Effect of foam quality on effectiveness of hydraulic fracturing in shales

• Gas production correlates with propped fracture area for low k shales.
• Gas production is a non-monotonic function of foam quality in most cases.
• High quality foams can place partial monolayers of proppants in fractures.
• Using water as initial pad followed by a foam with sand is effective.
• Using finer sand improves the fracture effectiveness for low quality foams.

The goal of this work is to study the effect of foam quality on the gas productivity of fractured wells in shales. A numerical model of fracture propagation is developed which incorporates a 2D fracture model, proppant transport, and foam rheology. The fracture conductivity distribution is computed from the local proppant concentration, which is incorporated in a reservoir simulation model to predict the gas production by depressurization. Given the rock mechanical properties, the fracturing fluid, proppants, and rock permeability control the shape and conductivity of the fractures created during hydraulic fracturing. The shape and conductivity of the fracture, in turn, control the gas productivity of such a well. Foam quality, proppant size, proppant concentration, and rock permeability are varied. The results show that, in most cases, the gas production is optimum at an intermediate foam quality (of about 65%) because it creates the largest propped fracture area and minimizes leak-off. Only for the case of 0.1 micro-Darcy rock, low proppant concentration and large proppant size, the gas production increases monotonically with foam quality because high quality foam can place a partial monolayer of proppants. Fracturing with water as the initial pad and a high quality (65–85%) foam as the proppant carrier is more effective than using the same foam for both pad and slurry.