The Impact of Different Shaly Sand Models on In Place Volumes and Reservoir Producibility in Niger Delta Reservoirs - The Dual Water and Normalized Waxman-Smits Saturation Models

Abstract

Abstract This paper compares hydrocarbon saturation (and their impact on in place volumes and producibility) from two different saturation models in use in the industry for saturation calculations in shaly sands in the Niger-Delta. For many of the leases where there are either JV partnerships or unitisation requirements, the different E&P companies approach hydrocarbon saturation estimation using different models, and it is important to understand where differences are expected to occur and what impact these differences make to both in place and recoverable hydrocarbon volumes. This paper reviews two of the more popular shaly sands models used – The Schlumberger Dual Water Model (DWM –Clavier, Coates & Dumanoir, SPE 19772) and the recast Waxman-Smits model by Juhasz - the Normalized Waxman-Smits (NWS –I. Juhasz, SPWLA 19811) model. The Dual Water model approaches the impact of clay minerals on the conductivity of reservoir rock by proposing the presence of two different ‘waters’; one within the free pore space and the other representative of the additional conductance path created by the cation-exchange process, due to the presence and morphology of clays. The Waxman-Smits model, on the other hand, is premised on the understanding that only ‘one’ water (the saturating brine) is present, and provides a semi-empirical basis for capturing the effect of the additional conductance path created by the presence of clays. The Juhasz modification (the Normalized Qv approach) casts the Waxman-Smits in a form that is directly comparable with the Dual Water Model. This paper summarizes the theoretical and experimental foundation of both shaly sands models and aims to quantitatively investigate the actual differences in the derived saturations, in selected Niger Delta reservoirs, in order to clearly show how much results from the methods differ. As mentioned, this will be extremely useful in comparing results for unitization purposes between oil companies who traditionally use different methods for hydrocarbon saturation calculations. To reduce the uncertainty introduced by parameters used in the models, the comparison has been done on Niger Delta reservoirs with core-derived parameters. In addition, core-derived saturation-height functions (from drainage capillary pressure curves) are compared to the log-derived saturation-height functions from both shaly sand models, to understand the impact on in place volumes and dynamic saturation functions (absolute and relative permeability derived from end point water and oil saturation).