Abstract

Optimizing 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.

Introduction

The Wara Burgan Sand is a sandstone reservoir in the Wafra Field (Figure-1) and is the fourth of six producing reservoirs in sequence (Figure-2) in the Main Area producing oil from depths of 2,900 and 3,700 feet below sea level. Wafra Field is located onshore in the Partitioned Neutral Zone (PNZ), an area of land jointly governed by the state of Kuwait and the Kingdom of Saudi Arabia. The Wara Burgan reservoir is a Middle Cretaceous estuarine to tidal-influenced deltaic sequence of fine to very fine grained sands and siltstones. Oil is structurally and stratigraphically trapped in a complex northwest and north trending anticline. It is sealed by the overlying Ahmadi Shale (caprock) and based by the Mauddud (Orbitolina) Limestone formations.

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