CARBONATE SCAL: CHARACTERISATION OF CARBONATE ROCK TYPES FOR DETERMINATION OF SATURATION FUNCTIONS AND RESIDUAL OIL SATURATIONS

摘要

This paper presents a special core analysis(SCAL)study aimed at carbonate rock characterisation and measurement of saturation functions for modelling water-oil displacement of a heterogeneous reservoir.A particular focus is made on the measurement of water-oil capillary pressure curves using the centrifuge and CAPRICI-an in-house technique combining capillary pressure and resistivity measurements in multiple drainage and imbibition cycles.The basic rock characterisation includes thin section,SEM,NMR and mercury-air capillary pressure(Pc)measurements.Capillary pressure has been obtained in three cycles: oil displacing water starting from 100% water saturated plugs(primary drainage),water displacing oil starting from connate water after aging the plugs to restore reservoir wettability(imbibition)and finally oil displacing water starting from residual oil saturation(secondary drainage).The data show that,for the particular carbonate reservoir under investigation,the fluid flow properties such as residual oil saturation and imbibition capillary pressure curves do not show consistent correlation with conventional rock typing or facies classification.For example,imbibition capillary pressure showed significant variations for a set of samples having similar permeability,porosity,and drainage capillary pressure curves.Insights into pore geometry and pore-scale physics are essential to explain the fluid displacement characteristics.Dynamic SCAL data(i.e.,water displacing oil capillary pressure and relative permeability data)need to be included in the identification of rock types during reservoir characterisation.The results of this study have important implications in the design,interpretation and application of laboratory SCAL programme and consequently on field development planning.Assigning saturation functions based on permeability or conventional rock typing is shown to be inadequate.Further research is needed to establish improved classification schemes for such types of heterogeneous carbonate reservoirs.