Experimental Observations of Calcite Precipitation in Fractures: The Role of Physical and Chemical Heterogeneity on the Persistence of Preferential Flow Paths

Abstract:

Mineral precipitation significantly alters the transport properties of fractured rock. In an idealized parallel-plate fracture with uniform surface reactivity, the injection of a supersaturated fluid leads to faster precipitation near the inlet and rapid sealing of the fracture. However, it is likely that physical and chemical heterogeneities will perturb local reaction rates. Predicting the evolution of transport properties in heterogeneous fractures requires an improved understanding of the feedback between mineral precipitation and fluid flow. We present results from three experiments in transparent analog fractures: i) uniform aperture with uniform surface reactivity, ii) uniform aperture with heterogeneous surface reactivity, and iii) variable aperture with heterogeneous reactivity. We controlled surface reactivity by exposing one fracture surface to a solution supersaturated with calcium carbonate for different durations. We then injected a metastable mixture with log-saturation-index of 1.44 at a constant rate of 0.5 ml/min. Transmitted light techniques provided quantitative measurements of fracture aperture and fluid transport over the flow field. In the homogeneous fracture (i), as expected, aperture decreased most rapidly along the inlet. In the variable aperture fracture (iii), we observed enhanced reduction rates in small aperture regions; these local reactions organized flow into thin pathways that connected regions of low surface reactivity and grew to span the length of the fracture over the six-month duration of the experiment. An ongoing experiment (ii) will clarify the relative importance of chemical heterogeneity versus aperture variability on the formation of these preferential pathways.