comprehensive research work on micro-emulsion fracturing fluid explored a standardized protocols for the later research.