A correction method of deep laterolog responses under under-balanced logging conditions

Under-balanced drilling and under-balanced logging are very important techniques for directly identifying and protecting hydrocarbon reservoirs and enhancing productivity. The wellhead assembly under under-balanced logging is greatly different from that under conventional logging. Therefore, the length of cable electrode for dual laterolog should be reduced to adapt to the wellhead assembly, which causes the deep laterolog responses to drop and cannot truly reflect the reservoir property. The rule of electrode coefficient varying with the cable electrode length under under-balanced conditions has been computed using finite element method, that is, the longer the cable electrode, the smaller the electrode coefficient. The deep laterolog resistivity correction method under under-balanced condition is introduced. Field data examples are used to test the new method and shows that this method is reliable. This method can be used to any length of cable electrode and can be applied to field pretreatment directly. The corrected deep laterolog resistivity can be used in logging interpretation and quantitative hydrocarbon evaluation. Key words: under-balanced logging, hydrocarbon reservoir protection, dual laterolog, electrode coefficient, apparent resistivity correction, finite element method