Summary
There have been many different approaches to quantifying cutoffs, with no
single method emerging as the definitive basis for delineating net pay. Yet
each of these approaches yields a different reservoir model, so it is
imperative that cutoffs be fit for purpose (i.e., they are compatible with the
reservoir mechanism and with a systematic methodology for the evaluation of
hydrocarbons in place and the estimation of ultimate hydrocarbon recovery).
These different requirements are accommodated by basing the quantification of
cutoffs on reservoir-specific criteria that govern the storage and flow of
hydrocarbons. In so doing, particular attention is paid to the relationships
between the identification of cutoffs and key elements of the contemporary
systemic practice of integrated reservoir studies. The outcome is a structured
approach to the use of cutoffs in the estimation of ultimate hydrocarbon
recovery. The principal benefits of a properly conditioned set of petrophysical
cutoffs are a more exact characterization of the reservoir with a better
synergy between the static and dynamic reservoir models, so that an energy
company can more fully realize the asset value.
Introduction
In a literal sense, cutoffs are simply limiting values. In the context of
integrated reservoir studies, they become limiting values of formation
parameters. Their purpose is to eliminate those rock volumes that do not
contribute significantly to the reservoir evaluation product. Typically, they
have been specified in terms of the physical character of a reservoir. If used
properly, cutoffs allow the best possible description and characterization of a
reservoir as a basis for simulation. Yet, although physical cutoffs have been
used for more than 50 years, there is still no rationalized procedure for
identifying and applying them. The situation is compounded by the diverse
approaches to reservoir evaluation that have been taken over that period, so
that even the role of cutoffs has been unclear. These matters assume an even
greater poignancy in contemporary integrated reservoir studies, which are
systemic rather than parallel or sequential in nature, so that all components
of the evaluation process are interlinked and, therefore, the execution of any
one of these tasks has ramifications for the others (Fig. 1). A particular
aspect of the systemic approach is the provision for iteration as the reservoir
knowledge-base advances. For example, simulation may be used in development
studies to identify the most appropriate reservoir-depletion mechanism, but,
once the development plan has been formulated, the dynamic model is retuned and
progressively updated as development gets under way.
The principal use of cutoffs is to delineate net pay, which can be described
broadly as the summation of those depth intervals through which hydrocarbons
are (economically) producible. In the context of integrated reservoir studies,
net pay has an important role to play both directly and through a net-to-gross
pay ratio. Net pay demarcates those intervals around a well that are the focus
of the reservoir study. It defines an effective thickness that is pertinent to
the identification of hydrocarbon flow units, that identifies target intervals
for well completions and stimulation programs, and that is needed to estimate
permeability through the analysis of well-test data. The net-to-gross pay ratio
is input directly to volumetric computations of hydrocarbons in place and
thence to “static” estimates of ultimate hydrocarbon recovery; it is a key
indicator of hydrocarbon connectivity, and it contributes to the initializing
of a reservoir simulator and thence to “dynamic” estimates of ultimate
hydrocarbon recovery.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
5 April 2004
- Revised manuscript received:
6 May 2005
- Manuscript approved:
31 May 2005
- Version of record:
15 August 2005
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