METHODOLOGICAL ASPECTS OF ELECTRICAL LOGGING DATA INTEGRATION FOR INVERSION BASED ON TWO-DIMENSIONAL AXISYMMETRIC FORMATION MODELS
K.V. Sukhorukova1, A.M. Petrov1, A.A. Lapkovskaya1, S.V. Ananyev2
1Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences,Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
Keywords: well logging, electrical resistivity, joint numerical inversion, two-dimensional geoelectric model, galvanic logging, induction logging, Achimov deposits, Jurassic reservoirs.
Abstract
This work is dedicated to
the development of a numerical inversion methodology for electrical logging
data in sub-vertical and slightly inclined oil and gas wells. It examines the
specific features of joint inversion of galvanic and induction logs combined in
various configurations, depending on the equipment used. The axisymmetric
two-dimensional geoelectric model of medium consists of homogeneous blocks
separated with horizontal and coaxial cylindrical boundaries. The blocks are
characterized by horizontal and vertical electrical resistivity, as well as
dielectric permittivity. Numerical inversion is carried out until a minimal
discrepancy is achieved between the measured signals and those simulated within
the interpretive formation model. Unlike the traditional layer-based approach, this
inversion method involves not only adjusting the sounding curves but also
accounting for signal variations along the borehole. In this case, the
resulting model aligns as closely as possible with the actual measurements.
Inversion based solely on galvanic measurements can potentially yield unstable
results near boundaries with high resistivity contrasts, underestimating the
resistivity of layers adjacent to highly resistive ones. The ambiguity can be
reduced through joint inversion of galvanic and induction logs, although this
often requires the base formation model to be more complex. In particular,
during drilling with fresh clay-based mud in reservoirs with mixed oil and
mineralized formation water saturation, a low resistivity annulus forms. This
zone is more electrically conductive than both the invaded zone and virgin
formation due to the higher content of saline formation water. It cannot be
detected through galvanic well logs analysis, but its influence on induction
logs is significant. Therefore, the presence of an annulus may be a crucial
factor to consider when constructing a geoelectric model of the formation that
aligns with resistivity logs obtained with different logging methods. These
specifics are illustrated with the results of inversion of real data measured
in vertical wells in the intervals of Lower Cretaceous and Jurassic deposits of
the Shirotnoe Priobie oil fields.
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