Study on Adsorption Characteristics of Micro-Nano Proppants in Fractures and Related Numerical Simulation

In hydraulic fracturing, micro-nano proppants can easily enter microfractures for support and have strong adsorption properties, so it's very important to explore their adsorption in micro-fracture support. By analyzing the forces of the proppant in fractures, a mathematical model of the microscopic interaction energy between the objects of action was established to calculate the microscopic interaction energy level of hydrophobic micro-nano proppant and water molecules, and then reveal the adsorption process of micro-nano proppant on fracture wall and the competitive adsorption mechanism between the fluid (water). Finally, FLUENT-EDEM was used for numerical simulation verification. The forces of micro-nano proppant in microfractures mainly include fluid drag force, settling force, van der Waals force, electrostatic force and hydrophobic resistance. The competitive adsorption process with water can be divided into three processes: closing to the hydration shell, breaking through the hydration shell and being adsorbed on the wall. To overcome the resistance to break through the hydration shell mainly depends on electrostatic attraction, and being firmly adsorbed on the fracture wall depends on hydrogen bonding. The above research can provide theoretical guidance for the design of micro and nano proppants and the application of effective microfracture support technology. 1. MICRO-NANO PROPPANT At present, many reservoirs, especially the dense unconventional reservoirs, the fracture size formed after fracturing is very small, even up to the micron, nanometer level. Conventional proppant particle sizes are unable to enter induced and naturally occurring microfractures, so the microfractures close, reducing the conductivity of the reservoir fracture network and inhibiting its extension and expansion. To achieve effective fracture support, micro-nano proppants can be used. Micro and nano proppants were first defined by Bose (2015) in sizes ranging from 100 nm to 1mm. Guo et al. (2022) defined it as less than 140 mesh proppant, while most defined it as less than 100 mesh proppant (Calvin et al. 2017; Danso et al. 2021). Micro-nano proppants are usually SiO2, small in size and light in weight. Scholars (Khanna et al. 2013; Keshavarz et al. 2014; Dahl et al. 2015; Li et al. 2020) have proved through experiments that micro-nano proppants can enter micro-fractures for effective support. Dahl et al. (2015) used ESEM microscopy to capture microproppant clusters or aggregates adhering to shale fracture surfaces, forming partial monolayers. This shows that micro-nano proppants have adsorption properties during migration in fractures, so it is of great significance to study the adsorption properties of micro-nano proppants for their application in oilfield.