This study illustrates a novel near wellbore rock typing and upscaling approach to obtain fit-for-purpose rock typing scheme to improve the characterisation and simulation of a highly heterogeneous offshore carbonate field, Field X. The key geological heterogeneities present in Field X were caused by late burial (mesogenetic) dissolution, such as chalky micro-porosity, macro-porosity including vuggy and moldic pores, leached stylolites and associated tension gashes. The prime challenges addressed are to incorporate the influence of diagenesis on reservoir petrophysical properties, adequately represent the multi-scale and multi-modal pore types and integrate dynamic data through rock typing. Another major issue is to upscale the petrophysical properties of the rock types to the reservoir model using appropriate geostatistical tools. The rock typing and upscaling methodology we adopted involves the geological-petrophysical classification of these heterogeneities through systematic evaluation of the key paragenetic events, and considers the crucial aspects of near wellbore modelling and upscaling. The outcome of this study has significantly improved characterization of porosity and permeability distributions and dynamic calibration in Field X. As a result, a new alternative geomodel scenario was obtained, which is now much better constrained to the reservoir geology and shows improved match to historic production data.

Carbonate reservoirs host a major portion of the world's remaining conventional and unconventional hydrocarbon reserves, typically containing multi-scale geological heterogeneities varying over many orders of magnitude in size. Characterizing and representing them robustly in reservoir models is a prime challenge in carbonate reservoir simulation. One of the key aims of this paper is, hence, to present a novel near wellbore upscaling (NWU) workflow that addresses the challenges associated with conventional carbonate modelling workflows. The NWU workflow provides a systematic geostatistical approach to obtain more realistic representation of multi-scale geological-petrophysical heterogeneities in complex carbonate reservoir simulation models. Using well log and core data, near wellbore regions were recreated to represent the core scale heterogeneities via high resolution geostatistical models. These core/centimeter scale permeability models were then upscaled into wireline/decametre scale using flow-based upscaling. The results, coupled with wireline data were used to generate global porosity-permeability and vertical-horizontal permeability relationships for reservoir simulation. Importantly, the workflow mitigates sample bias, which is frequently observed in the core data for carbonate reservoirs. We have applied our approach to a mature carbonate field, to model and upscale crucial multi-scale heterogeneities ubiquitous in the reservoir. These heterogeneities, such as mechanically weak zones of enhanced micro- and macro-porosity, leached stylolites and associated tension gashes, were caused by diagenetic corrosion. Core plugs representivity is always an issue in carbonates and these highly corroded features were very difficult, if not impossible, to sample due to their fragility. As a result, the field suffers from inherent sample biasing and insufficiency of Routine Core Analysis (RCA) data, consequently underestimating the permeability in the simulation model. The workflow presented here has enabled the authors to re-evaluate the reservoir permeability model by accounting for as yet under-sampled geological heterogeneities. The paper represents a focused individual study addressing this specific issue and doesn't necessarily reflect the operator's full understanding of this multifaceted field. Our new permeability model has addressed the need for artificial permeability multipliers and provided insight on the potential causes of the original mismatch. As a result, a new alternative model scenario has been built to help guide the on-going development plans and forecasting incremental oil recovery.