Identifying the Remaining Opportunities in a Mature Field - A Case Study
Authors: Al-Amri Badr Ali , Al Bulushi Nabil Is-haq , Al-Busaidi Rashid , Wolters Frank , Wong Heng , Sadek Ehab A/Azim , Drammeh Lamin 
Published: February 2011

A comprehensive integrated study was performed in the Study Centre of Petroleum Development Oman (PDO) on an ultra-mature field located in South Oman. The field has 30 years of production history and more than 300 penetrations with a current recovery factor of 48%. The study aimed to develop robust subsurface (static and dynamic) models to identify the remaining and by-passed oil opportunities. Furthermore, the subsurface models are planned to be used for a potential future Enhanced Oil Recovery development (Alkaline Surfactant Polymer).

The field has two main clastic reservoirs (A and B) of glacial and fluvial deposits containing relatively light oil. Reservoir A is characterized by high Net-to-Gross whereas reservoir B is more heterogeneous with several sub-units (B1, B2 and B3) and lower Net-to-Gross. The overall production behavior is dominated by a high water cut.

This paper presents an overview of the key challenges and learning points on managing mature fields in a multidisciplinary context. The first key challenge is the identification of the remaining opportunities to sustain the production of such an ultra-mature field. Furthermore, handling large subsurface datasets, building a good understanding of the field and the regional geology, and understanding the production behavior are all key challenges to the success of the project.

The study used a number of key steps to overcome these challenges. The first step entailed consistency in utilizing and analyzing the full dataset in an integrated manner to enhance understanding of the geology in aspects like reservoir architecture and rock quality. Moreover, maximizing the use of horizontal wells in subsurface modelling helped in capturing facies distribution, saturation profile and structure. Understanding the production dynamics was another crucial element. This was accomplished through the analytical analysis of production data, well-by-well reviews and conceptual modelling. This was further improved by a proper understanding of the residual oil saturation through well logs, core data, SCAL and Single-Well-Chemical-Tracer Tests. Sector modelling came at a later stage as another technique to improve understanding of the coning behavior and remaining oil opportunities. During the study, the first pass static and dynamic models had to be re-iterated several times by creating multiple realizations to improve the history match of produced fluids, reservoir pressure and formation pressure data (RFT).

In conclusion, the multidisciplinary approach applied throughout this study resulted into in-depth understanding of the subsurface key issues and identification of the remaining development opportunities. Results of recently drilled wells came as per expectation which supports the validity of the subsurface realizations generated by this study. Overall, this study has confirmed that the field’s recovery factor can be improved up to 53% by conventional infill drilling. In summary, a profound understanding of reservoir performance behavior is a key driver to produce the limit of an ultra-mature field.