Correction of the anisotropy in resistivity: application to pore pressure prediction

Dissertação para obtenção do Grau de Mestre em Engenharia Geológica (Georrecursos)

This dissertation is based on a curricular training period done at company Total EP Angola between July and December 2013. The data presented relate to a real case study of an exploration block, which for reasons of confidentiality is designated by Block Michocho. The fluids pressure measurement in the geological formations can be inferred from the formation resistivity log. In not perpendicular wells to the layers, resistivity curves show higher values than the expected due to the anisotropic effect of the formation thus the inference of the pressure of fluids from resistivity logs can lead to unrealistic values. Most of the developments wells drilled on Block Michocho in Angola are highly deviated, if not sub-horizontal, in the reservoir section. The objective of this work is to correct the anisotropic effect of the resistivity of Block Michocho due to non-perpendicularity of the wells when intersect the geological formations. In this study, the correction of the resistivity is based on the formula proposed by Moran and Gianzero in 1979 and involves the dipping angle of the induction logging tool and the coefficient of anisotropy of the rock formation. Prior to application of this formula for the corrections of resistivity of the Block Michocho wells logs, a set of validation tests were made. Due to lack of data on development wells (highly inclined wells) the validation test was carried out in five exploration wells where resistivity is available in the two principal directions. It was assumed that the formula would be approved for resistivity corrections if the horizontal resistivity obtained by the formula had a good correspondence with the horizontal resistivity obtained by the induction logging tool. After this validation step, the coefficient of anisotropy to be used in the formula was calibrated as well as the correction of the curves of resistivity of the remaining development wells, those much more diverted regarding the rock layers. The corrected resistivity can be applied for pore pressure prediction in low permeability rock formations, in which the main objective is to identify regions where fluid pressure is higher than normal pressure, i.e. overpressure regions. For illustration purposes, a resistivity curve from an exploration well was chosen and the pressure of the fluids in low permeability rocks was computed by using the formula proposed by Eaton in 1975. With this well data, a potential overpressure region was identified and should be avoided in drilling activities.