A novel quantitative imaging method for oil-based mud: The full-range formation resistivity

摘要

Oil-based mud cake with its high resistivity affects severely the quality of borehole microresistivity imaging and has limited the use of microresistivity imaging logging in oil-based mud environment. We have utilized an equivalent model of a resistor and a capacitor to emulate the electrical responses of the high-resistivity mud cake and the formation, respectively, and then developed the electrical coupling relationship between the mud cake and formation, including vertical coupling and parallel coupling. We used the coupling relationship with a formation model to measure both the apparent resistivity and apparent relative permittivity of the formation simultaneously. Furthermore, we have imaged two formation models, one low-resistivity and the other high-resistivity using the vertical and parallel coupling, compared with the standard images obtained in the water-based mud environment corresponding to the same formation models. We have found that the vertical coupling and parallel coupling conquering the bad effect made by oil-based mud cake, can measure quantitatively the resistivity of low-resistivity and high-resistivity formation, respectively, and describe qualitatively the relative permittivity of low-resistivity and high-resistivity formation, respectively. The joint use of the vertical and parallel coupling can deal with the full-range microresistivity imaging of formation.