Time-Lapse Saturation Monitoring and Its Applications to Time-Lapse Mapping

Abstract

Abstract Oil production and water injection involve extensive fluid movement through a reservoir. Monitoring of these fluid movements, and in particular water flooding of the hydrocarbon zone, is essential to optimization of reservoir performance. Fluid movements in carbonate reservoirs are complicated by their heterogeneous pore systems, which result in wide variations in permeability and cause uneven sweep and non-uniform depletion. High permeability streaks can promote fingering and channelling within the reservoir. Development of these irregular flow patterns is a function of porosity and permeability distribution, proximity to the original oil/water contact, activity of the underlying aquifer, vertical and lateral distance from injection points, and production and injection rates and distribution. The combined effects of all these parameters cannot be reliably modelled and predicted: their inter-relationship can only be determined by measurement of fluid saturations field-wide throughout the production life of the field. The normal technique for monitoring water saturation changes behind casing, when water salinity is high, is to survey using neutron decay time logs. These measure the neutron capture cross-section (Σfor) of the formation, which can be interpreted - together with open hole logs - to obtain values for water saturation. This paper illustrates an application of this technique on a field-wide basis over an extended period. Open and cased hole log data are combined into a data base of long term variations in water saturation. All the interpreted data are represented on maps which show the changes of water saturation with time. Maps of permeability distribution provide additional information which can explain anomalies and irregularities in the saturation maps. Such a field-wide study of reservoir behaviour enhances understanding of fluid movements, identifies high permeability intervals, and indicates zones of undisplaced hydrocarbon. The final objective is to use the derived model and understanding in simulations which will optimize and accurately forecast reservoir performance.