Quantitative gas saturation monitoring have always been a desired target for fluid monitoring in hydrocarbon bearing reservoirs. Over the years many techniques were attempted for monitoring gas saturations which involved the use of both open hole and cased hole logs. None of these techniques gave a reliable measure of gas saturation, and their results were considered at best qualitative. In many reservoirs in the Middle East, the need for an accurate technique of gas saturation monitoring became more acute because of gas cap expansion or the formation of secondary gas caps, which are on the whole the product of reservoir depletion. Moreover, high production rates in permeable reservoirs resulted in gas coning and the production of free gas.

With the introduction of the Dual-Burst Thermal Decay Time (TDT-P) Tool, accurate through tubing measurements are now available that have opened new horizons for gas saturation monitoring. Such data was used successfully to monitor quantitatively gas saturations in various reservoirs in the Middle East. The technique involves the use of the two measurements from the TDT-P: capture cross-section (∑) and porosity (øtdt). These two measurements were combined with results from open hole log evaluations, such as lithology, porosity and clay volume, to solve for values of water, oil and gas saturations. This technique was tried on a large number of wells, and the accuracy of the results, for most of the reservoirs under study (porosity >15 pu), is of the order of 10 saturation units.

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