Unstructured Gridding for Full-Field Simulation of a Giant Carbonate Reservoir Developed With Long Horizontal Wells
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper IPTC 11512, "The Application of Unstructured Gridding Techniques for Full-Field Simulation of a Giant Carbonate Reservoir Developed With Long Horizontal Wells," by H. Vestergaard, SPE, H. Olsen, SPE, and A.S. Sikandar, SPE, Maersk Oil Qatar, and I.A. Al-Emadi and R. Noman, SPE, Qatar Petroleum, prepared for the 2007 International Petroleum Technology Conference, Dubai, 4–6 December. The paper has not been peer reviewed.
A case history is presented on reservoir simulation of a giant, complex, low-permeability, carbonate reservoir completed with many long horizontal wells. The multiple radial layouts of 75 very long horizontal wells in the field posed a challenge in modeling individual-well performance with a manageable grid size and acceptable run time for history matching. The gridding dilemma was solved by use of 2.5D perpendicular-bisector (pebi) grids around each individual wellbore. This method provided sufficient resolution between wells and aligned the grid with the well paths, thereby avoiding grid nonorthogonality issues.

Introduction
The Al Shaheen field is offshore Qatar, covering an area of 800 sq miles. The Kharaib formation is part of the Lower Cretaceous Thamama group. The dome-shaped reservoir has a net oil-column thickness of 23 ft at the crest of the field, thinning toward the flank. The field has relatively low permeability, ranging from 1 to 15 md. Developed wells have no pressure support from the distant flank aquifer or the trapped water in the central part of the field. Oil gravity varies significantly: saturated low-viscosity (&lt;1 cp) oil to highly undersaturated higher-viscosity (35-cp) oil. The field has several local gas caps and a free-water level (FWL) dipping more than 500 ft across the field.
At the time of this study, the reservoir was developed with 75 ultralong horizontal wells, up to 30,000 ft long, drilled in alternating producer/injector patterns from five platforms. Production from the field began 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. Modeling issues detailed in the full-length paper were part of a history-matching study completed in 2004 that formed the basis to develop the reservoir with an additional 89 wells to infill existing well pat-terns and to develop the thinner oil column toward the flank of the reservoir.

Background
The Kharaib B formation is a laterally continuous limestone with a relatively constant thickness of approximately 80 ft within the subject area. Oil gravity ranges from 38°API at the crest of the field to 12°API at the northern flank. The saturation pressure varies from saturated near the gas caps to 1,300 psi undersaturated in low-gravity areas. There is not, however, a distinct correlation between saturation pressure and oil gravity. The large variation in saturation pressure indicates different natural-drive mechanisms in different areas of the reservoir.