Fresh Water Conundrum in Oil and Gas Reservoirs of Malaysia

Abstract

Abstract Positive identification of a water bearing interval is crucial to evaluating hydrocarbon bearing reservoirs in a well. Accurate estimation of formation water resistivity is also critical for proper computation of reservoir parameters used in hydrocarbon resource assessment. In reservoirs with high formation water salinity, it is relatively easy to differentiate water bearing zones from those bearing hydrocarbon. However, when the salinity of formation water is low, close to being fresh water, it becomes difficult to positively identify a water bearing reservoir, since the formation resistivity is relatively high. In some cases, a fresh water bearing interval may be misinterpreted as a hydrocarbon zone. In the Malay basin, offshore Malaysia, there are several reservoirs which contain relatively fresh formation water, making it difficult to evaluate the well logs from these reservoirs. Furthermore, some of these reservoirs are deep and have low porosity, which in combination with low formation water salinity, lead to relatively high formation resistivity. In certain reservoirs, there may even be some hydrocarbon shows, such as an oil stain on cuttings, which may have been due to residual hydrocarbon effects. These often cause confusion and contention in interpreting the fluid type in these reservoirs. Several case histories are presented in this paper, highlighting the importance of proper identification of fresh water bearing formation in evaluating well log data. In one of the case histories, a fresh water bearing interval was misinterpreted as a hydrocarbon zone and tested accordingly, resulting in substantial flow of formation water with extremely low salinity. In the others, the formations tested had low porosity and permeability, resulting in very small influx of formation water into the well bore with practically no flow at the surface. Since some of these zones had hydrocarbon shows and the formation water did not flow to the surface during the tests, there was much contention about the validity of the well tests and the conclusion on the fluid type. The authors have proposed a "what-if scenario" method, whereby a synthetic water saturation curve is computed using generic capillary pressure data, assuming that these zones are hydrocarbon bearing. The resulting water saturation curves are then compared with those computed using the resistivity logs from these zones. The possible presence of hydrocarbon may then be inferred from the difference between these water saturation curves.