An Integrated Approach for a Full-Field, Optimized Development of a Carbonate Reservoir on a Shallow Marine Environment, Abu Dhabi
Authors: El Mahdi Ahmed , Ayoub Mohammed , Daif-Allah Samy , Negahban Shahin , Ribeiro Maria , Bahamaish Jamal 
Published: December 2007

Full field development plan of a complex carbonate reservoir on land, islands, shallow and deep marine areas is a complicated issue at a high cost (building artificial islands, drilling from shore using highly deviated wells with long reach, etc.). This paper presents a multi-disciplinary approach for the full field development plan of the X field off coast Abu Dhabi.

The reservoir under study was discovered in 1969 being an oil bearing reservoir with a thin gas cap. It has good porosity development (14-18 %) with low permeability (0.4 to 5 mD). Two strike-slip fault patterns (W-E and NW-SE) were mapped seismically. Image logs and dynamic data suggest that fractures are rare across the reservoir. The field exhibits a low relief structure (250 ft) with dips of less than 2 degrees down flank and a pool area closely to 500 sq. km. The reservoir shows lateral variation in fluid and petrochemical properties. A full field compositional model is being used to evaluate the field reservoir performance under various development scenarios. The field has been in Early Production Scheme (EPS) since early 1990 and has been on stream since December 2005.

Due to reservoir tightness, coupled with the fact that the field is located on an environmentally sensitive area offshore Abu Dhabi, it is being developed with long horizontal wells drilled from clusters (land, island and artificial islands). Accordingly, most of the drilled wells are highly deviated and some of them are in the category of ERD (Extended Reach Drilling). For this reason, depth uncertainty associated with large departures and complex trajectories revealed to be an important component of the reservoir models uncertainty. Uncertainty analysis and risk mitigation were therefore tackled by the team as a priority for the field development.

This paper addresses the uncertainty related to a low relief structure and the masking of shallow velocities, the depth uncertainty associated with high departure horizontal wells and the spatial petrochemical heterogeneity. Static and dynamic reservoir models have been built using information from wells and 3D seismic data. An integrated modeling approach involving Geology, Geophysics, Petrophysics and Geostatistics were applied to build the frameworks and property models. Both deterministic and stochastic techniques were used to populate the models with porosity, permeability and saturation. Multiple realizations were generated for the uncertainty analysis.

Also discussed in this paper is the integrated work associated with the development plan optimization for this reservoir. A full field development plan was set using a 3-D compositional simulation model and calls for five spot gas injection patterns on the crest and staggered line drive on the flank using WAG injection. This required the drilling of 57 horizontal wells (23 injectors and 34 producers) in a shallow marine area. Subsequently, a detailed drilling strategy /development plan was set to develop the reservoir. The subject reservoir was put on production using a combination of Gas/Water/WAG injection schemes in late 2005 early 2006.