Reconstruction of Resistivity and Permittivity Profiles Using Electromagnetic Well Logging Data

There is low resistivity contrast between reservoir zone and host formation in geothermal fields, especially in areas where the technique of water injection is being carried out. Therefore, in low contrast area, permittivity can be one of the important reservoir indicators besides resistivity. In this study, one EM tomography method of reconstructing the resistivity and permittivity from electromagnetic well logging data was proposed. It was used to derive the distributions of resistivity and permittivity in nearby area around the borehole. The inversion procedures were carried out in two steps: first the low frequency EM well logging data was used to determine the resistivity, then it was regarded as the initial value, and the permittivity and resistivity were reconstructed from high frequency EM well logging data. The forward modeling at each inversion iteration was fulfilled by the numerical approximation of finite element method (FEM). Results of different anomalous bodies show that well logging data is only sensitive to nearby area around the borehole, the resolution of parameters is better in vertical direction than that in horizontal direction though it is also related to the distance between transmitter and receiver. Numerical simulation also suggests that as the frequency increases, the sensitive range becomes small. From our results, it can be said that the method proposed in our study can simultaneously reconstruct permittivity and resistivity from multi-frequency EM well logging data, and it may be a valuable means of evaluating low resistivity contrast reservoir in geothermal fields as well as in oil fields.