The Array Induction Tool (AIT) has significant advantages over standard induction tools in terms of vertical resolution, depth of investigation and the ability to detect moved oil. India is generally considered Laterolog country. However in many areas the conditions are such that either Induction or Laterolog is appropriate; the Laterolog has often been chosen because of its higher vertical resolution compared to the standard Induction.
The AIT offers vertical resolution down to 0.3 Mts. and depths of investigation up to 90 inches in many conditions. This is significantly better than the Laterolog. In order to understand the response of the AIT, the resistivity characteristics of five Indian reservoirs have been examined. This study includes cases of resistive and conductive invasion, invasion profiling in shaly sandstones, invasion seen by standard induction, thin beds and response in a layered reservoir. In each case the response of the standard logs (Laterolog or Induction) have been modeled and compared with the response of the modeled AIT
The results show that in the most situations the AIT provides a better estimate of true formation resistivity, Rt. The improvement is towards reading resistivity values close to the Laterolog resistivity in the oil zone, and also reading a correct resistivity value in the water zone as expected by the Induction tool. The combined improvement enhances confidence in the computation of hydrocarbon saturation significantly. The AIT, therefore, detects moved oil more reliably, and hence facilitates decision on further investment.
The theoretical modeling work has been validated by two field cases in a sandstone environment. Comparison with the available Laterolog data clearly brings out the advantage of the AIT in terms of vertical resolution as well as in describing the invasion profile. Using the field data, the AIT logs were reconstructed through modeling. The reconstructed values match the field data, except in cases where highly irregular profiles are seen on the field data.