Reservoir water flooding evaluation based on electromagnetic wave logging permittivity and conductivity inversion

Abstract

Electromagnetic wave logging at the measurement frequency of megahertz provides two kinds of information namely conductivity and permittivity, which is helpful to solve the problem of identifying water flooding situation of oil layers after long-term water displacement development. The electromagnetic wave combination logging tool in Daqing Oilfield measures two phase difference curves at frequencies of 60 MHz and 2 MHz. At present, there is still a lack of permittivity and conductivity inversion methods for this tool that are suitable for the interpretation of water flooding situation. Based on the magnetic dipole theory of uniform medium, the study found that for the combination logging tool, the 60 MHz phase difference is affected by both the permittivity and conductivity of the formation, while the 2 MHz phase difference is mainly affected by the conductivity. The reasonable and conveniently procedure for calculating the permittivity and conductivity from the inversion of the phase difference curves is constructed. Firstly, the shale content and porosity parameters of the layers are calculated from the curves of natural gamma and acoustic time difference, while the permittivity is calculated by a formula linearly constrained by shale content based on the permittivity of mudstone and oil-bearing clean sandstone. Secondly, for 2 MHz phase difference curve, a formula for calculating conductivity with known permittivity is established. Thirdly, set the calculated permittivity as the known parameter, a formula for inversion of the 2 MHz phase difference curve to calculate the conductivity is derived. Fourthly, using the inversion calculation of the conductivity as a known input, establish a formula for inversion of the 60 MHz phase difference curve to calculate the permittivity. In the process, the permittivity calculated by the shale content constraint method is used as the initial value of the iteration, which weakens the influence of mudstone on the permittivity and ensures the accuracy of the iteration. Finally, the permittivity and conductivity calculated by inverse calculation for two sealed coring wells can reflect the flooding situation of layers. The identification standard of flooding levels was established by using the conductivity and permittivity of inversion calculation, and the coincidence rate reached 90.7%, which is suitable for the effective identification of remaining oil in the long-term fresh water displacement layers. At the same time, based on the simplified theoretical model, the inversion of 3 combination of (2 MHz, 20 MHz), (60 MHz, 20 MHz), (2 MHz, 100 MHz) is carried out respectively, and the calculated values of remaining oil saturation are consistent with the injected water sweeping area, so the universality of the research method is verified.