TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractOptimizing production from existing boreholes requires the detection and evaluation of bypassed hydrocarbon and the ability to track fluid movement in the reservoir. Logging through casing is an important acquisition technique for this application. There are a number of measurements which provide reservoir saturation monitoring through casing, however each technology has limitations. The factors limiting the application of different measurement types relate to formation properties, completion type and production characteristics of the well. The selection of the most suitable technology requires consideration of the effect of the completion, well dynamics and formation characteristics on different measurements in addition to an understanding of the measurement physics.This case study is from Wara sandstone reservoir Wafra Field located in partitioned neutral zone (PNZ) between Kuwait and Saudi Arabia. The wells are producing with high water cut and reservoir surveillance requires an efficient method for saturation monitoring. The saturation monitoring in Wara is challenging due to relatively low formation water salinity, varying amount of clay content, and the depleted state of the reservoir. Application of slim, cased-hole formation resistivity measurements is successfully used to evaluate the saturation in challenging conditions. A lesson learned from this study is that proper job planning is critical for cased-hole logging. The planning phase is divided into two steps.The first part simulates the responses of the various tools using available reservoir information to identify the most appropriate saturation monitoring technique. The second part analyzes the logging conditions and implements appropriate well preparation measures to ensure representative data acquisition. The criteria developed for the selection of cased-hole resistivity measurements and the appropriate steps taken to ensure reliable data acquisition are presented in this paper.Proper job planning provided good quality time lapse formation resistivity logs in several cased-hole wells without the need to kill the wells. The study also contains a comparison between resistivity and sigma log evaluation of fluid contacts and quantification of water saturation in this shaly sandstone reservoir. It was found that due to low contrast and shallow depth of investigation, the sigma measurement does not provide a robust saturation analysis in the Wara formation in some cases. The cased-hole resistivity is used to evaluate time lapse saturation, which helped to identify by-passed oil zones. In addition, the zones which are most prone to high water production were identified and a work over plan is proposed and implemented. The time lapse cased-hole resistivity logs and the proper job planning has helped in successful saturation monitoring in this shaly sandstone reservoir.
The PNZ (Partitioned Neutral Zone) has four onshore producing fields bordering Kuwait and Saudi Arabia and is being operated by Joint Operations of Kuwait Oil Company and Saudi Arabian Texaco. The four fields are Wafra, South Umm Gudair (SUG), South Fuwaris and Humma. The majority of production in the PNZ comes from the Ratawi Oolite in the Wafra and South Umm Gudair fields. Production from these fields began in the early 1950s and as maturing of these fields with the passage of time the associated water production has significantly increased which has resulted an environmental hazard in handling and disposal of this vast produced water. This challenge has been met in the past utilizing first surface evaporation pits and then augmented with peripheral water injection into the Ratawi producing reservoir. Today JO is meeting this challenge with the innovative method of horizontal Mega disposal wells completed in the karsted Shuaiba formation. These horizontal Mega wells are capable of taking up to +/- 80,000 BWPD on gravity feed. So currently, +/- 750,000 BWPD of produced water in JO is handled by injection of +/- 220,000BWPD into the peripheral of the Ratawi Oolite in the main Wafra field for the Pressure Maintenance Project (PMP) and through disposal of +/- 530,000 BWPD into the karsted Shuaiba formation. This paper highlights the integrated approach of studying the 3D Seismic interpretation, geologic, and engineering data resulted in the identification of Shuaiba karst Structure as the target disposal zone. The construction and completion of Horizontal Mega wells with 9 5/8″ tubing completion in the Shuaiba karst has resulted in achieving the company target of Zero water disposal in surface pits. It is a remarkable achievement and this technique is uniquely introduced for the first time in Kuwait and could have a potential to be used in the other areas in the Middle East Region. Introduction Until January, 1996 for SUG and January, 2000 for main Wafra about +/- 350,000 BWPD produced water in PNZ was disposed in evaporation pits. The surface pits are environmental hazardous and is undesired practice, therefore JO Management decided to stop this practice and move toward down hole water disposal schemes to achieve Zero Disposal of produced water in surface pits. Shuaiba and Zubair are Lower Cretaceous formations (Fig No.1) that have been penetrated by every Ratawi producer in Wafra Field. Drilling through the Shuaiba has been a concern of severe loss circulation and numerous bottom hole assemblies have been lost due to stuck and hence resulted most of these wells in side tracking the hole. After having study of the characteristics of the Shuaiba formation which represents as wide spread with thick sandstone beds of the Zubair below and Wara / Burgan above. It is Dolomitic limestone, medium to coarse crystalline in texture. It is vuggy to cavernous at time and its thickness changes from 102–180 ft and is spread through out PNZ Field in Wafra. These characteristics appealed to use this formation in benefit of disposing of vast quantities of water produced from different formations rather than disposing in surface pits.
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