Calcite is the single most important reactive mineral in the context of permeability evolution of fractures because it is highly soluble, fast-reacting, and often abundant. Calcite dissolution is promoted not only by low pH such as from injected CO2 but also by low concentration of calcium such as in hydrofracking fluids. We have investigated reaction-induced permeability alterations in fractured calcite-bearing rocks using reactive transport modeling and core flooding experiments. The general finding is that fracture permeability can increase substantially as a result of calcite dissolution, but the extent depends on a complex interplay of the initial fracture geometry, mineral spatial variation, fluid composition and flow rate. Even in simple models that account for only calcite as a reactive mineral, complex dynamics arise. For example, fractures in rocks with larger amounts of calcite may show smaller permeability change as buffering retards the advance of the dissolution front. Complex buffering behavior is also responsible for the counter-intuitive finding that a far-from-equilibrium solution may produce less permeability change than a nearer-to-equilibrium solution at the same pH. In the lab, we have used x-ray imaging methods to observe the reactive evolution of fracture geometries, and have identified processes that are currently not accounted for in any reactive transport model. For example, in rocks with calcite-dolomite mixtures, roughness and microporosity develop on fracture surfaces, making it difficult to apply conventional fracture flow models such as the Local Cubic Law. Also, because vertical transport is frequently orthogonal to sedimentary bedding layers, contiguous transverse features that do not dissolve may constrict flow even when there is extensive calcite dissolution. Another example is the case of mineral dissolution causing reduced fracture permeability, where mobilized clay particles clog the flow path. Clearly, reaction-induced permeability evolution presents new challenges for computationally tractable reactive transport models.

Challenges for predicting permeability evolution of fractured carbonate-bearing rocks

247th National Meeting (2014)

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