A rapidly-growing energy demand in recent years has made shale gas more attractive than ever. Although shale-gas formations contain a large amount of hydrocarbons, their low-permeability characteristics have been a major impediment to economic development of the fields. Once properly designed, foam fracturing has advantages over the conventional hydraulic fracturing – for example, by using less water, it works better for water-sensitive shale-gas reservoirs; a smaller quantity of water involved makes the fracturing job more environment-friendly due to the reduced amount of chemical additives; and it offers a superior capability to carry and distribute solid proppants over the newly-created factures. In spite of its unique advantages, optimum foam fracturing treatment requires a good understanding of foam rheology.

A series of recent experimental studies revealed that foam flow can be represented by two distinct flow regimes in general: low-quality regime showing stable plug-flow pattern, and high-quality regime showing unstable slug-flow pattern. This study, for the first time, presents how to develop a comprehensive foam model that can handle a variety of bubble-size distributions and flow patterns by using two-flow-regime concept for fracturing.

Analyzing experimental data of surfactant foams and polymer-added foams shows that (i) in the low-quality regime, foam rheology is governed by bubble slippage at the wall with no significant change in its fine foam texture and (ii) in the high-quality regime, foam rheology is governed by the relative size of free-gas segment to fine-textured foam-slug segment. By using these governing mechanisms, this new foam model successfully reproduces foam flow characteristics as observed in the experiments, including almost horizontal pressure contours in the low-quality regime and inclined pressure contours in the high-quality regimes. Although the model is built with a power-law fluid model, the same procedure can be taken for Bingham-plastic or yield-power-law fluids.

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