Spontaneous Switching between Permeability Enhancement and Degradation in Fractures in Carbonate: Lumped Parameter Representation of Mechanically- and Chemically-Mediated Dissolution

Abstract

Abstract. Principal mechanical and chemical processes contributing to the observed spontaneous switching from net decrease in permeability to net increase in a fracture in carbonate are examined. The evolution of permeability, and related fracture aperture, is represented through a lumped parameter model. The significant processes of pressure solution beneath bridging asperities, transport of dissolved mass to the fracture void, and subsequent precipitation or dissolution within the fracture void enable the principal characteristics of observed behavior to be followed. The evolution of dissolved mass concentration in the pore fluid is followed for arbitrary applied stress, temperature, and pH conditions, with appropriate feedback to the evolution of fracture permeability. Comparisons with experimental measurements in limestone , Water Resour. Res. Vol. 40, W03502, doi: 10.1029 show satisfactory agreement for the evolution of fracture aperture and to a lesser degree in calcium concentrations in the effluent pore fluid. Importantly, the spontaneous switching in permeability change, from aperture reducing to aperture increasing, with no change in environmental conditions, is replicated without the need for an ad hoc trigger. Although this switch is accurately replicated, the lumped parameter model is incapable of replicating the rapid observed growth in permeability that directly follows. This inability results from the assumed form of the lumped asperity model, that is incapable of representing the spatially distributed change in aperture that is seen to occur within the fracture. Despite this inconsistency, the model is shown capable of representing the principal behaviors evident in the response.

A numerical model simulating reactive transport and evolution of fracture permeability

“…Prior to applying the current model, the lumped parameter model previously developed [35,37] is first adopted to predict the progress of mean aperture closure and evolution of Si efflux, for the same experiment. Then, the current model is applied to quantify the experimental observations, and predictions given by the two different models are compared and examined, relative to the experimental measurements.…”

Chemically and mechanically mediated influences on the transport and mechanical characteristics of rock fractures

“…Sealing has been suggested in some instances of vapor and fluid transport under stressed conditions [9][10][11][12][13], and gaping has been suggested in carbonate rocks , e.g., [14]. The controlling processes and feedbacks apply, in varying degrees, to a variety of disciplines, and at a variety of spatial and temporal scales.…”

“…The controlling processes and feedbacks apply, in varying degrees, to a variety of disciplines, and at a variety of spatial and temporal scales. The changes result mainly from dissolution and precipitation of minerals within the fracture [12][13][14], both in free-face dissolution and by stress-mediated effects.…”

“…Recently, some models have been developed in studies of the chemo-mechanical effects on transport properties, and these models have been applied to quantify the rates of fracture closure at a given stress condition [12][13][14]. The current work focused on different aspects of chemo-mechanical effects on fractured rock by considering the steady-state aperture changes that occur during the loading and unloading history of mechanical stress and temperature.…”

“…It is also recognized that there can be an increase of permeability by free face dissolution, but this process is not applicable for the proposed model. [14].…”