Unsteady-State Coreflooding Monitored by Positron Emission Tomography and X-ray Computed Tomography

Abstract

Summary The forced-imbibition displacement process—a complex function of multiple physical properties of rock, fluids, and flow conditions—is of vital importance. For this paper we used imaging techniques to detect the process in three dimensions with high temporal resolution. Positron emission tomography (PET) is a powerful tool for imaging fluid transport in porous media because it has high temporal resolution and is not affected by complex lithology and pore structures. In this study, we conducted a detailed experimental and numerical analysis to prove how PET can be used with microcomputed tomography (micro-CT) to complement the imaging of water displacement in the porous space of sandstone. To characterize the rock properties, a dry sample was imaged with a high-resolution micro-CT scanner. The image was reconstructed and segmented into two phases (solid and void) to calculate the porosity/permeability relationship on cubic subsets. The Navier-Stokes equation was solved for permeability calculation. The porosity/permeability relationship was used as the input to simulate the forced imbibition process on an upscaled micro-CT image using the Matlab Reservoir Simulation Toolbox (MRST) (Lie et al. 2011; Krogstad et al. 2015; Lie 2016; Bao et al. 2017). The result was the evolution of invading-fluid saturation in every upscaled voxel. In the following step, water tracer labeled with fluorodeoxyglucose (18F-FDG) was injected into the dry sample and a series of PET images was acquired to detect fluid pathways. Fluid-front topology and a propagation procedure were captured. We integrated saturation data across the core cross section for both data sets (direct PET-based and MRST-simulated on upscaled tomograms) to reduce the complexity of the analysis. Comparison of the resulting 1D saturation profiles demonstrated the capability of PET imaging to monitor and understand dynamic flow processes in reservoir rocks.