Characterisation and 3D Numerical Modelling of Multiphase Flow in Carbonate Rocks

Abstract:

Characterisation of multiphase flow properties, such as relative permeability and capillary pressure, is crucial in understanding fluid behaviour in the subsurface e.g. predicting CO₂ plume migration. Heterogeneity in the underlying rock structure such as vugs in carbonates, can cause large variations in porosity and permeability which manifest as capillary pressure heterogeneity. At the low flow potentials typically encountered during CO₂ injection, these heterogeneities can significantly impact fluid flow behaviour, typically observed as large saturation variations within the rock. With advances in non-invasive experimental techniques a method to characterise capillary heterogeneity in sandstones was first developed by Krause et al. (2011), and later refined by Pini et al. (2012) and Jackson et al. (2018). The method combines observations from X-Ray core-flood experiments with numerical simulations in an iterative optimization scheme. Driven by these successful results, we extend and advance this workflow, focusing on the fluid behaviour in Carbonate samples, Estaillades and Indiana limestone, compared to that in sandstones. After initial routine core characterization, the CMG IMEX^TM fully implicit flow simulator is used to simulate the core-flood experiments. By comparing the resultant voxel-specific saturations with those from the experiment, a 3D model of the capillary heterogeneity can be built, where the model is iteratively updated through minimization of the mismatch between the simulation and experiment observations. The existence of micro-porous, vug and matrix regions within the carbonate samples significantly complicates the 3D characterisation, with heterogeneity impacts more prevalent than in sandstone systems. We present a generalised workflow, applicable to a wider range of rocks with multi-scale heterogeneity and facies. Overall, the modelling of the carbonate cores has shown that capillary heterogeneity generally raises the gas relative permeability at low capillary number, thus favouring gas flow. After validating the characterisation workflow, we also use these 3D digital models to simulate unsteady-state experiments, allowing for direct comparison of the relative permeability curves obtained from steady and unsteady-state experimental methods.