Evaluation of Residual Oil Saturation in the Secondary Gas Cap of a Mature Carbonate Reservoir from Multi-Dimensional Magnetic Resonance Logging

Abstract

Tatweer Petroleum is a joint venture established in the Kingdom of Bahrain with the mission to reactivate the Bahrain field. One of the main oil producing reservoirs in the field is Mauddud, a shallow-water Cretaceous limestone. The reservoir has been producing for several decades with pressure support from crestal gas injection, thus a large secondary gas cap has developed covering the upper half of the reservoir. There is evidence that this section is preferentially oil-wet, with high Residual Oil Saturation (Sor). Tatweer's challenge is to quantify the Sor in the gas cap and design an EOR scheme to improve the oil recovery.Conventional resistivity, neutron, and density log methods can be used to compute the water and hydrocarbon volumes. However, the low formation water salinity combined with the uncertainty in the Archie coefficients and neutron-density endpoints result in insufficient saturation accuracy. In this paper we will focus on the integration of conventional logs with advanced multi-dimensional nuclear magnetic resonance (NMR) analysis to overcome the limitations.Modern NMR logging tools perform specialized acquisition sequences featuring a combination of polarization times, number of echoes and variable echo spacings in order to characterize the formation fluids simultaneously in T 2 (Transverse Relaxation Time), in T 1 (Longitudinal Relaxation Time) and D (Diffusivity). The result is reliable, continuous log quantification of fluid saturations based on the resulting D-T 1 or D-T 2 maps to subdivide the total NMR porosity into water, oil and gas volumes. In addition a crude oil viscosity can be derived from the oil signal. The water can be further divided into free and bound water. The methodology was applied in several Mauddud wells and shows results that are consistent with core data and a new petrophysical model developed by Tatweer based on rock types and saturation height modeling.