Hydraulic fracturing has been proved as a successful technology to increase productivity of ultra-tight shale oil and shale gas reservoirs. Although higher concentrations of polymer were traditionally used for conventional fractures, “linear gels”, “waterfracs”, “slick-water” and ”hybrid” fluids have been typically applied for tight shale plays as we produce from formations with lower permeability and higher brittleness. Fracturing jobs in tight shale plays tend to generate or extend a network of fractures while a bi-wing fracture was typically generated in conventional reservoirs. This network of fractures includes a large network of micro-fractures opened during the injection of fracturing fluids. Small fractures tend to close under closure stress unless a nano-sized proppant with significant stress resistance is injected to keep these micro-fractures opened. Although very high conductivity is not required for very low permeability formations, an open fracture or micro-fracture performs better than a collapsed fracture.
Proppants with different mesh sizes of 20/40, 30/50, 40/70, 70/140 and 80/200 with grain diameters ranging from 0.033 inch (0.8382 mm) to 0.0041 inch (104.14 μm) have been used during hydraulic fracturing of tight shale formations. These proppants are large enough to create conductivity in the larger generated or existing fractures but not small enough to penetrate into the existing or generated micro-fractures. This will cause the closure of micro-fractures at the end of a fracturing job thus reduction in the length and conductivity of the complex fracture network. This reduction in the fracture network extension will reduce production from tight shale formations.
The objective of this work is to investigate size, nano-hardness, reduced elastic modulus, fluid loss prevention capabilities as well as their induced fracture conductivity by nano-proppants from a currently known waste product.
Transmission Electron Microscope (TEM) images showed that nano-proppants had particle sizes varying from 100 nm to 1 μm. Particles showed hardness and reduced elastic moduli of 1.3 GPa and 20 GPa, respectively. These properties show potential for these nanoparticles to be used as proppants to keep fractures open under stress.
Fluid loss tests were conducted using 1% (w/w) concentrations of nanoparticles mixed with 2% (w/w) of KCl, cross-linked guar solutions, and cross-linked guar solutions mixed with 1 % concentration of nanoparticles and significant fluid loss reduction was observed for one of several types of nanoparticles. These nanoparticles generated significant conductivity when used as proppants in an API fracture conductivity test. Fracture permeability values of 27-33 mD were generated using these nano-proppants.
Use of nanoparticles prior to the placement of larger proppants is recommended in order to prevent fluid loss into the formation, and also increase the conductivity of the fissures and micro-sized fractures.