Rapidly increasing global energy demand and the limited global resources have renewed interest in unconventional natural oil and gas. Economical hydrocarbon production from unconventional resources requires source stimulation by fracturing to create conductive paths for hydrocarbon production. Proppants, which are essential for successful fracturing treatment, are injected with fracturing fluids to provide conductive pathways from the reservoir to the well.

Since the first fracturing was done with silica sand as a proppant in 1947, many other materials have been used as proppants including walnut hulls, natural sands, glass, resin coated sands, sintered bauxite and kaolin, and fused zircon. High-grade bauxites are preferred for achieving the high strength required of proppants in deep wells, where closure stresses can exceed 8,000 to 10,000 psi or even higher. The ability to manufacture high-quality proppants from a broad array of alternative raw materials offers significant strategic benefits. Ultra-lightweight proppant is also desirable since it reduces proppant settling, requires low viscosity fluids to transport. Additionally, it enhances the potential for increased propped fracture length. Perfectly spherical proppant with narrow size distribution provides fracture with the highest conductivity. Proppant resistant to diagenesis under downhole condition will maintain the long-term fracture conductivity. Multifunctional proppants filled with additives, particularly scale inhibitors for long-term well performance have been applied in the field as well.

The main objectives of this paper are to review: (1) different types of proppants used in the oil and gas industry, (2) standard testing methods for proppants performance, (3) the literature about the diagenesis/geochemical reaction of proppants and formation face under heat and pressure, and (4) proppant selection criteria for unconventional resources. Finally, the paper sheds light on the current challenges and emphasizes needs for new proppant development for unconventional resources.

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