A Novel Method for Extracting Resistivity Anisotropy from EM Resistivity Logging While Drilling

Abstract

In horizontal wells or highly deviated wells, the electromagnetic wave resistivity logging has been widely used in LWD service. Most of lithologic formations are macroscopically anisotropic, and the horizontal resistivity $(R_{h})$and vertical resistivity $(R_{v})$can describe the anisotropic characteristics of the transversely isotropic media. However, for EM resistivity logging of LWD, R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ad</inf> (attenuation resistivity) and R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</inf> (phase shift resistivity) are usually obtained from the amplitude ratio(EATT) and phase shift $(\triangle\Phi)$independently, which cannot well reflect the true anisotropic formation resistivity. As we all know, when LWD is used, the drilling fluid invasion is not so significant, and the impact of formation resistivity anisotropy on formation evaluation and geosteering should be mainly considered, therefore, one new method is proposed to accurately extract resistivity anisotropy in this paper.The conclusion that R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ad</inf> and R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ps</inf> have different detection properties (i.e., different DOI and VR) could be questionable. In this paper, the detection property of the same signal (EM wave) is considered consistent, thus, on the basis of homogeneous and anisotropic formation model, the amplitude ratio (EATT) and phase shift $(\triangle\Phi)$equations are firstly derived in the frequency range of electromagnetic wave resistivity logging while drilling; Then, R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</inf> and R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</inf> are simultaneously and jointly calculated from the combination of these two equations, if in the inhomogeneous medium, corresponding to apparent $R_{h}(R_{ha})$and apparent $R_{v}(R_{va})$, respectively.In order to verify above method, several formation models including homogeneous anisotropic formation model and multilayer horizontal formation models, are presented here based on numerical simulations. All the calculated results of R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</inf> and R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</inf> have shown that the new method proposed in this paper can be reliable and accurate in horizontal or highly deviated wells, when the formation is thick. Certainly, the resistivity inversion may be still necessary if the formation is very thin.We also proposed several other methods (for example, by changing tool frequency, tool spacing, etc.) to jointly solve R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ha</inf> and R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">va</inf> , and carried out initial verifications and analyses.