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Laboratory and discrete element study of proppant crushing and embedment and their influence on fracture conductivity Zheng, Wenbo


Proppants are widely used in hydraulic fracturing for unconventional resources to increase the recovery rate of hydrocarbons in the oil and gas industry. More than 55 million tonnes of proppants were supplied for hydraulic fracturing in 2015 to fracture more than 35,000 wells. During hydraulic fracturing, proppants are transported by fracturing fluids into fractures to form a permeable proppant pack, which acts as a path for hydrocarbons to flow to wellbores. When high closure stresses exist, proppants can be crushed into smaller fragments and embed into fracture surfaces, which reduces the propped fracture conductivity by decreasing the proppant pack permeability and thickness. Choosing a suitable proppant for hydraulic fracturing is key to success in recovering hydrocarbons. Compared with time-consuming laboratory tests and simplified empirical/numerical models, the discrete element method (DEM) is a natural approach for simulating proppant crushing and embedment into fracture faces. However, current DEM simulations have not considered proppant crushing under high closure stresses. In addition, the mechanical properties of different proppants have not been fully characterised thus modelling of proppants in DEM lacks verification. Moreover, DEM simulations of proppant embedment into rock surface have not been calibrated/compared with laboratory hardness tests on rock faces. Finally, the influence of proppant crushing and embedment on proppant selection has not been thoroughly studied. With these in mind, this research presents comprehensive laboratory experiments on proppant crushing which were used to calibrate discrete element models of proppant crushing and proppant embedment. Three objectives are accomplished within this research: (1) characterize the mechanical behaviour of proppant crushing and proppant embedment into fracture surfaces; (2) model proppant crushing and proppant embedment with DEM; (3) study the influence of proppant crushing and proppant embedment on hydraulic fracture conductivity and recommend optimal proppant usage for hydraulic fracturing. The results deliver a better understanding of the crushing characteristics of different proppants, and improve DEM modelling of proppant crushing and embedment into hydraulic fractures, which provides a cost-efficient approach for evaluating the proppant performance for hydraulic fracturing without the need to conduct problematic hydraulic conductivity experiments.

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