Multi-Scale Heterogeneity Modelling in a Giant Carbonate Field, Northern Oman

Volker C. Vahrenkamp¹*, Anouk Creusen², Simon Tull², Andrew Farmer², Abhijit Mookerjee², and Alia Al Bahry^2
1ADCO
²PDO
*[email protected]

A giant carbonate reservoir in northern Oman has been producing for some 40 years under primary depletion and secondary development schemes. Now the next brown field development phase is being prepared. Costs and potential rewards are substantial and require sound decision making. Geological, petrophysical and production data has revealed a multi-scale heterogeneity system. The driving forces for heterogeneity are sedimentary fabrics, depositional environments, large and small-scale depositional cyclicity, diagenesis, and fracturing/faulting. Hydrocarbon storage and flow through the matrix is fundamentally influenced by variations in pore types on a scale much below that of normal static and dynamic reservoir models. With the goal of building reservoir-size dynamic models the extensive heterogeneity on a centimetre-decimetre scale provides a significant upscaling problem both for static and dynamic properties. In order to populate reservoir-size models with realistic properties reflecting small-scale heterogeneity, both statically and dynamically, an approach has been chosen to define and model heterogeneities hierarchically. Small-scale heterogeneities were captured and simulated in mini-models (less than 1.0 cubic m in size), which provided pseudo-properties for volume cells in reservoir-size models. Principle rock types (PRT) are the fundamental building blocks of the matrix system. They cover and categorize the full range of pore types, sizes, pore-throat size distributions, capillary entry pressures, relative permeabilities, etc. PRTs were organised into Rock-type associations (RTA) based on sedimentary fabric (bioturbation, cross-bedding, layering, etc.). Mini-models were constructed using borehole image logs (BHI) data for all RTAs and their internal property variations. The distribution of RTAs in the reservoir is driven vertically by depositional cyclicity and laterally by facies dimensionalities. The key tools to distribute RTAs in the larger reservoir models are seismic data and the BHI logs, which provide control on both fabric and property heterogeneity.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain