Fluid Mechanics of Hydraulic Fracturing: a Review

Abstract

Although hydraulic fracturing is a mature technology that has been used commercially since the late 1940s, the development of unconventional hydrocarbon fields with the combination of directional drilling and multistage hydraulic fracturing in the last two decades gave rise to a substantial progress in both operations and associated modeling. Numerical simulators, based on those models, are key to the design and evaluation of hydraulic fracturing treatments. Though hydraulic fracturing is a truly coupled phenomenon, the solid mechanics part of the problem has typically received more attention than the fluid mechanics part. Yet, that fluid mechanics field is a very rich multidisciplinary domain, presenting a number of challenges posed by the contemporary technology advancement, most of which being still unresolved. This paper aims to review the state of the art in multiphase fluid mechanics modeling of hydraulic fracturing, highlighting gaps in the body of knowledge and clarifying the questions that are still open.This review sheds light on critical phenomena peculiar to hydraulic fracturing treatments, which are grouped into three categories (according to subsequent stages of the stimulation treatment): (i) proppant transport down the wellbore, (ii) proppant placement into the fracture, (iii) flowback from fractures into a well after the end of stimulation treatment (which is particularly important for preserving the integrity and conductivity of the fracture network). To support the modeling in these areas, constitutive relationships calibrated by experiments are of paramount importance. The list of phenomena, still not fully covered by modeling, includes: slugs dispersion in the well during alternate-slug fracturing, impact of fibers and visco-elasto-plasticity of the fracturing fluid on proppant placement in fractures, effects of complex rock fabric and real fracture morphology (roughness, steps, ledges, turns, and junctions), transition from dense suspension to close packing, dynamic bridging and mobilization, particle sedimentation to form a packed bed and re-suspension, dune transport in fracture network, overflush, and flowback into the near-horizontal well from fractures, to name a few. All these effects need to be properly accounted for in the hydraulic fracturing simulators in order for the contemporary technology of multistage fracturing to be designed, executed, evaluated, and optimized properly and safely to yield optimum production, especially in unconventional reservoirs.