AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Carbonate Rock Characterization and Modelling for Improved Properties Predictability: Capillary Pressure and Permeability in Multimodal Rocks
(1) Shell International, Rijswijk, Netherlands.
Carbonate rocks are known for their heterogeneity exemplified by the spread observed in measured permeability as a function of porosity that commonly span orders of magnitude for a given porosity within one geological sequence or reservoir. It is well established in general terms that permeability variability is a function of diverse pore types that occur in carbonate rocks. What is less well established are the precise porosity-permeability relationships for particular pore types and the relationships that result from multiple pore types that (more often than not) occur in measured samples. In this paper focus is given to characterization of pore systems at core plug scale to provide improved models for permeability and saturation prediction. A companion paper addresses heterogeneity and property upscaling. These methods fall under a wider Rock Typing workflow.
In this work we focus on the analysis of a number of heterogeneous rock types. Petrographic observations of pore types are analysed alongside mercury injection capillary curve data and air permeability data. Pore throat size distributions derived from the capillary pressure (Pc) data are correlated with the petrographically observed pore types. Multimodal pore throat size distributions provide a measure of the pore volume represented by different pore types. An important result of this work is that partitioning of the pore throat sizes via histogram analysis allows a distinction of those pore types and pore volumes that contribute most to permeability. Segregation of the pore types in this manner provides a much-improved correlation of porosity to permeability. The partitioning is consistent with the Pc fitting workflow routine and leads to improved Pc models describing the data with higher accuracy.
For the analyzed pore types/ rock types some universality of relationships can be defined. This is demonstrated by pore throat modal analysis and consistent porosity-permeability correlations. Sufficiently large data sets are required to provide statistical robustness. These universal relationships are contingent on an understanding of spatial heterogeneity and property upscaling for them to be applicable to reservoir flow simulation modelling.