Pore system characterisation in heterogeneous carbonates: An alternative approach to widely-used rock-typing methodologies

Abstract

Rock typing is used in carbonate reservoir characterisation to group petrophysically similar rocks for reservoir modelling purposes. The focus is often upon bringing together samples with similar porosity, permeability and capillary pressure. To achieve this, rock typing studies are usually conducted primarily on core plugs, using routine core analysis data and thin sections. However, such an approach results in a tendency to focus on grouping samples with consistent petrophysical properties, ignoring the geological controls on pore evolution. This means that modelling rock types predicatively using 3D geocellular models is not easily accomplished. Furthermore, where geological observations are made, they tend to consider only depositional rock properties, and not the effects of diagenesis. In doing so, a key control on the evolution of the carbonate pore network is ignored. This paper addresses these issues using a giant carbonate reservoir in Northern Oman. Rock types were defined on the basis of pore geometry, whilst retaining distinct, geological descriptors. The aim was to ensure that each rock type could be defined on the basis of both its petrophysical properties and behaviour during hydrocarbon recovery. The results show that characterisation of pore heterogeneity is critical to the prediction of flow behaviour under reservoir conditions, and that routine petrophysical parameters are often not good indicators of sweep efficiency.