Modelling of Morphology and Petrophysics of Nummulitic Carbonate Rocks

Carbonate rocks usually show a complex distribution of heterogeneities at different length scales that are associated with depositional processes and complex diagenetic history. Very often, it is desired to calculate the petrophysical properties at a prior state of diagenesis, which might be used as input in basin modelling, for instance. Here, we describe numerical methods that allow us to understand the possible petrophysical states at key moments in the geological history of a study reservoir.
We focus on a fossiliferous, partly-dolomitised carbonate. Dolomitisation has completely replaced the micrite (“mud”) that existed between the fossils enhancing reservoir quality. The Nummulites (fossils) are distributed within the micrite in some characteristic patterns. Dolomitisation has partly or completely dissolved the Nummulites creating local macro-porosity. The pore system of this nummulitic rock thus has two different length scales ([1]). In order to get insight into how the rock morphological structure governs the petrophysical properties, we need to consider how the pore systems are connected within each scale and across these scales. We employ a new methodology ([2]) combining multi-scale pore structures, derived from several imaging methods, into a single pore-scale network. The major petrophysical properties (e.g. porosity, permeability, capillary pressure) can then be calculated, and two-phase drainage/imbibition simulations can be carried out.
To address the variability of the diagenetic processes, we generate three synthetic micrite models, composed solely of micro-pores, that cover the range of porosity and permeability suggested by the literature at the depth at which dolomitisation is believed to occur. We obtain 3D XCT images of the two Nummulites fossil forms, allowing us to generate models (see Figure 1) containing multiple Nummulites via a set of control arguments such as density, orientation, size and types of interaction with neighbouring micro-pores. The latter issue is addressed by creating a connection region between each Nummulites and its neighbouring micrite/dolomite matrix, with pores in both materials being available to be connected or not, based on some simple geometric measures. Using this approach, we create multiple, realistic models of Nummulitic rocks with a combined two-scale pore structure.
Our flow simulations show that in the micrite models, Nummulites act as baffle to flow. In models using the dolomite matrix, partly-cemented or non-cemented Nummulites have practically no impact, whereas biomouldic porosity created by Nummulites corrosion enhances reservoir quality. This behaviour is confirmed by comparison with a routine core analysis dataset.