Lattice Boltzmann Methods Applied to Three-Dimensional Virtual Cores Constructed from Digital Optical Borehole Images of a Karst Carbonate Aquifer

Recovery of whole-core samples from macroporous karst carbonate is nearly impossible with conventional drilling technology. Thus, the most porous part of coreholes drilled in karst systems rarely yield whole-core samples. The consequent lack of samples for measurement of fluid-flow properties in karst carbonate aquifers impedes characterization of ground-water flow within these systems. This study uses advanced modeling techniques together with geophysical corehole data acquired from the karst carbonate Biscayne aquifer of southeastern Florida, USA, to explore a combination of innovative technologies designed to compensate for the lack of macroporous whole-core sample data. Specifically, these methods are being used to better understand the ground-water flow regime in the Biscayne aquifer. In this study, digital optical borehole image logs were compiled for test coreholes that penetrate the rocks of the Biscayne aquifer. The borehole image data were then processed to map the 3-D distribution of macropores and rock matrix present on the borehole walls using Stanford geostatistical software (SGeMS). The SGeMS program was used to compute variograms that were used as input for a computer simulation. The simulation results provided virtual 3-D renderings of the complex karst macropore network of the Biscayne aquifer that statistically replicate the borehole wall image data. These renderings provided 3-D visual records of areas of the aquifer that are composed of a carbonate eogenetic macropore system dominated by centimeter-scale vugs produced by fossil molds and voids associated with trace fossils. The vugs can coalesce over broad areas in the Biscayne aquifer to form laterally persistent zones of preferential ground-water flow. Lattice Boltzmann methods (LBMs) were used to measure the intrinsic permeability of the 3-D aquifer renderings. When using LBMs the rock matrix was assumed to be a nonporous media, thus permeability was only measured within the network of macropores. Comparison of LBM-derived permeabilities to those obtained from conventional laboratory techniques show that the measured permeability of whole-core samples are substantially challenged in areas where centimeter-scale vuggy macroporosity is present and for this type of porosity, LBMs are preferred. The results obtained using LBMs closely conform to the analytical solutions for pipeflow, providing the impetus and justification for its use in obtaining intrinsic permeability values for virtual macropore systems. LBMs were also used to simulate 3-D fluid flow through the renderings of macropores and rock matrix. LBMs were especially useful for simulations of inertial (non-Darcian) fluid flow, which may dominate flow in macroporous zones in parts of the Biscayne aquifer. The methods being developed in this study are providing a means for estimating and correlating the permeability of macroporous zones, as well as determining whether flow is primarily laminar or turbulent.