Abstract

This paper presents the results of a reservoir characterization and modelling study based on reservoir rock typing (RRT) of Lower Cretaceous carbonate reservoirs in one of Abu Dhabi Onshore oil fields. The final goal is to obtain multiple realizations of 3D descriptions of the petrophysical properties, namely porosity and permeability, which match and are consistent with the underlying RRT scheme, at the grid block level. The RRT were described in all sections of the reservoir for all cored-wells.

The established reservoir rock types were based on depositional facies sequences, diagenetic overprints and petrophysical properties, including pore throat size distribution, porosity and permeability. The model reveals that the vertical changes in the rock types are a function of depositional facies, while the lateral variation down structure across the same lithofacies unit are controlled mainly by diagenesis.

Considering the limited number of cored wells compared to the total number of loged-wells, the characterization started by predicting both permeability and rock type at the non-cored wells. Permeability was predicted using a combination of regression analysis and geostatistics. The use of geostatistics not only has been usefull in capturing the high variability of permeability but also has ensured that the core data is fully honored at plug locations. Rock type was estimated at the non-cored well using discriminant analysis.

Consistency checks were been applied to the results of both prediction to ensure consistency between properties and rock types. Non-consistent results were assigned as unestimated-points. A geological conceptual model, in the form of iso-rock type maps, was used to QC the results of the prediction at the non-cored well and as a tool in deriving soft information about the spatial relationship of the different rock types.

The 3D descriptions of the properties were generated using a geostatistical technique. The technique not only honors the conditioning data and spatial relationship of each property, but also honors the local relationship between each property and the rock type. Additionally, some constraints derived from the diagenetic model were implemented in the modeling process to ensure that the model follows the geological conceptual model as much as possible. In transforming the well-log data into the model grid, appropriate scale-up methods and grid-block thickness were selected to ensure that reservoir heterogeneities are maintained. Multiple realizations of the properties were generated in order to capture and to quantify the intrinsic variability of the model. The results of this characterization study will be used for flow performance evaluation.

Introduction

Simplified models for describing reservoirs with complex geology are not able to adequately represent the reservoir heterogeneities and their impact on connectivity patterns and flow mechanisms. A critical step in the construction of reservoir simulation models is the description of the underlying reservoir geology. This is even more mandatory in the case of carbonate reservoirs, frequently characterized by an intensive diagenetic overprint of the original carbonates. This process frequently includes assessing the complex reservoir internal architecture reflected in the internal discontinuities within the lithological units, vertical and lateral variation of rock properties and similar heterogeneities within the reservoir layers.

The state of the art technology in generating reservoir models is to start with proper characterization of the underlying geology. The result of the characterization should be used in generating the 3-D model. The challenge that needs to be considered from the start is how the model can replicate the reality, i.e., what is defined during the characterization process.

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