Capillary pressure can be viewed as the pressure required to drive a fluid through a pore throat, with greater pressure being required as the pore throat becomes smaller. The size and distribution of pore throats within a host rock control its capillary pressure characteristics, which in turn control fluid behavior in the pore system. The pore geometry of carbonate rocks has as great influence on the ability of the rock to contain and produce oil and gas as do porosity and permeability. This has been demonstrated by examining a large number of samples from a Saudi Arabian carbonate reservoir. By studying the lithology and genesis of the rocks, and comparing families of capillary pressure curves, it was possible to establish very clear mutual relationships.

The mercury injection technique followed by ejection and reinjection was used to determine capillary pressure behavior for each sample. Experiments were also performed to test the plug size and surface effects on capillary pressure characteristics. A detailed interpretation of the data was carried out in order to extract various correlation parameters. These data were combined with basic rock properties and incorporated in generating families of capillary pressure curves for several carbonate rock types, and in the computation of pore throat size distributions.

Experimental data are presented using J-Functions for the correlation of capillary pressure and saturation relations for all samples. It is shown that the correlation improves significandy when the samples are grouped together based on the rock lithology.

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