Spontaneous switching of permeability changes in a limestone fracture with net dissolution

Abstract

Results are reported for water flow-through experiments conducted on an artificial fracture in limestone at room temperature and under ambient confining stress of 3.5 MPa. Tests are concurrently monitored for mineral mass loss or gain and for changes in differential pressure between the inlet and outlet, throughout the 1500-hour duration of the experiment. Periodic imaging by X-ray computed tomography augments the fluid and mineral mass balance data and provides a third independent constraint on dissolution processes. The sample is sequentially circulated by water of two different compositions through the 1500-hour duration of the experiment, the first 935 hours by sampled groundwater (pH ≈ 8), followed by 555 hours of distilled water (pH ≈ 6). Large changes in the differential pressure are recorded throughout the experiment, for the constant flow rate of 2 cm2/m; these are used as a proxy for recorded changes in fracture permeability, under invariant effective stress conditions. Mass of Ca and Mg were net-removed throughout the experiment. During the initial circulation of groundwater, the differential pressure increased almost threefold and is interpreted as a net reduction in permeability as the contacting asperities across the fracture are removed and the fracture closes. With the circulation of distilled water, permeability initially reduced threefold and ultimately increased by 2 orders of magnitude as a "wormhole" developed in the sample. This spontaneous switch from net decrease in permeability to net increase occurred with no change in experimental conditions of flow rate or applied effective stress, and Ca was net dissolved throughout. This behavior is attributed to the evolving localization of mass removal, triggered as free-face dissolution outcompetes stress-mediated dissolution at the asperity contacts.