The complex fracture network generated from hydraulic fracturing is of great importance for well productivity in unconventional reservoirs. However, numerous natural or induced microfractures are closed without proppant placement after hydraulic fracturing is completed. Micro-proppants have small enough particle sizes and longer transport distances, which can access microfractures and keep them open. The objective of this work is to investigate the particle size distribution, crushing rate as well as microfracture conductivity by micro-proppants from fly ash. Microproppants have a large range of particle size from 1 μm to 300 μm, and the average particle diameters are 22.84 μm and 51.13 μm. The crushing rates of micro-proppants are related to the particle size distributions and mineral contents. More concentrated distribution of particle sizes and higher contents of high-hardness minerals lead to a lower crushing rate of micro-proppants. The fracture conductivity is exponentially decreased with closure stress, and it is controlled by average particle sizes and crushing rates of micro-proppants. The results demonstrate micro-proppants from fly ash are sufficient to the requirements of hydraulic fracturing and improve hydrocarbon production of unconventional reservoirs.
Hydraulic fracturing is considered the most effective stimulation method for unconventional reservoirs to improve well productivity. The complex fracture networks are created by pumping a large number of fracturing fluids during hydraulic fracturing, which consists of primary fractures, secondary fractures, and induced microfractures. The induced microfractures have small openness of less than 0.1 mm (Danso et al., 2021). For unconventional reservoirs with extremely low permeability like shales, induced microfractures have a large contribution to the conductivity of the fracture network (Sharma & Manchanda, 2015; Grieser et al., 2016; Keshavarz et al., 2016). The 20/40 mesh proppants are usually used to support the primary fracture for high fracture conductivity. The 100-mesh proppants in recent use cannot reach the induced microfractures. Therefore, micro-proppants with particle diameters less than 150 μm (100-mesh) are selected to reach the induced microfractures (Nguyen et al., 2013; Dahl, Calvin, et al., 2015; Calvin et al., 2017a; Tian et al., 2022). The micro-proppants are generally added to the pad fluid before placing the conventional proppant in primary fractures. Field trials have indicated that micro-proppants can improve well productivity up to 50% in unconventional reservoirs such as Wolfcamp, Woodford SCOOP, and Barnett Shale formations (Calvin et al., 2017b; Dahl, Nguyen, et al., 2015; Lau et al., 2019; Radu et al., 2020).
