Developing Methodologies for Gravity-Assisted Solution Mining, 20-R8423
Gary R. Walter
Inclusive Dates: 10/01/13 – 09/30/15
Background — The objective of this project was to develop a numerical approach for geomechanical analyses of solution mining activities by combining modeling of the evolution of a horizontal solution cavity with a model that predicts the onset of caving and the extent of caving. This approach can be used to design and optimize undercut solution mining projects in bedded evaporites, particularly for trona deposits such as those in the Green River Basin of Wyoming. This new simulation approach can help mine operators in designing and operating solution mining projects of this type rather than relying on intuition and experience from conventional underground mining.
Approach — The project produced a simplified cavity development model for simulating the gross aspects of evolution of the solution cavity geometry. The model can be used as input to the geomechanical model of the caving process. The model assumes a well-mixed fluid at each cross section of the cavity so that the fluid flow rate can be computed from the known injection-extraction rates and the cross-sectional area of the cavity. The one-dimensional advective-dispersion equation is used to compute local, average concentrations. Cavity wall dissolution rates are based on published, empirical relationships between fluid concentrations and dissolution rates. Changes in cavity geometry are simulated using geometric relationships between evaporite dissolution and cavity wall geometry. Two approaches were used to simulate cavity stability and fracturing. In the first approach, a caving model was developed to simulate the gross aspects of caving initiation and cave front progression. The evaporite mass above the undercut was discretized into Voronoi blocks. In addition to gravity-driven deformation, these blocks can undergo creep-related deformation, which is simulated by assigning viscoplastic material behavior to these blocks. The second approach used a finite-element-based modeling technique to simulate cavity wall creep and development of fractures in the overburden.
Accomplishments — A computer code capable of simulating the evolution of a solution cavity for time periods up to approximately three years was developed. This code was then used to generate representative solution cavity geometries for simulating the stability of the cavity and fracture development. Combining solution cavity evolution simulations with geomechanical modeling provided valuable insights into the processes important to solution mining of bedded evaporite deposits. In addition to developing the solution cavity evolution modeling code and testing the efficacy for the geomechanical modeling approaches, this project produced two manuscripts for presentation at Solution Mining Research Institute technical conferences. The first manuscript focused on development and collapse of solution cavities in thin trona beds. The second addressed the stability of solution cavities developed in thick potash deposits.