Comparing petrophysical properties and pore network characteristics of carbonate reservoir rocks using micro X-ray tomography imaging and microfacies analyses

Abstract

AbstractCarbonate rocks have complex pore structures as a result of sedimentological and diagenetic processes. We investigated the pore network of four carbonate rock samples originating from an oil well in Hungary. Two samples were from the productive part of the Sarmatian limestone, and the other two were from the dry interval. We employed X-ray computed tomography (micro-XCT) in combination with lab measurements and microfacies analysis. To achieve accurate X-ray image segmentation we investigated and compared several segmentation techniques, including entropy and clustering. We then employed the Naïve Bayes classifier and tenfold cross-validation to assess the accuracy of our results. Our study achieved high accuracy with the type-2 fuzzy entropy technique on various metrics, including precision and recall. The comparison between the measured helium porosity and image-derived porosity showed a close match. Micro-XCT measurements revealed connected pore structure in the productive interval, compared to isolated pores in the dry interval. Microfacies analyses indicated that both the rocks of the productive and dry intervals were deposited in a marine environment, but different diagenetic processes altered the sediment into productive and dry rocks, respectively. The diagenesis of the productive rocks occurred in a meteoric phreatic environment, where secondary pores were created (porosity enhancement). In contrast, the dry rocks underwent meteoric vadose and marine diagenesis, where all previously created pores were filled with cement (porosity destruction). Our study highlights the importance of accurately characterizing the pore network of carbonate rocks, which can aid in understanding reservoir properties and predicting fluid flow behavior.