Quantitative Monitoring of Gasflooding in Oil-Bearing Reservoirs by Use of a Pulsed Neutron Tool

Abstract

Summary We developed a new method to monitor gasflooding of oil reservoirs. The method is based on computing two porosities: the base true porosity determined before gasflooding and monitor-apparent neutron porosity determined after gasflooding. The base and monitor porosities provide determination of a hydrogen index of the reservoir fluid after the flooding. The hydrogen index is then used to determine saturation of the flood agent after flooding and, consequently, saturation of water and oil after flooding. An example of quantitative analysis of base and monitor logs illustrates monitoring of CO2 flooding by use of the described method. Introduction The method of monitoring gasflooding presented in this paper is intended mainly for quantitative log interpretation at the well sites or remote computing centers. It requires a minimum of information about reservoir rock and fluid properties, most of which can be derived from the logs. The method is based on the computation of two porosities: the base porosity determined before gasflooding and monitor-apparent neutron porosity determined after the gasflooding. The primary disadvantage of this approach is that the monitor porosity has to be corrected for the excavation effect. The formulas proposed in Ref. 1 can be used for this purpose. However, these formulas take only the hydrogen index of fluid, porosity, and lithological composition of a reservoir into account. Specific flood agent, hydrocarbon types and gravity, different clay types, and accessory minerals are not taken into consideration. Thus, the correction for the excavation effect cannot always be accurate. If one desires higher-accuracy computations and if the specific chemical and physical properties of matrix, shale, formation water, hydrocarbons. and flood agent are known, one can use more comprehensive techniques for gasflood monitoring, such as the TMDFLOOD model described in Ref. 2. TMDFLOOD is a sophisticated log-analysis and modeling program more suitable for in-house, mainframe processing. It uses known chemical and physical properties of the reservoir, hydrocarbons. and the flood agent to model both the capture cross section, s, and ratio/porosity responses of a pulsed-neutron logging tool. Because this model is based on total neutron migration length and on direct ratio computations for the calculation of volume fractions of the flood agent, hydrocarbons, and formation water, it does not require a correction for the excavation effect. Higher accuracy of the monitoring method presented in this paper can also be achieved if chemical and physical properties of the reservoir and the flood agent are known. The neutron migration length and direct ratio calculations can be used to compute corrections to the monitor porosity and to derive corresponding formulas for the specific conditions. Once derived for a specific flood agent and a specific reservoir, these formulas can be used as part of the method presented here to correct monitor porosity for the "excavation effect" during the total life of the gasflood monitoring project. Refs. 3 through 6 also consider the quantitative monitoring of gasflooding by use of logging methods.