Elsevier

Fuel

Volume 229, 1 October 2018, Pages 79-87
Fuel

Full Length Article
A novel CO2 and pressure responsive viscoelastic surfactant fluid for fracturing

https://doi.org/10.1016/j.fuel.2018.04.081Get rights and content

Abstract

Hydraulic fracturing is playing a more and more important role in rapid shale oil and gas recovery development. Meanwhile, damage to the reservoir fracture and environmental contamination caused by fracturing fluid flowback water has raised serious concerns. In this study, a CO2-responsive surfactant, erucamidopropyl dimethylamine (EA), was synthesized and used as the thickening agent to develop a novel viscoelastic surfactant (VES)-based fracturing fluid. This fluid was responsive to the presence and pressure of CO2. It was not only rock formation friendly and environment benign but also readily reusable attributed to a gelling-breaking process that can be easily controlled by manipulating CO2 gas conditions. The viscoelastic behaviors of this VES fluid were investigated through rheological measurements under ambient and elevated temperature and pressure. The fluid-rock interaction was studied in core flooding tests. Results showed a good CO2-responsiveness, switchable viscoelastic performance, high shear tolerance, thermal stability, good salinity tolerance, and low core damage of the fluid. The switchable rheological behavior implies this fracturing fluid can readily be reused. It is expected that this fluid finds applications not only in enhanced oil and gas recovery, but also in other areas such as CO2 sequestration reservoir leakage sealing.

Introduction

Over the past decade, the rapid rise of unconventional shale oil and gas development through horizontal drilling and high volume hydraulic fracturing has expanded the extraction of hydrocarbon resources [1], [2], [3]. Because the oil and gas are distributed in ultra-low permeable rock formations, hydraulic fracturing is required in order to economically recover the hydrocarbon resources from reservoirs. Hydraulic fracturing is a stimulation treatment to greatly improve well productivity by creating high conductivity fractures in ultra-low permeability reservoir formations by injecting a fracturing fluid [4], [5]. The property and performance of the fracturing fluid are critical to the success of reservoir fracturing. The main functions of the fracturing fluid are to create fractures and transport proppants into the fractures, which is critically based on their viscoelastic properties. Meanwhile, minimizing the adverse influence of the fluid on reservoir formation pore structures and gas and oil seepage after fracturing is another important consideration. Furthermore, mitigating the negative environmental impact of fracturing fluid is so crucial that it cannot be overemphasized.

Water-based fracturing fluids are currently playing very important roles in enhanced oil recovery (EOR). Guar gum and its derivatives are widely used in hydraulic fracturing fluids [6], [7]. Insoluble residues produced from the breaking down of guar gel always cause damage to the reservoir rock permeability and it is a growing concern over the contamination of hydraulic fracturing flowback water [1], [8], [9]. These issues encourage the development of more reservoir rock friendly and environment benign hydraulic fracturing fluids.

Studies in improving properties of hydraulic fracturing fluids have found that viscoelastic surfactant (VES) solution can improve the productivity of gas and oil and can be efficiently recovered after fracturing [10], [11]. Attributed to the dynamic reversible three-dimensional network structure, the VES fluids exhibit excellent viscoelastic properties [12], [13]. Compared with conventional polymer fracturing fluid, VES fracturing fluids have many advantages, such as fewer components, easy preparation, good proppant transport capacity, low treatment friction, low residue, limited damage to cracks and formations, and no addition of breakers [14], [15]. VES fracturing fluids require no chemical breakers, yet can be better cleaned up than crosslinked polymer fluids, leading to lower skinning in formation pores and greater well productivity.

Nevertheless, high cost, relatively low temperature resistance and potential environmental risk have been reported as the main shortcomings for the VES fracturing fluids [1], [8]. One strategy for overcoming these shortcomings is to reuse the VES fracturing fluids [9]. CO2-responsive VES solution presents superior advantages, because environmental benign and abundant CO2 can potentially be a green trigger in practical applications. Compared to other triggers such as temperature, light, and electric field, CO2 has many advantages that can be applied to hydraulic fracturing fluid, including inexpensive, nonhazardous, and can be easily removed from the system. Meanwhile, the interaction among the surfactant molecules and the dynamic reversible entanglement of wormlike micelles in the VES fluid bring up desirable viscoelastic properties for proppant transport. The good CO2 responsiveness and switchable system viscoelasticity portend the achievable reuse of this fluid. By offering far more flexibility than traditional surfactants, CO2-responsive switchable VES make it possible for the one molecular to fulfill two roles, i.e. the high viscosity for fracking and proppant transport and low viscosity for recovery after fracking. Since Jessop et al. [16] reported reversible nonpolar-to-polar solvent upon exposure to CO2, numerous researches on the performance and application of CO2-triggered systems have sprung up. Lestari et al. [17] reported a novel microfluidic strategy for fundamental researches of liquid–liquid phase separation mediated by CO2. Xu et al. [18] introduced switchable particle-stabilized emulsions by addition of CO2 gas and removal of dissolved CO2 by sparging with air. It was reported that the temperature resistance of VES system was always not sufficient for practical applications [19], [20], [21], and limited researches on the influence of high temperature on the performance of CO2-triggered systems has been reported. Therefore, a new VES fluid is in demand in order to overcome the shortcomings of the existing CO2-responsive viscoelastic fluid systems. To our best knowledge, no VES fracturing fluid that is viscoelastic-switchable based on CO2 presence and pressure has been reported.

The main objectives of this study are (1) to introduce a reusable CO2-responsive VES fracturing fluid; (2) to examine the rheological properties, temperature resistance and shear resistance of this fluid; (3) to evaluate the permeability damage of the fluid by characterizing the interaction of the fluid with reservoir rock. First, surfactant erucamidopropyl dimethylamine (EA) was synthesized and used as a thickening agent for VES hydraulic fracturing fluid. Second, the influence of temperature, the presence and pressure of CO2 on the viscoelastic behaviors of the fluid was investigated by rheological measurements. Third, fracturing fluid performance and interaction with rock were studied. It is expected that this CO2-responsive VES fluid would find applications not only in enhanced oil and gas recovery, but also in areas such as flow pass manipulation in microfluidics [17] and leakage sealing in carbon sequestration revoir caprock formations.

Section snippets

Materials

Potassium chloride, sodium chloride, magnesium chloride, calcium chloride, sodium salicylate (Nas, NaC6H4CO2, 99.5%), erucic acid, N,N-dimethyl-1,3-propanediamine (DMPDA), NaF, and Al2O3 were purchased from Aladdin Reagent CO, Ltd. (Shanghai, China) and used as received. Carbon dioxide (99.99%) and nitrogen (99.999%) were purchased from Oxarc Inc. USA.

Synthesis of EA

EA with purity greater than 99.0% (HPLC) was synthesized following to a previous procedure [22], [23] with modifications. Erucic acid was reacted

Viscosity behavior

The EA synthesized in this work is an amphiphilic surfactant. It contains a long carbon chain, an amide function group and a tertiary amine headgroup. We reported that molecules with analogous structure had a CO2-responsive property in aqueous solution [24]. The EA aqueous solution was a milky and low viscous fluid at room temperature (Fig. 1a). This fluid quickly changed into a transparent viscoelastic liquid system (Fig. 1a) when the fluid was in contact with CO2 under certain pressure or

Conclusions

In this work, a CO2-responsive amphiphilic surfactant EA was synthesized and used as the thickening agent in VES solutions to form a novel fracturing fluid that was not only rock formation friendly and environment benign but also reusable. This CO2-responsive VES fracturing fluids exhibited good viscoelastic which is critical for proppant transport. The fluids were easily switchable between high (up to thousands of mPa.s) and low (less than 3 mPa.s) by manipulating the presence of CO2. Rapid

Acknowledgements

The work was funded by the National Natural Science Foundation of China (51425406), the National Science and Technology Major Project (2017ZX05023003-003), the Chang Jiang Scholars Program (T2014152), the Fundamental Research Funds for the Central Universities (15CX08003A, 16CX06031A), the China Scholarship Council. The Pacific Norwest National Laboratory is operated by Battelle for the U.S. Department of Energy under contract DE-AC06-76RLO 1830. The rheology work was partially supported by the

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