Modeling particulate transport to optimize hydraulic fracturing and refracturing



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Coupled particle-fluid flow happens in almost all types of particulate processes which are widely observed in many industries, including pharmaceutical, chemical, food processing, and the petroleum industry. An example in the petroleum industry is the employment of proppants in hydraulic fracturing stimulation treatments to develop unconventional reservoirs. A uniform fluid and proppant distribution among multiple clusters in a fracturing stage will contribute to the highest unconventional well productivity. In contrast, a non-uniform treatment distribution increases the risk of frac-hits and results in some formation regions remain under-stimulated. This dissertation aims to enhance our understanding of the underlying reasons for ineffective stimulation treatment distribution to optimize hydraulic fracturing and refracturing. Three specific topics are addressed in this dissertation, namely (1) proppant transport in perforated wellbores, (2) shear-thinning fluid flow and particle transport modeling in realistic rough fractures, and (3) advancements in step down tests and integration with fracture modeling to guide field perforation design. For Topic 1, first, the proppant transport through one perforation in a horizontal wellbore is simulated by coupling computational fluid dynamics with a discrete element method (CFD-DEM) under various suspension flow conditions that are most relevant for practical applications. Proppant transport efficiency (PTE) and perforation flow ratio (PFR) are defined to quantify proppant transport behavior in perforated pipes. A comprehensive study on the effect of casing, perforation, fracturing fluid and proppant properties is conducted, resulting in 65 PTE versus PFR curves with 374 CFD-DEM simulations. A linear relationship between the proppant transport efficiency and perforation flow ratio is identified with PFR ranging from 0 to 0.2, which is consistent with the experimental work. With the updated CFD-DEM dataset, an improved correlation for proppant transport efficiency is obtained to predict the proppant distribution in a fracturing stage more accurately. Then, the effect of wellbore inclination on proppant transport efficiency is examined. The difference between proppant transport efficiency in a horizontal wellbore and a slightly deviated wellbore can be over 20% for low flow rates (< 2 bbl/min) and as low as 5% for high flow rates (> 40 bbl/min). This indicates that proppant inertia dominates proppant transport behavior in the heel-side clusters. The situation is reversed for toe-side clusters where inclination can play a crucial role in determining proppant transport into perforations. Finally, for the first time, the proppant transport in a fracturing stage with multiple clusters is simulated by the CFD-DEM approach. A detailed analysis of proppant distribution at both the cluster level and the perforation level is conducted and direct guidelines for perforation design are provided. The all-low-side perforation design gives the most uniform proppant distribution among four clusters resulting in the lowest proppant concentration at the toe-side of the wellbore, while the 60° phasing performs the worst. For Topic 2, first, this dissertation presents a 3D micro-scale flow simulation for both Newtonian and Cross power-law shear-thinning fluids through a rough fracture over a range of flow regimes, thus evaluating the critical Reynolds number above which the linear Darcy’s law is no longer applicable. The flow domain is extracted from a computed microtomography image of a fractured Berea sandstone. We confirm the previous conclusion in the literature that the non-Darcian Forchheimer’s law could be extended to shear-thinning fluid flow in porous media. Both the inertial coefficient β in Forchheimer’s equation and the critical Reynolds number Re [subscript c] only depend on the fracture geometry and have no obvious dependence on the fluid rheology property. A new correlation for shift factor α for shear-thinning fluid flow in a rough fracture is proposed. Then, this dissertation proposes a simulation workflow to study proppant transport in realistic rough-walled rock fractures to optimize the proppant placement efficiency. Proppant placement, proppant concentration profile along the flow direction, pressure difference build-up across the fracture and total proppant volume retained within the fracture are quantified under various suspension flow conditions. For Topic 3, an improved methodology is presented to better estimate the pressure drops in perforations and in near-wellbore tortuosity using step down tests before and after pumping of proppant slurry in typical plug-and-perf hydraulic fracture stimulations. A workflow is presented to show how the uncertainties in the magnitude of near-wellbore complexity and perforation size, along with uncertainties in hydraulic fracture propagation parameters, can be incorporated in the perforation cluster design.


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