Sensitivity Study on the Impact of Sub-Volume and Image Resolution on the Prediction of Petrophysical Properties Using Micro CT Technology

Abstract

Abstract The introduction of high-resolution micro-CT scanners in oil & gas industry has provided a cost effective approach to numerically calculate the important petrophysical properties of reservoir rocks. High-resolution rock images are acquired and later processed to capture all the pore types and mineralogy to enable accurate prediction. This study focuses on estimating the representative elementary volume (REV) of porosity and absolute permeability from micro-CT images of carbonate and sandstone formations. Using topological representative networks and robust pore scale simulator, the effects of different volumes (100-1024 voxels) and resolutions (2.7-27 μm) on the estimation of petrophysical properties are examined. Four different formations are analyzed: Berea (medium-permeability sandstone), Doddington (high-permeability sandstone), Estaillades (medium-permeability limestone), and Ketton (high-permeability limestone). Simulation results are benchmarked to core measurements. The results of this study are very encouraging as sandstone porosity estimates were not affected by resolution coarsening and their absolute permeability was predicted accurately at a resolution up to 18 µm in Berea and between 8-27 µm in Doddington. In limestones, the ratio of micro-pores to macro-pores can be an indication of the accuracy of pore-scale simulation results. Micro-pores in limestone are not well captured with a 2-3 μm resolution. Therefore, pore-scale simulations can be run successfully if the flow is dominated by the macro-pores. In general, results can be improved by increasing sample volume beyond 1024 voxels then coarsening it to be computationally feasible. This study gives insights and guidance on defining the appropriate micro-CT scan sub-volumes and resolutions for different rock types when designing and testing pore-scale simulation experiments. Ultra-high resolution imaging and enhanced pore-scale simulation efficiently provide basic and advanced petrophysical data that are comparable to core lab measurements.