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

Abstract

[1] 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 cm 2 /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.

A numerical model simulating reactive transport and evolution of fracture permeability

“…Conversely, precipitation-driven sealing is observed in tuff at a range of temperatures [9]. Gaping is observed in hydrocarbon reservoir rocks [10,11], and spontaneous or induced switching from sealing to gaping is reported at ambient temperatures ð208CÞ in limestone [12] and at modest temperatures (20-1208CÞ in novaculite [13]. These limited studies on fractures provide no conclusive view of the effects controlling the evolution of the transport properties, and the evolving rates and magnitudes of fracture permeability driven by interaction between the mechanical and chemical processes remain poorly constrained.…”

“…Correspondingly, processes are innately averaged, and no account is made for the spatial structure. Such models typically make adequate predictions of homogeneously distributed behaviours, but not of localized effects, such as the evolution of a through-going dissolution conduit (wormhole) [12,37]. The digitized fracture obtained through the profiling data constrains the relation between the fracture aperture and the contact-area ratio as shown in Figure 8.…”

Short-Timescale Chemo-Mechanical Effects and their Influence on the Transport Properties of Fractured Rock

“…These characterizations explain the enigmatic observation that fractures gape with net dissolution (DURHAM et al, 2001;POLAK et al, 2003). Specifically in this work, process-based models are applied to quantify rates of fracture closure observed in prior experimental data where the role of free-face dissolution is shown important (e.g., the experimental results of POLAK et al, 2004).…”

Influence of Layering on the Formation and Growth of Solution Pipes

“…Recent years have seen a growing number of experimental studies of dissolution-induced morphology evolution [26][27][28][29], many of them in the context of CO 2 sequestration [30][31][32][33][34][35][36][37][38]. Since many of the potential reservoirs for CO 2 storage are in carbonate strata, it is crucial to understand how the flow of CO 2 -acidified brine impacts the long-term changes in porosity and permeability of the reservoirs and how it affects the caprock properties.…”