Gas Saturation Detection Model Applied To Heterogeneous Reservoir Using (TDT)
Authors: Hiekal S.A. , Hamada G.M. , Khalil M. 
Published: December 2007

Abstract
The Zeit Bay field reservoir units consist of sandstone and carbonates, partially overlaying a tilted block of fractured basement reservoir with a complex drive mechanism. A secondary recovery scheme of gas re-injection into the original gas cap was initiated to maintain reservoir energy and minimise pressure decline. Hence accurate detection of gas movement is very critical. Several difficulties to monitor gas-oil contacts were encountered in a considerable number of wells. Some of these difficulties were, gas channelling behind the casing, gas coning, wellbore fluid changes, porosity and lithology changes, wellbore fluid invasion into the reservoir and the presence of formation stimulation fluid (HCL). The application of conventional methods using the response of gas indicator curves could result in a false indication of formation gas-oil contacts.
This paper discusses the approach adopted in order to determine the gas-oil contact in wells where such problems occur. A database was established including more than 70 TDT runs, open hole log and pressure data of 12 infill wells, and production performance records of all Zeit Bay wells. The approach follows the Polyachenko model of functional relationship between count rates and gas saturation. Several cross plots for the same range of porosity and connate water saturation, e.g. formation capture cross section (SIGM), total selected near detector counts (TSCN), total selected far detector counts (TSCF), the capture cross section of the borehole (SIBH), and inelastic far detector counts (INFD). Each cross plot gives a definite diagnostic shape around the depth of the formation gas-oil contact. By using these cross plots it would be possible to calculate gas saturation from a stand alone run. The model was validated by RFT and open hole log data from infill wells. Also it was successfully applied in wells which showed an ambiguity in the detected formation gas-oil contact.
The field gas-oil contact in Zeit Bay was revised using the results of the model. This revision lead to an accurate definition of the oil leg and to the drilling of three additional wells in the field. The open hole log results of these three wells verified the gas-oil contact determined by the model.

Introduction
The Zeit Bay field has a NW-SE trending structure comprising of clastic and carbonate rocks overlaying a tilted block of granitic basement. It measures about 2.5×4.5 km in size and is situated at the SW corner of the Gulf of Suez (Figure 1). The field was discovered in 1981, about 40 development wells were actively depleting the reservoir; and 25 of these wells are still active producers. The reservoir in Zeit Bay field is in complete hydraulic communication. The initial GOC detected in this field was 4020 ftTVDss (Figure 2), and the oil accumulation, with an 830 ftTVD (Shell 1988).
Production from Zeit Bay field commenced in 1984, and reached around 200 MMSTB, in 1996. The average field GOR started to increase with the end of 1987 (Figure 3). The primary recovery mechanism is a solution gas drive supported by gas cap expansion, gravity drainage, aquifer support and gas injection optimised to a level of more than 70% of total gas production.
The gas movement in Zeit Bay field is affected by a large number of factors: cumulative production, well location, formation dipping, fracture direction and the degree of communication between producing and gas injection wells, (Hamallawy et al, 1993).
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