Joint inversion of induction/lateral/normal logs, case studies at Shenli field site, China

The behavior of local convergence and nonunique solutions in electric-logging inversion problems make logging interpretation difficult. These negative effects are improved by joint inversion of induction- and galvanic-logging measurements. A two-dimensional joint inversion algorithm is suggested for the simultaneous estimation of formation resistivity, invasion depth and invasion-zone resistivity from field data containing a combination of induction array, lateral and normal devices. Compared with single-log inversion of lateral and normal logs, the joint inversion not only improves the unfavorable local convergence of single inversion, but also constrains the range of possible solutions. Even if the initial guess values vary within a wide region, the joint inversion is fast to converge close to true values; it weakens the dependence of inversion results on initial guesses. In addition, these nonunique solutions are constrained in a limited domain by the present joint inversion technique, thus, the inconsistent interpretations that are usually caused by traditional single-log approach can be avoided. Case studies also illustrate that the present joint inversion of induction/lateral/normal logs provides a reasonable and efficient algorithm that allows for improving the accuracy of the results, enhancing the reliability of formation parameter estimation, and in most cases reducing the uncertainty of hydrocarbon saturation.