3-D PRINTING POROSITY MODELS OF BEREA SANDSTONE

For the geosciences, 3-D printing is a novel tool that allows the manufacturing of pore networks by transforming digital models into lab-testable samples. We present a new approach to measure porosity of Berea sandstone by combining digital rock imaging, 3-D printing, and helium porosimetry. Berea sandstone samples with 22% porosity were subjected to computed tomography (CT) and segmented into digital models of pore and grain phases. The resolution of CT images was 4.5 microns; the resolution of 3-D printing was 140 microns for the grains and 300 microns for the pores. Pore radii of the Berea sandstone range from one to 100s of microns. Therefore, grain phase models of Berea sandstone extracted from CT volumes were upscaled at least 10 times to meet the resolution of a 3-D printer and reproduce the smallest elements of the pore network (pore throats). Each solid phase model was 3-D-printed in a black resin using laser stereolithography. The 3-D printed models and original rock samples were compared using two approaches: 1) porosity calculations based on helium porosimetry of natural rock and 3-D printed copies and 2) digital measurements of porosity on digital pore network models and CT scans of 3-D printed samples. Because the porosity represents a volume fraction, uniform upscaling should not change the porosity values. Comparison of porosity measured on 3-D printed models and natural samples indicated a difference of 1-1.5 p.u. While the most significant challenge in the application of 3-D printing to pore network analysis is to reproduce the pore throats, our results show a good match between designed and 3-D printed porosity. While current resolution of 3-D printers is insufficient to build micron-scale porosity, mercury injection analysis will be used on upscaled models of sandstones to measure the pore throat size distribution and estimate permeability of the 3-D printed models. Experiments with reproduction of petrophysical data such as transformation of mercury porosimetry results into 3-D porosity models is the main direction where 3-D printing can be advanced with a high potential to replicate flow, acoustic, electrical, and mechanical properties of natural porous rocks.