Electromagnetic fields in a non-uniform steel-cased borehole

摘要

Since most oil wells are cased in steel, electromagnetic (EM) signals undergo severe attenuation as they diffuse across the casing. This paper examines an effect of non-uniform casing properties on EM fields measured in a steel-cased well embedded in a layered formation. We use a finite-element method for computing secondary azimuthal electric fields in a cylindrically symmetric model, and analytically obtain primary fields for a homogeneous casing in a homogeneous whole space. Although steel casing largely masks EM signals induced into a layered formation, the induced signal is more pronounced in phase than in amplitude. The effect of casing non-uniformity is quite large in measured fields but is highly localized. When electrical conductivity varies rapidly in the casing wall, the resulting EM fields also vary rapidly. A cross-correlation function of these variations has strong peaks at two points, the interval between them being equal to the source-receiver distance. The high-frequency coherent noise event caused by the non-uniform casing can be greatly suppressed by low-pass filtering to enhance EM signals indicating formation conductivity.