Summary
A user-friendly type chart has been constructed as an aid to the evaluation
of water saturation from well logs. It provides a basis for the inter-reservoir
comparison of electrical character in terms of adherence to, or departures
from, Archie conditions in the presence of significant shaliness and/or low
formation-water salinity. Therefore, it constitutes an analog facility. The
deliverables include reservoir classification to guide well-log analysis, a
protocol for optimizing the acquisition of special core data in support of log
analysis, and reservoir characterization in terms of an (analog) porosity
exponent and saturation exponent.
The type chart describes a continuum of electrical behavior for both water
and hydrocarbon zones. This is important because some reservoir rocks can
conform to Archie conditions in the fully water-saturated state, but show
pronounced departures from Archie conditions in the partially water-saturated
state. In this respect, the chart is an extension of earlier approaches that
were restricted to the water zone. This extension is achieved by adopting a
generalized geometric factor—the ratio of water conductivity to formation
conductivity—regardless of the degree of hydrocarbon saturation. The type chart
relates a normalized form of this geometric factor to formation-water
conductivity, a “shale” conductivity term, and (irreducible) water saturation.
The chart has been validated using core data from comprehensively studied
reservoirs.
A workflow details the application of the type chart to core and/or log
data. The analog role of the chart is illustrated for reservoir units that show
different levels of non-Archie effects. The application of the method should
take rock types, scale effects, the degree of core sampling, and net reservoir
criteria into account. The principal benefit is a reduced uncertainty in the
choice of a procedure for the petrophysical evaluation of water saturation,
especially at an early stage in the appraisal/development process, when
adequate characterizing data may not be available.
Introduction
One of the ever-present problems in petrophysics is how to carry out a
meaningful evaluation of well logs in situations where characterizing
information from quality-assured core analysis is either unavailable or is
insufficient to satisfactorily support the log interpretation. This problem is
especially pertinent at an early stage in the life of a field, when reservoir
data are relatively sparse. Data shortfalls could be mitigated if there was a
means of identifying petrophysical analogs of reservoir character, so that the
broader experience of the hydrocarbon industry could be utilized in
constructing reservoir models and thence be brought to bear on current
appraisal and development decisions. Here, a principal requirement calls for
type charts of petrophysical character, on which data from different reservoirs
can be plotted and compared, as a basis for aligning approaches to future data
acquisition and interpretation. This need manifests itself strongly in the
petrophysical evaluation of water saturation, a process that traditionally uses
the electrical properties of a reservoir rock to deliver key building blocks
for an integrated reservoir model. The solution to this problem calls for an
analog facility through which the electrical character of a subject reservoir
can be compared with others that have been more comprehensively studied. In
this way, the degree of confidence in log-derived water saturation might be
reinforced. At the limit, the log analyst needs a reference basis for recourse
to capillary pressure data in cases where the well-log evaluation of water
saturation turns out to be prohibitively uncertain.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
27 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
30 May 2007
- Manuscript approved:
20 June 2007
- Version of record:
20 December 2007
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