Claude Boyeldieu
Schlumberger Inland Services, London
Szabolcs B. Horvath
OMV, Vienna
ABSTRACT
Before planning a tertiary recovery project, it is necessary to accurately evaluate the quantity of oil remaining in the reservoir. As such projects concern fields which may have been drilled decades ago, when logging programs were very limited, drilling new in-field wells provides an opportunity to make a complete evaluation of the reservoir in its present state.
A case history is presented of the evaluation of residual oil after water flooding in a well of the
Maustrenk Field in Austria. The well was continuously cored with a well-conditioned mud to ensure the best conditions for open hole logging. The mud had been prepared in such a way that a “smooth” invasion of mud filtrate would take place. The mud cake was thin and the depth of invasion some 10 to 12 inches. The mud salinity and composition were such that the mud filtrate had a resistivity equal to the original formation water before flooding. As the formation water was rather fresh and as the computation technique used on this method requires the use of the Thermal Decay Time Log response in open hole, it was necessary to add boric acid to the mud. This increased the nuclear capture cross section of the mud filtrate, thereby making the Thermal Decay Time Log measurement more sensitive to water saturation in the invaded zone.
Open hole logging consisted of the Dual Laterolog, Micro-Spherically Focused Log, the
Compensated Neutron Log, the Formation Density Log, the Gamma Ray Log and the Thermal Decay Time Log. The computer-processed evaluation was made by comparing the results obtained from the microdevices in the invaded zone with those derived from the Thermal Decay Time Log. Parameters were adjusted until agreement was obtained, and final results were compared with the porosity and saturation measurements performed on cores.