Digital Pore Space Study of a Middle-Eastern Carbonate Rock Sample

Abstract

While Image processing is still an area of research, standard workflows have emerged and are routinely used in Oil&Gas companies. However, while hardware capabilities have increased consequently, allowing large samples to be scanned with a high fidelity, permeability simulations are still limited to small samples unless having access to HPC. Direct simulations are known to be more flexible in terms of type of rocks, but limited in terms of sample size, while Pore Network Model based allow much larger sample sizes but less rock types. In this study, we will focus on the pore space analysis of a middle-eastern carbonate sample. The rock sample is 7.5 cm tall and has a diameter of 3.8 cm. It has been acquired at 3 different resolution: a microCT scan at 16μm, a microCT scan of a 10 mm of diameter subsample at 5 μm, and a 10 mm of diameter SEM section at 2μm. This study will propose a methodology to mix the different scales in order to get an accurate pore space analysis of the largest possible sample size. As micro porous regions are visible at every scale, bringing uncertainty to the segmentation step, the first part of our analysis will consist of determining the most accurate pore space at the three different resolutions. We will rely on image registration (2D to 3D and 3D to 3D) and image based upscaling methods, further validated by simulation results. Given the large numerical size of the samples, specific workflows involving large data 3D visualization and processing will be presented. Then, different measures will be conducted: porosity and connected porosity, absolute permeability with three different methods (Lattice Boltzmann, Finite Volume, Pore Network Modeling), relative permeability curves using a Pore Network Model simulator. A new pore network model generation applicable to highly concave pore spaces such as carbonates ones will also be introduced. A scalable method using automation will be presented, so that repeating the simulations on different samples of different space origins and size is easy. We will expose the results and limits of every method and will determine which size is bringing a convergence of the results. We will especially look at the convergence of direct based simulations and pore network model based ones, such that expanding the size prior to Pore Network Model generation can be reliable. In addition to the benchmark of the different simulation methods and their associated limits, the results will help us determining the representative elementary volume at different resolutions and the associated uncertainty depending on whether sub-resolution acquisitions are available or not.