Development of a three-dimensional compositional hydraulic fracturing simulator for energized fluids
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Current practices in energized treatments, using gases and foams, remain rudimentary in comparison to other fracturing fluid technologies. None of the available 3D fracturing models for incompressible water-based fluids have been able to capture the thermal and compositional effects that are important when using energized fluids, as their constitutive equations assume single-phase, single-component, incompressible fluid flow. These models introduce a bias in fluid selection because they do not accurately capture the unique behavior of energized fluids. The lack of modeling tools specifically suited for these fluids has hindered their design and field implementation. This work uses a fully compositional 3D fracturing model to answer some of the questions surrounding the design of energized treatments. The new model is capable of handling any multi-component mixture of fluids and chemicals. Changes in fluid density, composition, and temperature are predicted using an energy balance equation and an equation of state. A wellbore model, which relates the surface and bottomhole conditions, determines the pumping requirements. Fracture performance is assessed by a fractured well productivity model that accounts for damage in the invaded zone and finite fracture conductivity. The combination of the fracture, productivity, and wellbore models forms a standalone simulator that is suitable for designing and optimizing energized treatments. The simulator offers a wide range of capabilities, making it suitable for many different applications ranging from hydraulic fracturing to long-term injections for enhanced oil recovery, well clean-up, or carbon sequestration purposes. The model is applicable to any well configuration: vertical, deviated, or horizontal. The resolution of the full 3D elasticity problem enables us to propagate the fracture across multiple layers, where height growth is controlled by the vertical distribution of the minimum horizontal stress. We conducted several sensitivity studies to compare the fracture propagation, productivity, and pumping requirements of various fluid candidates in different reservoirs. The results show that good proppant placement and high fracture conductivities can be achieved with foams and gelled fluid formulations. Foams provide a wide range of viscosities without using excessive amounts of gelling agents. They also provide superior fluid-loss control, as the filter-cake is supplemented by the presence of gas bubbles that reduce liquid-flow into the porous medium. CO₂, LPG, and N₂ expand significantly (by 15% or more) as the reservoir heats the fluid inside the fracture. These fluids show virtually no damage in the invaded zone, which is a significant improvement upon water-based fluids in reservoirs that are prone to water blocking. These results, however, are contingent on an accurate fluid characterization supported by experimental data; therefore, our work advocates for complementary experimental studies on fluid rheology, proppant transport, and fluid leak-off. A comprehensive sensitivity study over a wide range of reservoir conditions identified five key reservoir parameters for fluid selection: relative permeability curve, initial gas saturation, reservoir pressure, changes to rock mechanical properties, and water-sensitivity. Because energized fluids provide similar rheology and leak-off behaviors as water-based fluids, the primary design question it to evaluate the extent of the damaged zone against costs, fluid availability, and/or safety hazards. If the fluid-induced damage is acceptable, water-based fluids constitute a simple and attractive solution; otherwise, energized fluids are recommended. Notably, energized fluids are well-suited for reservoirs that are depleted, under-saturated, and/or water-sensitive. These fluids are also favorable in areas with a limited water supply. As water resources become constrained in many areas, reducing the water footprint and the environmental impact is of paramount concern, thereby making the use of energized treatments particularly attractive to replace or subsidize water in the fracturing process.