A New Approach to 3D Imaging of Multi-scale Pore Systems in Carbonates using Confocal Microscopy
A Hassan, V Chandra, M Yutkin, TW Patzek
Microporous carbonates host a significant portion of the remaining oil-in-place in the giant carbonate reservoirs. For example, intragranular microporosity in carbonates is usually surrounded by large pores that provide efficient flow bypasses and make oil recovery from the microporous grains very difficult. In this work we use confocal laser scanning microscopy (CLSM) to produce high-resolution 3D images of the pore space to characterize and quantify the multi-scale pore types and their interconnectivity.
In this work, we critically review the main challenges involved in confocal image acquisition and signal processing to obtain the high-resolution 3D images of the micropore space. We follow the protocol described in our earlier study (Hassan et al., 2019) to fabricate epoxy pore casts of microporous carbonate rock samples. After dissolving the rock grains with HCl, we use CLSM to produce 3D images of the fluorescent epoxy conformed to the pore space of the rock sample. We evaluate different imaging conditions to optimize the quality of the pore cast imagery with respect to various experimental factors including the choice of fluorophore, objective lens and medium of imaging. Furthermore, we experimentally measure the resolution for confocal imaging system to ensure our pore cast images capture the realistic geometric attributes of the rock pore space. We determine the “true” resolution by evaluating the Point Spread Function (PSF) from imaging standard sub-resolution fluorescent microspheres. We demonstrate that the deviations from the theoretical values of resolution, which can be as high as 60% in some systems, are common since the true resolution depends on imaging optics, sample geometry, signal-to-noise ratio, and dynamic range of the detector.
We applied our protocol to characterize a standard limestone sample, and were able to identify a bi- modal porosity distribution at lateral and axial resolutions of 1 μm and 9.2 μm, respectively. Moreover, we visualized the sub-micron scale 3D interconnectivity between macro- and micro-pores at lateral- and axial-resolutions of 0.36 μm and 2 μm, respectively. The high-fidelity 3D images of the pore space allowed for a more reliable petrophysical interpretation and prediction of transport properties, as we verified the estimated values for porosity, pore-size distribution and permeability with experimental measurements. The imaging methodology we have developed in this study can be used as a standard protocol for obtaining high quality 3D images of epoxy pore casts using confocal microscopy, and can contribute to
improved characterization of micritic carbonate reservoirs and oil recovery methods therein.
