Preparation and properties evaluation of novel silica gel-based fracturing fluid with temperature tolerance and salt resistance for geoenergy development

Various polymers are the most widely used product to provide rheology for water-based fracturing fluid, however, they have weaknesses in terms of temperature resistance and salt resistance. The change from organic-based to inorganic thickeners may be a meaningful attempt. Inorganic silica gel is a potentially alternative viscosifier with high temperature resistance, excellent proppant carrying capacity, and can even be used to prepare fracturing fluids with high salinity of produced water. In this paper, the silica gel viscosifier was firstly prepared using sol–gel method. Then, the gelation time under different influencing factors and rheological properties of the silica gel viscosifier were studied in detail. Subsequently, the silica gel-based fracturing fluid was prepared by adding the desired amount of drag reducer to the silica sol solution, and the properties of novel fracturing fluid were systematically evaluated with regard to the temperature and shearing resistance, drag reduction, and static proppant suspension. The potential mechanism of gelation process by syneresis of silica gel was revealed at last. Results showed that the microstructure of silica gel is synthetic, amorphous and consists of a three-dimensional network of SiO2 particles. The effect of SiO2 concentration on the gelation time is more pronounced than other factors such as temperature and pH level. In addition, the silica gel viscosifier exhibits strong salt resistance, whether monovalent ions (Na+, K+) or divalent ions (Ca2+, Mg2+), and the gelation time decreased significantly with the increase of salt concentration. The shear viscosity of the silica gel viscosifier increased with an increase of SiO2 concentration, showing shear thinning behavior as well. Meanwhile, the silica gel-based fracturing fluid prepared by adding drag reducer into silica sol solution also presented excellent thermal stability and shear resistance, drag reduction and proppant suspension performances. The retained viscosity can be maintained above 50 mPa·s after shearing at 180 °C for 60 mins; the drag reduction rate shows a declining trend at high displacement due to the gelation process; the settling rate of 40/70 mesh sand proppant with 35 % sand ratio is less than 30 % after standing at 90 °C for 4 h. In addition, the gelation process is essentially the formation of Si-O-Si linkage by dehydration between SiO2 particles, which is gradually extended at both ends and sides of the chain, and eventually forms a rigid, highly porous, entangled network. Findings found in this study provide a research basis for the popularization and application of silica gel-based fracturing fluid.