An Innovative Workflow for the Dynamic Modeling and History Matching of a Mature Complex Carbonate Oil Field in the Middle East
Authors: Tomio Caroline , Fernagu Jeremie , Sullivan Luther Thomas , Al Naimi Abdulaziz Rashid 
Published: March 2015

A large Middle Eastern oil field, located in Arabian Gulf, is producing a 28° API oil from heterogeneous Upper Mishrif Cenomanian carbonates. The mature field is experiencing high water-cuts arising from a complex combination of different water production mechanisms. Despite the high water production, remaining stakes have been identified and will be targeted for future development. The aim was to build a new reservoir model to be used as an evaluation tool for future field redevelopment. Successful field history matching required the combination of four water production mechanisms at field and local scale: natural depletion of tighter interbedded layers, water-drive, small-scale dual medium effects and large scale conductive natural fractures. The major problem was that from the outset, matches were found with different relative contributions of these mechanisms and possibly with less than four mechanisms modelled. In this challenging context, an innovative history matching workflow has specifically been developed and implemented in order to match the full field behaviour. In outline, the work started with a conventional simulation model which is sufficient to handle the material balance of the system, the natural depletion and the water-drive mechanisms. Complexity was then increased by adding small scale dual-medium effects or explicit large scale conductive features using dual porosity – dual permeability modeling to create two separate models. Finally combination of these two models together gives a full field model which captures all the water production mechanisms. This approach has had two main advantages. It allows (1) a progressive understanding of water production mechanisms and their varying contribution towards water cuts across the field and (2) an optimization of running times of individual simulations as the bulk of the pressure matching work, troubleshooting and broad stroke matching were carried out on computationally inexpensive models. The workflow has also allowed an important parallelization of the matching work using several reservoir engineers working on different models. Finally, a successful history match has been achieved and the final model is currently being used to evaluate future development options.

Introduction

An oil field, located offshore in Arabian Gulf, has been producing since the late 90s from the complex Upper Mishrif Cenomanian carbonates. The maximum oil column thickness is about 80m from top structure to Free Water Level (FWL) and most of the oil in place lies in the transition zone. Because of reservoir heterogeneity and complexity, the field is still being developed using a phased approach. In addition to the main development phases, infill well drilling has been continuous to maximise production and reduce uncertainties.

Since the start of the development, the field has experienced a rapid increase in water cut, passing from 10% to close to 90% in less than 15 years. This water cut evolution results from different reservoir heterogeneities which have impacted more or less across the field: main features evidenced to date result from diagenetic imprints (dissolution, karstification and cementation) as well as structural elements (fracture corridors).

The current study was launched to build a new history matched reservoir model, integrating the most recent wells results, with the aim of being one of the tools to be used for the evaluation of future development possibilities over the field. The innovative and pragmatic workflow used to build the model has proved successful for the history matching of this complex field and could provide a useful template for other studies of multi-component carbonate fields.