Fracture penetration and proppant transport in gas- and foam-fracturing

Gases show promise as an alternative to water-based fracturing fluids because they are non-damaging to water-sensitive formations, show some potential to create complex fracture networks, flow back to the well rapidly after treatment, and deliver some environmental benefits. However, the ability of gases to transport proppant has been questioned due to their relatively low viscosity and density. The fracturing then proppant-carrying capacity of various gases is investigated to determine the form and function of the emplaced proppant pack. First, fracture propagation and proppant transport driven by several commonly-used pure gases (CO₂, liquified petroleum gas, ethane, and N₂) is simulated and compared against slickwater fracturing – generally identifying inferior reach and functionality. Several methods are then investigated to improve the proppant-carrying capacity of the pure gases. Results show that, compared with slickwater, gases create shorter and narrower fractures and carry proppant shorter distances due to their lower viscosity and faster leak-off. Among the gases examined in this study, liquified petroleum gas and CO₂ return the deepest proppant penetration along the fracture, followed by ethane, and with N₂ unable to carry proppant into the fracture due to the resulting narrow fracture. However, elevating injection rate of gases could improve their fracture-inducing potential and proppant-transport capability to a level competitive with that of slickwater. A near-uniform proppant distribution may be achieved by using a gelled gas, with an approximately two order-of-magnitude enhancement in viscosity, or a foam-based fluid with a high quality. The fracture length may also be extended by limiting leak-off due to the increased viscosity. Moreover, ultra-light-weight proppants (ULWPs) perform better with gases than commonly-used sands in terms of uniformity of distribution, due to decelerated proppant settling. Well performance is improved significantly by fracturing with gelled gases or foams instead of pure gases or by pumping ULWPs instead of normal sands.