Encapsulation of potassium persulfate with ABS via coacervation for delaying the viscosity loss of fracturing fluid Fanbin Meng , 1,2 Miao Li, 1 Shujun Wang, 2 Xiang Liu, 1 Weiying Gao , 3 Zhuang Ma, 2 Chaopei Kong, 1 Xuejiao Ma, 1 Jiacheng Li 1 1 School of Environment, Tsinghua University, Beijing 100084, China 2 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, China 3 Department of Science and Technology, Chinese People’s Armed Police Force Academy, Langfang 065000, China Correspondence to: M. Li (E-mail: miaoli@tsinghua.edu.cn) ABSTRACT: Encapsulation of persulfates is an important and effective method for sustained release to delay the gel breaking of a fracturing fluid in a high-temperature reservoir. For a gel breaker with sustained release performance, the coacervation of Acrylonitrile-Butadiene- Styrene (ABS) induced by polydimethylsiloxane (PDMS) was conducted to encapsulate potassium persulfate (KPS). Moreover, the compo- sition, KPS loading, morphology, structure, release property, and gel-breaking performance of the obtained microcapsules were investigated thoroughly. The results show that KPS was successfully encapsulated by the ABS, and the KPS/ABS microcapsules consisted of micron- sized particles with a porous surface, core/shell structure, and significant sustained release property that were affected by the core/shell mass ratio during the preparation. Moreover, the apparent viscosity of gel fracturing fluid at 70  C obtained using the KPS/ABS microcapsules as the gel breaker could be maintained above 50 mPas for 100 min. Therefore, the encapsulation of KPS by the coacervation of ABS is a feasible method to achieve sustained release property, and it could effectively reduce the rate of viscosity loss of fracturing fluid at a high temperature. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47734. KEYWORDS: coatings; oil and gas; composites; non-polymeric materials and composites Received 9 October 2018; accepted 19 February 2019 DOI: 10.1002/app.47734 INTRODUCTION Hydraulic fracturing is an excellent technology for the increase of oil well production, 1–4 and the gel breaking of fracturing fluid is a key and difficult step of fracturing construction. 5–7 Persulfates, which are generally used as the gel breaker, often cause the untimely loss of fracturing fluid viscosity, especially in the fracturing construction for a high-temperature reservoir. 8–10 Microencapsulation of active ingre- dient is a remarkable technology to control the release of encapsu- lated ingredient. 11–18 Therefore, it is feasible to encapsulate persulfates to delay the release of gel breaker; this is of universal interest. 8,19,20 It is challenging to encapsulate a water-soluble active ingredient, 21–24 and encapsulation methods for persulfates are relatively less avail- able. As the most water-soluble active ingredient, persulfates are mainly encapsulated via fluidized bed coating. 25–27 Besides the above method, Zuo et al. 28,29 also achieved the encapsulation of persulfates by in situ polymerization and successfully delayed the release of per- sulfates. Previous studies show that polymerization and fluidized bed coating are mainly used for the encapsulation of APS or KPS, and the aforementioned encapsulation of persulfates effectively reduced the viscosity loss of fracturing fluid. However, a gel breaker has rarely been encapsulated via coacervation. Inexpensive and widely used Acrylonitrile-Butadiene-Styrene (ABS) can be used as the shell of sustained release microcapsules. 30,31 The increase in polydimethylsiloxane (PDMS) concentration in an ABS dichloromethane solution will induce the coacervation of ABS, then the coacervate ABS droplets will deposit on the surface of core particles. Finally, the encapsulation process is achieved by the solidification of ABS with its nonsolvent. Moreover, the encapsula- tion of a water-soluble ingredient with the shell of ABS via coacerva- tion induced by PDMS not only produces sustained release microcapsules but also has the advantage of mild reaction condi- tions and even can achieve the recycling of most reagents. 31 For the sustained release performance of KPS, in this study, the coacervation of ABS was conducted for the encapsulation of KPS under mild conditions. Then, the morphology, composition, KPS loading, structure, release property, and the gel-breaking perfor- mance of encapsulated KPS (KPS/ABS microcapsules) were inves- tigated. Moreover, the preparation and release mechanism of KPS/ABS microcapsules was studied. The coacervation successfully achieved the encapsulation of KPS, and the obtained KPS/ABS microcapsules exhibited benign sustained release property and maintained the apparent viscosity of gel fracturing fluid above 50 mPas for 100 min at 70  C. © 2019 Wiley Periodicals, Inc. 47734 (1 of 6) J. APPL. POLYM. SCI. 2019, DOI: 10.1002/APP.47734