Petrophysical properties analysis of a carbonate reservoir with natural fractures and vugs using X-ray computed tomography

• X-ray CT is applied to analyze the effect of fractures and vugs on carbonate petrophysical properties.
• Fractures increase the stress sensitivity of permeability and vugs increase the stress sensitivity of porosity.
• Fractures and vugs increase gas flowability compared with matrix, and oil recovery is low due to gas channelling.

X-ray computed tomography (X-ray CT) is a nondestructive internal core structure characterization technique. With X-ray CT, natural fractures and vugs have been identified in the Y Field, which is a limestone carbonate reservoir in the Middle East. Based on CT images of all core samples, three main core types in the Y Field were identified: matrix, fractured and vuggy carbonate. For core samples with different pore types and structures, the petrophysical properties vary considerably. In this paper, the X-ray CT technique was used to investigate the effect of fractures and vugs on the porosity–permeability relationship, internal heterogeneity of the core samples, permeability and porosity stress sensitivity, and oil–gas relative permeability. The analysis results show that matrix carbonate displays a stronger porosity–permeability relationship compared with that of fractured and vuggy carbonate. Fractured carbonate is very sensitive to the net confining pressure compared with non-fractured carbonate, and vugs increase the porosity stress sensitivity. The oil–gas relative permeability experiment demonstrates that fractures and vugs can provide flow channels and increase gas flowability. In particular, fractures are unfavourable for gas injection and oil recovery is low due to gas channelling.