Dealing with Multi-level Property Heterogeneity in Carbonate Reservoirs

Volker C. Vahrenkamp
Abu Dhabi Company for Onshore Oil Operations ([email protected])

Many carbonate fields of the Middle East have some 40 years of production under primary depletion and secondary development schemes. Now most of them are getting ready to enter the next brown field development phase. Costs and potential rewards are substantial and require a sound understanding of what influences flow in the subsurface.
Geological, petrophysical and production data often reveal a multi-scale heterogeneity system affecting fluid flow and hence reservoir performance. 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, which usually contains the vast majority of the reserves 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 these heterogeneities must be understood in their architecture (the piping system) and quantified in terms of porosity, permeability and dynamic properties. Furthermore, a strategy must be at hand that allows small and intermediate heterogeneities to be represented in larger sized simulation models. An approach is presented to define and model heterogeneities hierarchically. Small scales heterogeneities are captured and simulated in dedicated “mini-models” (< 1m³ in size) which provide pseudo properties for larger volume cells in reservoir size-models.
Principle Rock Types (PRT’s) 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 and relative permeability characteristics. PRT’s are organised into Rock-Type Associations (RTA’s) based on sedimentary fabric (bioturbation, cross-bedding, layering, etc.). The construction of mini-models can be based on conceptual considerations or driven by high resolution log data such as image logs. The distribution of RTA’s in the reservoir in turn is driven vertically by depositional cyclicity and laterally by facies dimensionalities. Key tools to distribute RTA’s in the larger reservoir models are seismic, image logs and analogue data, which provide control on both fabric and property heterogeneity.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008