Research on cross-well pseudorandom electromagnetic detection method and extraction of response characteristics

A method is proposed that uses the metal casing of an oil and gas production well to construct a long dipole emission line source and applies pseudorandom multifrequency excitation signals to it to identify reservoir characteristics of interwell reservoirs. This method can significantly improve the work efficiency and precision of electrical prospecting. The cross-well pseudorandom response signal contains multiple main frequency components, as well as various measuring frequency points. Targeting the frequency components with different amplitudes and initial phases in the pseudorandom signal, the digital coherent detection method and its noise resistance are studied. Using this method can effectively extract the amplitude and phase information of each frequency component in different noise environments and has a strong ability to suppress signals of other frequencies. In the heterogeneous interwell reservoir detection model, the amplitude characteristics of each frequency component of the pseudorandom electromagnetic response at the measuring line with different well spacings are extracted. Numerical results show that the curve of equal-frequency measurement appears downwardly "concave" at the interface of the low-resistivity interlayer, and upwardly "convex" at the interface of the high-resistivity interlayer. With the increase in the resistivity of the low-resistivity interlayer, the dynamic range of the extraction component of the same frequency decreases gradually, and the family of equal-frequency response curves overlap with each other. By contrast, for the high-resistivity interlayer, as its resistivity gradually increases, the dynamic range of the extraction component of same frequency also increases gradually. Moreover, as well spacing varies, the response characteristics of the equal-frequency curve family on different measuring lines are also obviously different. The digital coherent detection method significantly simplifies the receiving circuit system. In practical application, response information of different discrete frequency components can be extracted according to requirements. This thus provides a theoretical basis for the extraction of resistivity and characteristic identification of interwell heterogeneous oil and gas reservoirs.