52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: The dissolution of rocks in underground flow paths is transport-controlled if their dissolution rates are relatively high, while the dissolution is reaction-controlled if their dissolution rates are relatively low. Transport-controlled dissolution is a common process in the formation of gypsum karst and oil reservoir acid stimulations. As an initial step to understand groundwater flow and dissolution in fractures and in wormholes, pipe flow is used as a simplified representation of groundwater flow. The extended Graetz solution was developed by Li and Einstein, 2017 to simulate the transport-controlled dissolution process when water flows through a tube in gypsum. This model can predict both the effluent concentration and the evolving geometry of the tube during the dissolution process. This paper focuses on the experimental validation of this model. An effluent chemistry monitoring system (ECMS) was developed and integrated in the triaxial system at MIT. Flow tests were conducted with cast gypsum tubes in this triaxial system. The experimental results of the effluent chemistry and tube geometries confirmed the validity of the extended Graetz solution.
The dissolution of rock minerals is a very common process that is of interest in a number of contexts: (1) underground gypsum and limestone karst formations (Benito et al. 1995; Johnson, 2008); (2) petroleum reservoir exploitation; (3) carbon dioxide sequestration (4) geothermal reservoir exploitation and heat storage; (5) mining and mineralization processes (in situ leaching); and (6) geotechnical applications (including effects on underground storage reservoirs, tunnels and other structures) (Cooper, 1986). The dissolution changes the geometry of the flow paths, which in turn affect the rate of dissolution in the rock-fluid system. For example, the dissolution of rock minerals in a rock matrix enlarges the pore space and concentrates flow in wormholes, which are the long, finger-like channels that form due to the nonuniform dissolution of the matrix. The dissolution of rock minerals in rock fractures enlarges the fracture and leads to channeled flow in rock fractures. The evolution of these cavities (wormholes and fractures) may lead to undesired consequences such as sinkholes, subsidence and cap rock leakage. Therefore, it is important to have a better understanding of the dissolution induced evolution of the underground cavities.
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