UC San Diego

This work focuses on providing a better understanding of the proppant enhancement process utilized in hydraulic fracturing of rock networks. During rock fracture enhancement, proppants (i.e., rigid particles), are typically mixed with a carrier fluid and injected into a fracture network that has been initiated/propagated by a ‘clean’ particle-free fluid under pressure. Once introduced into an opened fracture, these proppant particles become confined between fracture faces once pressure is released and dissipates. This ‘propped’ material in turn provides a porous pathway for production fluid (e.g., hydrocarbons, cycled geothermal fluid) through the fracture network, improving productivity and/or yield.

The design of proppant applications is commonly reliant on simplified particle-fluid behavior. Several of these simplifications and assumptions include taking proppant horizontal transport rate as equal to averaged Poiseuille flow velocity, basing settling behavior of particles on Stokes law, and considering slurry settling behavior to be analogous to that between smooth parallel plates. It has been observed however, that the variance between in-lab versus field achieved performance can vary by upwards of 90 %. To evaluate potential causes for this discrepancy, this work focuses on modeling of field-realistic conditions with high accuracy, including in-flow behavior of concentrated proppant slurries and proppant behavior within realistically rough fracture openings. The investigation is performed using combined computational fluid dynamics with the discrete element method (CFD-DEM). This method provides a detailed micro-scale representation of the complex interactions exhibited by this two-phase system, allowing for extraction and measure of difficult-to-capture fluid-solid interactions. Both the unresolved and resolved implementations of this modeling method are used to explore the meso and micro-scale behaviors of proppant slurries.

Findings reveal that complex particle clustering structures in flowing slurries can affect particle travel capacity and lead to variance from simplified slurry flow models. Proppant settling, flow, and transport within realistic rough fracture environments are also shown to exhibit notable fluid-particle-fracture surface interactions leading to significant variance from simplified modeling formulations that are based on smooth-walled domain behavioral simplifications. Overall, this work provides a greater understanding of proppant behavior in more field accurate conditions and contributes to better design considerations for future proppant treatments.