Relative permeability assessment in a giant carbonate reservoir using Digital Rock Physics

Representative relative permeability data are of great importance for accurate and more importantly valid reservoir simulation models. Relative permeability tests are conventionally performed in specialized laboratories throug h Special Core Analysis (SCAL) conducted on selected core plugs. The experiments can be time consuming and as such performed on a limited number of reservoir core plugs to represent specific reservoir rock types (RRT), flow units and/or reservoir zones. A detailed study to investigate the reliability and validity of water-oil relative permeability curves and end points were derived using the new discipline of Digital Rock Physics (DRP). Reservoir core plugs from a giant carbonate reservoir in the Middle East on which imbibition water-oil relative permeability were acquired were chosen for this huge validation study. The cores were selected and tested through a comprehensive SCAL program consisting of careful preserved core acquisition, selection/screening, detailed characterization, testing and QC/QA of laboratory results using numerical simulation and integration with multiple tests such as single-speed centrifuge and water-oil capillary pressure. The DRP based investigations were performed as part of a blind study (relative permeability data unknown to DRP contractor) on 16 different RRT samples, comprising a vast range of lithofacies with core porositi es from 11 – 34% and permeabilities from micro-Darcy to several Darcies. Relative permeabilities are quantified by adopting a multi-resolution integrated pore-scale modeling approach based on X-ray computed tomography images and numerical 3D digital rock models. This technique is able to capture the dominating pore classes present on the core plug scale: vuggy porosity, inter- and intragranular macro-porosity and micritic micro-porosity. Rock curves are generated for each of the pore classes and then used in a steady-state upscaling routine to obtain primary drainage (water saturation decreasing from 100% brine saturation) and imbibition (water saturation increasing) relative permeability curves representing the entire core plug. The comparisons between relative permeability curves derived from DRP and experimental steady state tests provide an assessment on the validity of the DRP based tests. Comparisons have been performed for each RRT and show an excellent match of laboratory measured relative permeability; typically withi n 90% of the test results if rock-fluid wettability can be correctly postulated. Wettability variation for different RRT and respective pore geometry can also be quantified, and thus significantly improve the uncertainties in the predicted relative permeabilities.