Natural Shuaiba Influx Into Arab Reservoirs of the Bahrain Field
- A.S. Khalaf (Bahrain Natl. Oil Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- April 1993
- Document Type
- Journal Paper
- 363 - 367
- 1993. Society of Petroleum Engineers
- 4.3.4 Scale, 4.1.2 Separation and Treating, 5.4.2 Gas Injection Methods, 5.2 Reservoir Fluid Dynamics, 4.1.5 Processing Equipment, 4.3.3 Aspaltenes
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Jurassic Arab D water salinity and composition vary considerably in theBahrain field. These lateral variations result from the introduction ofCretaceous Shuaiba fresh water into the Arab D reservoir. The section thatseparates the Shuaiba and Arab D formations is about 2,500 ft thick. Shuaibawater influx into Arab D has caused many reservoir and production problems,such as a rapidly advancing freshwater front, asphaltene buildup on surface anddownhole equipment, and the anomalous occurrence of water in structurallyhigher wells. These problems are observed in the southern part of the structurewhere Shuaiba water enters the Arab D. A collapse structure in the south isbelieved to serve as a conduit.
The Arab formation occurs in the lower part of the Riyadh group of the UpperJurassic Age and is penetrated 4,000 to 5,000 ft below mean sea level in theBahrain field. Lithologically, the formation consists of four carbonate zonesseparated by anhydrite beds. These zones are designated as Arab A through D indescending order. In the off-structure wells, Arab D carbonate becomes moredolomitic and loses the good reservoir characteristics of the crestal wells.The Shuaiba aquifer is a porous and permeable limestone of the Upper CretaceousAge. Its upper contact is unconformable with
ahr Umr siltstones while the lower is conformable with the Hawar shale. Fig.1 depicts the Mesozoic stratigraphic column of the Bahrain field and itsgeographic location. Arab A and C are gas reservoirs; Arab D is a major oilreservoir with a large gas cap. Arab B is a minor reservoir with a thin oilcolumn and a relatively large gas cap. Arab A and C were exploited during1938-76 and are considerably depleted. Oil production from Arab D started in1938. production from Arab D started in 1938. The water in the Arab formationin the Arabian Shelf oil fields and in exploratory areas around Bahrain Islandis supersaline (Fig. 1). The salinity of Arab D in the Bahrain field is verylow, especially in the south where it averages 7,000 ppm (NaCl). The anomalousfresh nature of this water has puzzled geologists and engineers for years. Theonly source of abundant fresh water is the thick, porous Shuaiba aquifer, whichlies about 2,500 ft above Arab D. Vertical communication between theseformations is believed to freshen the Arab D water in the Bahrain field.
Classification and Composition of Arab D Waters
In this study, Arab D waters are classified into three types. Type C. Thissupersaline water occurs regionally and in the off-structure areas aroundBahrain. It is characterized by low sulfate content (300 to 750 ppm) and veryhigh NaCl content (greater than 150,000 ppm). Type B. This fresh water occursonly in the southern part of the field. This type is characterized by highsulfate content (greater than 1,000 ppm) and very low NaCl content (less than10,000 ppm). Type A. This water, with a salinity of 12,500 to 38,000 ppm NaCl,occurs in the northern part of the Bahrain field. The sulfate content variesfrom 750 to 1,000 ppm. The Shuaiba water is slightly fresher than Type B water( 6,500 ppm NAC1), with a sulfate content of about 800 ppm. Tables 1 and 2 givethe chemical composition of Types A and B, and Type C and Shuaiba waters,respectively. Type C water is believed to be the connate water of the Arabreservoirs. Each Arab reservoir is overlain by a thick anhydrite bed. Anhydriteis deposited in closed basins under extensive evaporation conditions. Theevaporation during anhydrite deposition left heavier and more concentratedbrines behind, possibly accounting for the high salinity of the Arab connatepossibly accounting for the high salinity of the Arab connate water. Mostprobably, the Arab D water in the field area also was originally supersaline asin the surrounding areas. However, the mixing of fresh water reduced itssalinity. In the southern part of the field, which is close to the point offreshwater entry, flushing was more pronounced; the northern area had lessmixing of fresh water. This gave rise to Type B and Type A waters,respectively. Fresh Arab D and Shuaiba waters are considered to be meteoricbecause of their very fresh nature and high sulfate content. On the other hand,Type C water is considered to be the connate water of Arab D, and type A is amixture of the Arab D connate and Shuaiba meteoric waters.
Source of Fresh Arab D Water In the Bahrain Field
The Arab D water in the southern part of the field could have been freshenedby lateral advancement of a freshwater front within the Arab D reservoir or byvertical communication between fresh and connate waters. Lateral advancement offresh water from the Arab D aquifer to the field area is ruled out for thefollowing reasons.
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