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Carbonate reservoirs are generally highly heterogeneous and complex because many phenomena, such as diagenesis and tectonism have modified primary depositional structure and texture. Many carbonate reservoirs in the Shuaiba Formation of north Oman are characteristically heterogeneous with complex reservoir architecture. It is common to observe dual-media and natural fracture system behaviors with large variations in reservoir properties. Many of these reservoirs have been developed with horizontal wells, so they require application of advanced reservoir evaluation and management practices. Many of the horizontal wells produce at high water cuts, despite apparent initial high oil saturations. The need for data is paramount for effective management of these reservoirs, and to understand factors affecting the productivity of horizontal wells - both at the wellbore and within the reservoir itself. The first stage of this process is to define an accurate structural model incorporating all available reservoir, wellbore (2D modeling) and seismic data. The second stage, involves integrating this model with the inflow profile derived from advanced production logging measurements. We describe several field examples from a complex and heterogeneous carbonate reservoir in Oman, and discuss how they may be used to identify a range of important needs relating to horizontal well placement, improving completion strategy, and optimizing water shut-off activities. Introduction Short to long radius horizontal wells have been drilled in many types of oil and gas reservoirs, in both carbonate and siliclastic formations. A variety of technical advances in drilling, including logging while drilling and formation evaluation have meant that the application of horizontal wells for reservoir development in both primary recovery and enhanced oil recovery (EOR) applications is now a common practice. There are many options available for completion of horizontal wells, such as open hole (barefoot), cased hole (e.g. cemented, expandable and perforated liner), slotted liners and gravel-pack completions. In the Middle East and North America a large proportion of horizontal wells have openhole completions. Since horizontal wells have a much greater reservoir contact area than vertical wells, for reservoirs with lateral heterogeneity, fault and fracture networks or structural compartmentalisation, this can result in well performances that range from the spectacular to the very mediocre.

With an approximate STOIIP of 760 MMbbls, the Omar field is the largest field in Al Furat Petroleum Company's portfolio. The field -located in the Euphrates Graben 45km SE of DeirEzZor -was discovered in 1987 and holds a maximum undersaturated oil column of more than 500m with two original oil-water contacts of 3750 and 3778 meters subsea. The oil production almost exclusively originates from two sandstone formations: the Cretaceous sheet-like shallow marine Lower Rutbah (RUL) and the Triassic coastal fluvial plane Mulussa F (MUF) formation. The Omar Field is formed by an elongated, high relief tilted horst block, which is internally compartmentalised.Originally, the field produced naturally at a peak net oil rate of some 80kbpd but production declined rapidly because of the lack of any pressure support. Following the implementation of water injection from 1991 onwards a plateau production of around 60-70kbopd was achieved for some five years (1994)(1995)(1996)(1997) declining to the current net oil production of 20 kbopd.Despite the structural complications, the injector-producer connectivity in the laterally extensive RUL sands could be established rather confidently and recoveries in excess of 55% should eventually be achievable. Predicting water-flood efficiency in the Mulussa F 3D sand channel labyrinth turned out more complicated. As a matter of fact, it was demonstrated that the resolution achievable by static reservoir modelling was not sufficient to predict the water-flood efficiency meaningfully. As a consequence a statistical infill campaign was launched with a focus to infill the existing major gaps between the MUF wells and secondly to establish a line drive waterflood pattern while investigating the merits of a dense five spot.The results of this infill drilling campaign (executed in 2005-2006) and a new 600-fold high-resolution seismic survey gave a multi-disciplinary team the challenge to improve in identifying more attractive targets while reducing the downside drilling results observed during the infill campaign. A combination of the new structural data with a regional geological well correlation fully and iteratively integrated with dynamic well information and production data, indicates that the recovery in the MUF formation could well be optimized through a more deterministic instead of the previously adopted statistical infill drilling approach.

Two new internal multiple attenuation methodologies are presented. The first approach uses a 3D wave equation based multiple modeling technique followed by 3D adaptive subtraction. The recorded data are back propagated/propagated through a reflectivity model (obtained from a preliminary migration) of the overburden. All possible multiple raypaths are modeled by using different combinations of two sub-windows in this reflectivity model. Each combination results in a specific multiple model. The generated multiple models are then simultaneously adapted and subtracted from the input data. The second approach is a 3D dip extraction and filtering technique, taking advantage of any dip discrimination between the primaries and multiples. It works on post stack and pre stack volumes. We show how the two methods are complementary and how they can be combined.The first approach can be applied to any seismic data plagued with strong internal multiples and is perfectly adapted to modern dense, wide azimuth surveys. The second approach works on any seismic volume in which multiples and primaries have different dips in a given domain.Some real data examples from South Oman 3D seismic datasets are shown. All are characterized by a heavy multiple contamination, generated by strong shallow reflectivity sequences in a flattish overburden, and overlying deeper, weaker and heavily structured primaries. By revealing structures previously invisible, these techniques add a lot of value to existing seismic data sets.The generation mechanism of internal multiples is often quite complex making them difficult to predict. The main benefits of the methods described here are that they work in absence of move out discrimination and without the need of a precise identification of the internal multiple generators.