ABSTRACT

As the larger hydrocarbon accumulations on the UK Continental Shelf become depleted, geoscientists are challenged to develop new techniques to produce from complex smaller discoveries. This paper describes the development and application of novel formation evaluation techniques to enable production from complex gas reservoirs. The example used in this paper involved a large structure, which had long been recognised but was not developed due to a variety of technical challenges including the thin-bedded nature of the sediments and the presence of both mobile and immobile viscous residual oil. The oil is a highly viscous liquid, which if produced, could block production tubing due to the shallow depth of the reservoir and associated low pressures. To successfully produce dry gas, identification of both oil and gas zones was necessary to enable gas zones to be perforated, and oil zones to be excluded. During the development drilling campaign the reservoir was appraised by using a formation evaluation programme specifically designed to address the presence of oil within the thinly bedded reservoir. In conjunction with core data and high resolution electric logs, nuclear magnetic resonance tools were used to identify and avoid perforating zones with higher oil saturations. Over the reservoir interval, there were significant intervals of borehole washout. Badly affected logs were repaired using fuzzy logic. This technique finds relationships between electrical logs in order to create synthetic logs, which are used for quality control, to infill data gaps and to repair sections of poor log. In order to understand the variation in reservoir quality and to correlate between wells, litho-facies and permeability were predicted throughout all wells. Genetic algorithm and fuzzy logic techniques were used to find relationships between the electrical logs and the core results. These relationships were used to predict continuous litho-facies and permeability curves together with a visual and numerical comparison of their uncertainty. Formation fluid types were derived from the nuclear magnetic resonance measurement (NMR). A pattern recognition technique that analyses the entire shape of the T1 and T2 distributions was used to derive the volumes of gas, oil and water. The technique was calibrated using Dean and Stark fluid analysis data and the results were used to ensure that the perforation strategy avoided oil bearing sands. This paper describes the subsurface challenges and how, through the application of novel formation evaluation techniques, a complex tight gas field has been characterised. As the larger hydrocarbon accumulations on the UK Continental Shelf become depleted, geoscientists are challenged to develop new techniques to produce from complex smaller discoveries. This paper describes the development and application of novel formation evaluation techniques to enable production from complex gas reservoirs. The example used in this paper involved a large structure, which had long been recognised but was not developed due to a variety of technical challenges including the thin-bedded nature of the sediments and the presence of both mobile and immobile viscous residual oil. The oil is a highly viscous liquid, which if produced, could block production tubing due to the shallow depth of the reservoir and associated low pressures. To successfully produce dry gas, identification of both oil and gas zones was necessary to enable gas zones to be perforated, and oil zones to be excluded. During the development drilling campaign the reservoir was appraised by using a formation evaluation programme specifically designed to address the presence of oil within the thinly bedded reservoir. In conjunction with core data and high resolution electric logs, nuclear magnetic resonance tools were used to identify and avoid perforating zones with higher oil saturations. Over the reser

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