Accurate saturations required for carbonate characterization
Low-resistivity pay is a long-standing problem in formation evaluation;
using the Archie equation in such environments underestimates oil saturation
and overestimates water saturation. To overcome this challenge when drilling in
a low-resistivity carbonate reservoir in Libya, an operator collaborated with
Schlumberger petrotechnical experts to find a solution that would enable more
accurate measurement of saturations.
Water and oil saturation reevaluated
First, Schlumberger worked with the operator to suggest a methodology to
obtain more recent and reliable oil- and water-saturation measurements for
reevaluation. For this, Schlumberger recommended using the RSTPro reservoir
saturation tool, which determines reservoir saturation, lithology, porosity,
and borehole fluid profiles with an accuracy comparable to that from core
analysis. These petrophysical data were then input into GeoFrame software,
which delivers advanced interpretation and actionable information to help
operators make timely, better-informed decisions.
Then, the Schlumberger petrotechnical team reviewed previous approaches
taken to measure reservoir saturation, including resistivity modeling using
special core analysis (SCAL) data. SCAL interpretation variables include the
cementation factor (m), saturation exponent (n), and formation water salinity.
The team collaborated with the customer to test model resistivity in multiple
scenarios using different m and n values. Then, they tested different salinity
values until a good match was obtained between the modeled resistivity and true
resistivity by using the modified parameters of 60,000-ppt salinity, an m value
of 1.67, and an n value of 1.82.
Lastly, the Schlumberger team suggested reinterpreting the openhole logs
using the Archie equation with the modified m and n parameters that led to good
agreement between modeled resistivity and actual field resistivity.
Reevaluation showed that the new openhole saturation is higher than that with
default values of m and n. The accuracy of this approach was validated by the
good match of the data obtained using the RSTPro tool with that from laboratory
core saturation studies as well as the modified Archie calculation.
Net pay improved and oil recovery increased
Interpretation results revealed that the carbonate pays oil
saturation is actually higher than previously concluded from openhole log
interpretation. Processing the data took only 24 hours, allowing the customer
to confidently and quickly act upon the suggestions received.
Remediation based on these petrotechnical data and interpretation
resulted in improved operations by a number of measures.