Lattice Boltzmann Methods Applied to 3-D Virtual Cores Constructed from Digital Optical Borehole Images of a Karst Carbonate Aquifer

Recovering whole-core samples from macroporous karst carbonate is nearly impossible with conventional drilling technology; the most porous part of coreholes drilled in karst rarely yield whole-core samples. The result is an absence of samples for permeability measurement that impedes characterization of flow in karst aquifers.

This study uses advanced modeling techniques and geophysical corehole data from the Biscayne aquifer of southeastern Florida, USA, to surmount a lack of macroporous whole-core samples. These methods are improving understanding of the aquifer's flow regime. Digital optical borehole image logs were acquired in coreholes. Borehole image data provided the 3-D distribution of macropores and rock matrix. Geostatistical software computed variograms and allowed for computer simulation of virtual 3-D renderings of the macropore network that statistically honor the borehole wall data. These renderings simulate areas of the aquifer composed of a carbonate eogenetic macropore system dominated by centimeter-scale vugs produced by fossil molds and voids associated with trace fossils. These vugs can coalesce to form laterally persistent zones of preferential ground-water flow.

Lattice Boltzmann methods (LBMs) were used to measure the permeability of the aquifer renderings. The rock matrix was assumed to be nonporous; thus permeability was only contributed by macropores. Comparison of LBM-derived permeabilities to conventional laboratory measurements showed that permeability measurements of whole-core samples are challenged where centimeter-scale vuggy macroporosity is present. LBM results closely conform to analytic solutions for pipe flow, providing the impetus and justification for its use in obtaining permeability values for the virtual macropore systems.

LBMs were especially valuable for simulation of inertial (non-Darcy) fluid flow, which could 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 and investigating whether laminar or turbulent flow is the rule.