Abstract
Accurate water saturation estimation in thin carbonate reservoirs is one of the major formation evaluation challenges in horizontal wells. This is mainly due to the strong shoulder bed effect of thick dense sections on the resistivity logs recorded in thin porous reservoir layers, particularly in horizontal wells. The performed work implies correction of the high resistivity log data acquired while drilling horizontal well sections by means of resistivity modeling. This has been achieved by building a forward model for the resistivity data acquired in the deviated section of the overlying reservoirs in water injector wells, where the porous section is thick and the shoulder bed effect is minimal. The resistivity model construction is based on the relationship of the deep and shallow Laterolog resistivity to the neutron porosity log responses measured in the thick reservoir sections. The established model equations are used to construct deep and shallow resistivity logs in the horizontal sections of thin reservoirs by resistivity inversion using the neutron porosity log data.Several well data sets have been tested and the results were complimented by test data, formation tester fluid sample analysis, Dean Stark measurements and core data. The results obtained confirmed that the resistivity inversion technique is applicable to compute water saturation in both the flushed zone (Sxo) and the un-invaded zone (Sw) for horizontal well sections drilled across carbonate reservoirs. The application of the inversion process allows generating resistivity log responses corrected for the tight shoulder bed effects, and providing more accurate water saturation estimation.
