A review on non-aqueous fracturing techniques in unconventional reservoirs

Technical and environmental concerns, besides water conservation in remote areas, are associated with the utilization of conventional water, oil, and polymer based fracturing techniques for shale formations. On the other hand, non-aqueous fracturing techniques are green hydraulic fracturing methods that leave minimal impact on the formation and environment; and thus, could serve as a good alternative to water-based approaches. In this review, several waterless technologies for improving shale and ultra-tight rocks conductivity was studied. Some available simulation techniques for monitoring the performance of these techniques were also covered. Liquid and supercritical carbon dioxide (Sc-CO2) fracking method has a high initial execution cost, although it present a potential in mitigating global warming was discussed. The general understanding of the inherent limitations associated with each methods is required, and therefore have been addressed for better decision making. While waterless fracturing is characterized by a rapid flow back period, Sc-CO2 has low sand carrying capacity because of its low viscosity. Similarly, cryogenic fracturing with liquid N2 for instance causes rapid cooling of the rock that changes the petrophysical and mechanical properties of the formation. For these reasons, future research focus should be geared towards comprehending how the cooling process involved in employing cryogenic fluids could induces micro-fractures and alter rock permeability. Furthermore, the means to improve the carrying capacity of Sc-CO2 with low-medium viscoelastic surfactants need to be studied, in addition to the impact of adsorption forces between fracturing fluid and the rock on proppant transport. Long term reaction mechanisms between acidified water, CO2 and rock mineralogy need to be investigated alongside fracture propagation modeling for a successful carbon capture and storage in fractured low permeability formation.