Developing Methodologies for Gravity-Assisted Solution Mining, 20-R8423
Gary R. Walter
Inclusive Dates: 10/01/13 – 09/30/14
Background — The objective of this project is to develop a numerical approach to model the evolution and fate of horizontal solution cavities in solution mining projects. The approach combines a cavity evolution model that considers dissolution dynamics in a cavity and a geomechanical model that predicts the onset and extent of caving resulting from the ongoing cavity evolution. This combined model can be used to design and optimize undercut solution mining projects in bedded evaporates, particularly for trona deposits like those in the Green River Basin of Wyoming. This new tool will help mine operators design and operate solution mining projects using a simulation-based tool to augment intuition and experience gained from conventional underground mining.
Approach — The project uses a simplified cavity evolution model to simulate the gross aspects of the solution cavity geometry, and uses the evolving cavity geometry as input to a geomechanical model that considers the caving process. The cavity evolution model assumes the fluid is well mixed at each cross section of the cavity, calculating the fluid flow rate from the known injection-extraction rates and the evolving cross-sectional area of the cavity. The model uses the one-dimensional advective-dispersion equation to calculate average concentrations across each cross section. Changes in cavity geometry due to evaporite dissolution are simulated using published empirical relationships between fluid concentrations and dissolution rates, combined with geometric relationships considering cavity wall geometry. The caving model simulates the gross aspects of caving initiation and collapse front progression. The caving model discretizes the evaporite mass above the cavity into Voronoi blocks. These blocks can undergo gravity-driven deformation and collapse, as well as creep-related deformation due to viscoplastic material properties.
Accomplishments — Results from this project were presented at the Solution Mining Research Conference held in San Antonio in May 2014. These modeling results addressed the initial project scope, which considered evaporite beds featuring an insoluble overlying confining bed. Other evaporate configurations lack an insoluble overlying confining bed, such as potash deposits in Canada that occur within a thick sequence of other evaporite deposits like halite. In these geologic settings, economic-mineral extraction procedures seek to prevent dissolution of the overlying evaporite beds in order to minimize contamination of the high value solution and avert a potential collapse of the cavity and overlying land surface if the cavity were to penetrate too far into the overburden. Common practice uses an oil cap, formed by injecting a liquid hydrocarbon such as diesel fuel, to prevent dissolution of overburden beds. On-going work is extending the cavity evolution model to consider the more complex effects of an oil cap on cavity development.