Relationship Between Pore Geometry and Pore-Throat Geometry as a Means to Predict Reservoir Performance in Secondary Recovery Programs for Carbonate Reservoirs

Christina M. Dicus
Petrophysical Analysis Subsurface Technology, ConocoPhillips, Houston, TX

The purpose of this study was to develop a method by which a detailed porosity classification system could be used to understand the relationship between pore/pore-throat geometry, genetic porosity type, and facies. This study also investigated the relationships between pore/pore-throat geometry, petrophysical parameters, and reservoir performance characteristics. The focus was on the Jurassic Smackover reservoir rocks of Grayson field, Arkansas. This three part study developed an adapted genetic carbonate pore type classification system, and used petrographic image analysis and mercury-injection capillary pressure tests to calculate pore/pore-throat sizes. These were compared to facies, pore type, and each other showing that pore-throat size is controlled by pore type and that pore size is controlled primarily by facies. Pore size range can be estimated from pore type and median pore-throat aperture. Capillary pressure data was used to understand the behavior of the dependent rock properties. It was determined that size-reduced samples tend to show similar dependent rock property behavior, but size-enhanced samples show dispersion. Capillary pressure data was used to understand fluid flow behavior of pore types and facies. Oncolitic grainstone samples show unpredictable fluid flow behavior compared to oolitic grainstone samples, yet oncolitic grainstone samples will move a higher percentage of fluid. Size-enhanced samples showed heterogeneous fluid flow behavior while the size-reduced samples could be grouped by the number of modes of pore-throat sizes. Finally, this study used petrographic image analysis to determine if 2-D porosity values could be compared to porosity values from 3-D porosity techniques. The heterogeneous pore network found in the Grayson reservoir rocks prevents the use of petrographic image analysis as a porosity calculation technique.

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