Using Gamma-Ray and X-Ray Computed Tomography for Porosity Quantification of Reservoir Analogue Rocks

During the last few decades, X-ray micro-computed tomography (µCT) has been largely used to characterize rock properties and to create high-resolution 3D digital image volumes. It has allowed access to important information about porous systems in reservoir rocks. However, the reliable quantification of porosity of rocks which present porous volumes ranging from centimeter to nanometer scale remains a challenge. Assessment of nano scale porous volume is very difficult by image segmentation techniques, due to the intrinsic limits of the x-ray imaging method. Moreover, image processing for analysis of various types of porosity in the same sample, including microporosity could be computationally expensive. We present a method based in the Gamma-Ray computed tomography (axis attenuation) that can substantially improve the limits presented by conventional X-ray microtomography. This study compared the porosity values acquired by typical segmentation methods for microtomography images, and by the values obtained trough the proposed method of gamma-ray computed tomography to calculate the porosity. Results of both approaches were compared to porosity measurements obtained through experimental equipment (helium porosimeter). These analyses were performed in core samples of limestones and sandstones analogous of Brazilian oil reservoirs. The Gamma Ray Attenuation method (axis attenuation) presented a better correlation (R² = 0.9588) to the experimental measurements when compared to the image segmentation methods (R² = 0.9194). The results suggest that Industrial application of gamma ray tomography for precise evaluation of large number of core samples can be highly effective. Furthermore, the gamma ray data can be integrated with data provided by conventional µCT image processing to complement information regarding morphological aspects.

Keywords: Porous System, X-ray microtomography, Gamma Ray tomography,  Reservoir rocks