Effective and economic proppant placement has been one of the key objectives of hydraulic fracturing. Different proppant & fracture fluid characteristics, and placement methodologies have been historically applied based on learnings from standard proppant transport studies with parallel plate slots. The standard test setup represents a simplified planar fracture with constant width and confined height, incorporating only basic flow characteristics, and thus, is incapable of capturing unique phenomena of proppant transport in unconventional reservoirs.
In this study, proppant transport laboratory tests were conducted on a large-scale (10 ft × 20 ft) tortuous slot flow system. This novel setup incorporates many significant unconventional fracture features, including lateral and vertical tortuosity, variable width, leak-off, fluid dynamics replicating upward fracture growth, etc. Proppant transport behavior was investigated for multiple parameters such as proppant size, density and concentration; fracture fluid type and viscosity; pumping sequence; pump rate; and fracture properties (width, leak-off location and rate, fracture tortuosity profile, flow directions). The detailed parametric and integrated study of test results includes analysis of proppant dune evolution, dune shape, particle size distribution across dune, propped area, fluid, and proppant collected from leak-off and exit ports.
Multiple unique phenomena occurring at tortuous interfaces were observed, including the generation of isolated pockets of proppant pack, restriction of upward movement due to proppant bridging, and creation of discontinuous and sparsely distributed proppant pillars above the dune. The test results demonstrated a larger proppant dune angle in front of the dune peak during injection and a subsequent fall-off of proppant pack with a higher percentage of smaller mesh proppant back filling the area at and near the inlet (analogous to the wellbore). Self-segregation of proppant in slickwater as per mesh size resulted in higher percentage of smaller mesh proppant settled near the injection point, and a higher percentage of larger mesh proppant placed farther in the system. These observations and novel learnings highlight that it is critical to account for tortuous fracture pathway, leakoff effects and flow directions (both lateral and upward) to better understand proppant transport behaviors in unconventional fractures. A partially proppant-filled fracture area is recognized in unconventional fracture in addition to general classification of propped and unpropped fracture area. Utilizing proppant with large mesh size distribution range or pumping smaller mesh proppant first in slickwater helps achieve dual benefits of higher near wellbore conductivity and improved far-field transport.
This study demonstrates and physically verifies unique proppant transport behaviors in unconventional hydraulic fractures. It also provides novel learnings that will help the industry to optimize hydraulic fracture design through the selection of optimum proppant and fluid properties with enhanced pumping strategies for overall well productivity improvement in an unconventional reservoir.