Multi-scale characterization of porosity in naturally mineralized fractures

Abstract

Naturally occurring fracture systems contain a separate porosity and permeability that can impact the way fluid flow through a rock by either providing flow obstruction or avenues of preferential flow. Current techniques for quantifying fracture porosity and permeability are limited to length scales of 10’s to 100’s of microns up to millimeter resolution, yet we know that pore networks in rock are fractal systems with porosity in the micron to manometer length scales. Because nano-scale pores play an important role in tight rock matrices, it is possible that pores at these length scales in natural fractures could contribute to flow either within the fracture itself or between the fracture and matrix. Here we use a combination of electron microscope techniques, cathodoluminescence imaging and small-angle neutron scattering to test the hypothesis that natural fractures that appear sealed at macroscale have connected micro- and nano-porosity. Fracture material from sub-vertical and horizontal mineralized fractures was analyzed. Estimates of micro-porosity in the analyzed vertical fractures by SEM image analysis range between 2.0 and 9.8%, but a lack of continuous grain boundary porosity suggests that the analyzed vertical fractures are not potential pathways for fluid flow. Estimated micro-porosity in horizontal fractures by SEM image analysis ranges between 0.9% and 8.3 % of fracture area and does contain connected grain boundary porosity. SANS-porosity in the horizontal fractures ranges between 0.2% – 1.7%. Preliminary FIB-SEM and TEM imaging shows that the geometry of nano-porosity is influenced by the crystal habit of the calcite fracture material. This thesis provides the first quantitative look at nano-porosity in macroscopically filled fractures and gives evidence that these pores are potentially connected and could contribute to fluid flow. Additional studies with this same approach can help build a knowledge base of nano- and micro-porosity presence in natural fractures and help elucidate their contribution to formation permeability.