Multiple Discrete Pore Systems in Arab D Limestone

Edward A. Clerke and Johannes J. Buiting
Reservoir Characterization, Saudi Aramco, Dhahran, Saudi Arabia

The pore systems of the Arab D limestone, a formation that holds a large proportion of the world’s oil reserves, show abundant microporosity. Prior petrographic studies are continued here with an extensive mercury injection capillary pressure study. The MICP study demonstrates that the pore volume of the micropores (~25% of pore space) is volumetrically significant. Using Thomeer type curve analysis to fit the MICP data of our very large MICP data set, the pore systems’ volumes and entry pressures are quantified. The micropores are shown to be oil charged and do not contribute measurably to permeability.

The MICP entry pressure data show four distinct entry pressure modes or “porositons” three of which represent forms of microporosity. The Arab D limestone matrix pore system therefore requires a multimodal matrix storage model and a monomodal matrix permeability model.

Having determined the nature of the embedded carbonate pore system modes, the petrophysical model is rebuilt for the relationship of the entry pressure modes to the depositional geology, spectral porosity log response, hydrocarbon storage, permeability and multiphase transport and microscopic displacement efficiency. The conventional experimental data must be analyzed using new multimodal methods and algorithms.

Permeability is shown to be controlled by the properties of the mode containing the largest of the maximum pore throat diameters. The Type 1 microporosity is shown to shift the imbibition cycle oil relative permeability curve to the right and hence affect the determination of the microscopic displacement efficiency in the presence of macro and micropores.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas