The Al Khalij field, located offshore Qatar, was discovered in 1991 and put into production in 1997. The field is original in many aspects. It is one of the few carbonate reservoirs with a stratigraphic closure, the oil being trapped in the upper part of a monocline due to a lateral variation in reservoir facies. In addition, the reservoir lies within the capillary transition zone and consists of a succession of highly conductive oil bearing layers ("drains") in between matrix layers of poor permeability. Due to the complex reservoir architecture and the large uncertainties in the reservoir dynamic behavior, the field has been developed in several stages to improve understanding and minimize risks. This paper will discuss on the development of Al Khalij field and will illustrate how advanced technological solutions were applied to overcome complex geological and reservoir problems in order to enhance reservoir performance and well productivity. More specifically the paper will illustrate how:High-resolution 3D seismic has revealed additional areas for development, which were previously considered to be uneconomic.Innovative well design in a stair-step manner was implemented to intersect productive layers and increase drainage efficiency with a limited number of wells.Stimulation and completion techniques were adapted to optimize productivity while limiting water production.Water injection was implemented in the field after validation of its effectiveness through pilot water injection, including tracer test, and through reservoir simulation studies. ALKHALIJ: A Complex Field A subtle closure The Mishrif formation is the main reservoir and consists of Cenomanian limestones. The depositional environment corresponds to a carbonate platform with rudist shoals. The Al Khalij field is a stratigraphic trap (Fig. 1). The lateral stratigraphic seal, globally N-S orientated, is provided by lagoon mudstone deposits developed to the west and recognized on wells located a few kilometers west of Al Khalij field. The top of the Mishrif reservoir is partially eroded with maxima of erosion to the NE and the SE as a consequence of the uplift of the Rostam field salt structure. The cap seal is provided by the Laffan shale. A complex internal reservoir architecture Vertically, the reservoir is subdivided into two main stratigraphic units (Fig. 2), separated by a maximum flooding surface (MFS 7):The upper reservoir unit is currently being produced in the central and southeastern parts of the field. Towards the North, these layers are partially eroded.The lower unit is water bearing in the current producing area while it is oil bearing in the North and will be developed in 2003–2004. The top and base of the reservoir can be interpreted on 3D seismic (Fig. 3) which provides a control on reservoir structure, erosion limits and volumetrics.

fax 01-972-952-9435. AbstractThe Al Rayyan oilfield, offshore Qatar presents an interesting case of natural water drive. This paper presents the results of investigations to understand the production mechanisms in the main producing zone in the field (Arab C). This zone is a thin, low relief Arab carbonate reservoir with high porosity and permeability. The reservoir has a strong natural aquifer drive and the reservoir pressure has remained close to the original pressure during the 5-year initial production phase. The field was developed with horizontal wells to provide sufficient productivity in the thin reservoir. The main phase of field development, planned following these studies, is now underway with the drilling of additional sidetracks and further longer, carefully placed horizontal wells. Results of the first two new wells are described.

Accurate knowledge of oil saturation and its distribution is essential for any reservoir being considered for redevelopment or for secondary and tertiary operations. Determination of oil saturation forms the only rational basis for prediction of recoverable oil reserves and it is critical in the selection, design and economic evaluation of an oil-recovery process. As technology advances, more of this oil will come within our reach. However, it is essential that we understand the reservoir involved so that proper, economically viable technology can be applied. Thus, the importance of residual oil saturation (SOR) which in turn is the basis for the estimation of remaining oil-inplace. Five techniques to determine remaining oil saturation are currently used. They are coring, logging, chemical tracers, reservoir engineering studies and pressure transient tests. Normally, two or more of these methods are used together and the data evaluated as a whole since each technique has its own advantages and limitations. When evaluated together, a clear understanding of the total reservoir can be obtained. Conventional logging tools and procedures have been used extensively for many years for determination of reservoir and fluid properties. Although SOR determinations normally require a higher degree of accuracy than is commonly achieved by conventional logging methods, the incentive to use logs for SOR measurements remains high. They are often cheaper and easier to use than is the case for other methods which are currently available for measuring SOR.

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