The Application of Unstructured Gridding Techniques for Full Field Simulation of a Giant Carbonate Reservoir Developed with Long Horizontal Wells
Authors: Vestergaard Helle , Olsen Henrik , Sikandar Ali Shahbaz , Abdulla Ismail A. , Noman Rashed 
Published: February 2016

Abstract
This paper presents a case history on reservoir simulation modelling of a giant, complex, low permeability, carbonate reservoir which has been completed with many long horizontal wells. The lateral magnitude of the Al Shaheen field in Qatar and the multiple radial layout of the 75 very long horizontal wells in the field posed a challenge in modelling of individual well performance using a manageable grid size with an acceptable run time for history matching. The gridding dilemma was solved by use of 2.5-D PEBI grids around each individual wellbore. This allowed for sufficient resolution between wells and also aligned the grid with the well paths thereby avoiding grid non-orthogonality issues.
The initial fluid distribution and wettability dependent flow properties added further complexity to the reservoir simulation model. This paper describes how tilting free water levels, separate gas caps and large lateral variations of oil properties across the field were captured in the initialisation of the simulation model. Dynamic flow properties in the reservoir are heavily dependent on wettability and oil properties and the paper describes the rigorous implementation of a complex petrophysical model in the reservoir simulation model by use of endpoint scaling.

1. Introduction
The reservoir in subject is a major carbonate reservoir in the Al Shaheen field [Reference 1] in Block 5 offshore Qatar, covering an area of some 2080 km2. The Kharaib Formation forms part of the Lower Cretaceous Thamama Group that is widely present in the Southern Arabian Gulf.
The dome shaped reservoir has the thickest oil column of typically 23 hydrocarbon-feet at the crest of the field thinning towards the flank. The field has relatively low permeability ranging from 1–15mD. The developed wells enjoy no pressure support from the distant flank aquifer and the trapped water present in the central part of the field is not in communication with deeper layers. Oil gravity in the field varies significantly from saturated with low viscosity (&lt;1 cP) to highly undersaturated with relatively high viscosity (35 cP). The field has several local gas caps and a free water level dipping more than 500 ft across the field.
The reservoir was at the time of the study developed with 75 ultra long horizontal wells of up to 30,000 ft total length drilled in alternating producer injector patterns from 5 different platform locations. Production from the field started in 1994 and secondary recovery by water injection commenced in 1997.
Continuing evaluation of reservoir performance and assessment of further development potential of the field prompted the need for detailed history matching on a full field basis. The modelling issues discussed in this paper were part of a history matching study completed in 2004 which formed the basis to further develop the reservoir with additional 89 wells to infill existing well patterns and to develop the thinner oil column towards the flank of the reservoir.

2. Background

2.1. Geology
The Kharaib B formation is a laterally continuous limestone of Late Cretaceous age and the reservoir has a relatively constant thickness of some 80 ft within the Block 5 area. The lower water bearing part is mud-dominated with poorer rock properties whereas the upper oil bearing units are coarser grained.
A vertically cyclic nature with low porosity zones interbedded between the main reservoir units with higher porosity is characteristic for this formation. The permeability is in the range 1–15 mD.