Permeability evolution in carbonate fractures: Competing roles of confining stress and fluid pH

Abstract

[1] We explore the permeability evolution of fractures in carbonate rock that results from the effects of mechanical stress and nonequilibrium chemistry (pH of fluid). Core plugs of Capitan limestone are saw cut to form a smooth axial fracture that is subsequently roughened to simulate a natural fracture with controlled surface topography. Aqueous solutions of ammonium chloride (pH 5$7) transit these plugs at confining stresses of 3-10 MPa, with flow rates and mineral mass fluxes measured to constrain competing mechanisms of permeability evolution. The effluent calcium concentrations are always much lower than equilibrium calcium solubility, resulting in the dissolution-dominant permeability evolution in our experiments. Depending on the combination of confining stress and fluid pH, the fracture apertures either gape (permeability increase) or close (permeability reduction). We quantitatively constrain the transition between gaping (pH < 6.1) and closing (pH > 6.5) with this transition independent of confining stress up to 10 MPa. A transitional regime (6.1 < pH < 6.5) of invariant aperture represents a balance between the two mechanisms of free-face dissolution and pressure solution at the bridging asperities. We employ a lumpedparameter model to interpret the dissolution-dominant evolution of permeability. By considering different dissolution rate constants between noncontacting asperities and the stagnant water film at the contacting asperities, this model replicates the principal characteristics of permeability evolution of the fracture. Observed rates of aperture change are ill matched when the influent pH is 5-6, since wormhole formation is not accommodated in the model. These observations offer a promising pathway to index the switch from aperture gaping to aperture closing for reactive flow as reactivity is reduced and stress effects become more important.

Initial conditions or emergence: What determines dissolution patterns in rough fractures?

“…Understanding the processes underlying wormhole formation and development remains an important problem for the petroleum engineering community [ Panga et al , ; Cohen et al , ; Maheshwari and Balakotaiah , ; Ishibashi et al , ], usually with the aim of optimizing reservoir acidization. In recent years, further research has been initiated because of the relevance of wormhole formation to CO 2 sequestration [ Luquot and Gouze , ; Ellis et al , ; Elkhoury et al , ; Hao et al , ; Deng et al , ; Carroll et al , ; Smith et al , ; Elkhoury et al , ; Ott and Oedai , ].…”

Fundamental controls on fluid flow in carbonates: current workflows to emerging technologies

“…Studies of the impact of fluid pH and effective stress on fracture aperture changes in limestone have identified a transitional pH regime that, for a given effective stress, reflects a balance between the mechanism for free-face dissolution and pressure solution. At high pH values, the fracture aperture increases; however, at lower pH values, there is a decrease as stress effects become more important (Ishibashi et al 2013). These observations may help to constrain elusive fracture aperture evolution, a process that is commonly poorly constrained in fracture-flow simulations.…”

“…This work highlights an important area for future RTM research: specifically, the potentially significant modification of fluid pathways on production timescales by fluidrock interaction during EOR/IOR. Observational studies that are bringing knowledge of reactive transport and deformation processes closer together also signal opportunities for an expanded suite of complementary experiments to address changes in fracture permeability during production (Huerta et al 2012;Ishibashi et al 2013).…”

Influence of Layering on the Formation and Growth of Solution Pipes

“…Soon, however, the competition between the pipes for the available flow leads to the starvation of the shorter ones and the spacing between active pipes increases. Note that for the power-law length distribution (15) …”

“…In that context, it has also been studied experimentally [6,11] and numerically [12][13][14][15]. In the physics community, the interest toward these systems has been sparked by classical papers of Daccord and Lenormand, which analyzed the patterns formed in a dissolving plaster and related them to other pattern-forming systems, such as diffusion-limited aggregation [16,17].…”