Analysis of Sensitive Factors in Dual Laterologs for Cavernous/Karst Formations Near the Borehole

Igneous and carbonate reservoirs consist of formations characterized by containing extensive fractures, caverns (karst), and fractured karst. The challenges exist in the fact that there is obvious anisotropy and strong heterogeneity, resulting in huge difficulty in logging identification and quantitative evaluation. In order to provide a theoretical basis for the logging identification and evaluation of karst/cavernous formations, the numerical analysis has been used to analyze the effects of hole diameter, drilling fluid resistivity, matrix resistivity, cave size, cave fillings. Further, the distance between cave and borehole is calculated based on the response of dual laterolog using the finite element method. The results showed that the dual laterolog resistivity was significantly reduced due to the existence of near-borehole cave formation. With an increase of cave size, the reduced amplitude of dual laterolog resistivity could be increased as well. The dual laterolog resistivity is the smallest at the center of cave, and as the cave radius increases, both the deep and shallow lateral resistivity decrease. The dual laterolog resistivity increases with the increase of distance between the cave and the well wall, and the increase rate of shallow lateral resistivity is significantly faster than that of deep lateral resistivity. The research results can provide theoretical guidance for the interpretation of dual laterolog data in cavernous/karst formation.