Evaluation of Moved Oil Volume and Viscosity with NMR and Dielectric Dispersion Logs in a Heavy Oil Sandstone Reservoir Under Steam Injection

Abstract

Abstract We examine the vertical profiles of moved oil volume and oil viscosity in the monitoring well of a steam injection pilot project. The oil volume and viscosity were assessed with petrophysical logs during the drilling of a monitoring well located near the centre producer of the producers-injectors pattern. Prior to drilling the monitoring well, steam was injected for several months at the bottom of the sandstone heavy oil reservoir to increase the oil temperature and decrease its viscosity. As the monitoring well was drilled, water filtrate from the drilling fluid displaced a fraction of the reservoir oil proportional to the effectiveness of the steam flood; within the zones of elevated reservoir temperature, the oil viscosity was reduced and the mud filtrate invasion deeper. As the reservoir oil is viscous, the displacement induced by the water filtrate is small – the filtrate invasion is shallow. Monitoring this displacement therefore requires log measurements with a small depth of investigation, even smaller than the flushed zone resistivity Rxo from a micro-laterolog. In this work, we evaluate the moved oil volume and the viscosity of the reservoir oil from the combination of magnetic resonance porosity, T2 relaxation time and D diffusion constant, and of water-filled dielectric porosity at 2 depths of investigation. The water volume deep into the reservoir is compared to the NMR free fluid volume and to the deep and shallow dielectric water-filled porosities to identify the zones and calculate the volumes of oil displaced by filtrate. The NMR station measurements of T2 relaxation time and D diffusion constant provide a direct evaluation of reservoir oil and water volumes as well as an estimate of the in-situ oil viscosity after steam flood. The profiles of fluids volumes after filtrate invasion confirm the effective vertical extent of the steam flood at the monitoring well and also provide a proxy to the residual oil volume under dynamic reservoir conditions. These volumes, controlled by the heavy oil shallow displacement by filtrate are only observable and measureable from the NMR and dielectric dispersion logs. This method, applied for the first time in an offshore steam injection pilot, is applicable in the open hole conditions of drilling the monitoring well. However, derivatives of this method can be applied in cased hole – NMR logs have been recently acquired in radio-transparent glass-reinforced epoxy casing with suitable temperature rating for steam injection.