Wormhole Formation During Acidizing of Calcite-Cemented Fractures in Gas-Bearing Shales

Abstract

A relatively large number of calcite-cemented fractures are present in gas-bearing shale formations. During hydraulic fracturing, some of these fractures will be reactivated and may become important flow paths in the resulting stimulated fracture network. On the other hand, the presence of carbonate lamina on fracture surfaces will have a hindering effect on the transport of shale gas from the matrix toward the wellbore. We investigate numerically the effect of low-pH reactive fluids on such fractures, and show that dissolution of the cement proceeds in a highly nonuniform manner. The morphology of the emerging flow paths ("wormholes") strongly depends on the thickness of the calcite layer. For thick carbonate layers, a hierarchical, fractal pattern appears, with highly branched wormhole-like channels competing for an available flow. For thin layers, the pattern is much more diffuse, with less-pronounced wormholes that merge easily with each other. Finally, for intermediate thicknesses, we observe a strong attraction between shorter and longer wormholes, which leads to the formation of islands of carbonate lamina surrounded by the dissolved regions. We argue that the wormholeformation processes are not only important for the increase of shale-gas recovery, but also can be used for retaining the fracture permeability, even in the absence of proppant.2019 SPE Journal 1 is the main driving process behind the cave formation in karst systems (Dreybrodt 1990;Hanna and Rajaram 1998;Cheung and Rajaram 2002;Szymczak and Ladd 2011). All these studies show that the dissolution of rock fractures is a strongly unstable process, with spontaneous focusing of the flow in high-porosity channels (wormholes). The instability here is, in fact, stronger than the corresponding instability in the dissolving porous media (Szymczak and Ladd 2011), with much weaker stabilization caused by diffusion. It needs to be stressed that both experimental and numerical studies of fracture dissolution have considered homogeneous soluble samples, in which aperture growth is potentially unlimited. In shales, the situation is fundamentally different, because the amount of soluble cement at a given place is finite and, thus, after a certain time, the cement fully dissolves, and the dissolution front moves farther downstream. As it turns out, a limited amount of soluble material significantly influences the dissolution patterns in fractures. Nevertheless, as we show in the present study, the dissolution front is still unstable, with an intense wormhole formation in the system. Besides serving as a means to increase the fracture-matrix transport, such a nonuniform dissolution is potentially important for retaining the fracture permeability even in the absence of proppant Wu and Sharma 2015). Whereas a uniformly etched fracture may close tightly under the load of overburden once the fluid pressure is removed, the nonuniform etching will tend to maintain the permeability. The less-dissolved regions will then act as supports to keep more-dissolved regions op...

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