OilUnmixer™ software for allocation of commingled production has
been subjected to numerous blind
tests by numerous petroleum companies. A blind test is when an
outside laboratory prepares artificial mixes of multiple oils, and
then OilUnmixer™ software is used by OilTracers LLC to determine
the contribution of each oil to the mixes (with OilTracers LLC
having no advance knowledge of what the "answer" is). Numerous blind
tests have shown that OilUnmixer™ typically yields allocation
results that are within 1-5% of the actual result, even when the
oils being commingled are extremely similar in composition.
There are many advantages to using oil geochemistry (vs.
production logging) to allocate commingled production. At
OilTracers, we use geochemical methods to
solve two types of production allocation problems:
Assessing the relative production from multiple pay zones in a
given well
- Cost advantages: Geochemical techniques for allocating
commingled production from multiple zones in a single well
typically result in a >90% cost savings relative to production
logging (McCaffrey et al., 2006). Geochemical techniques cost $550
to $1650 per well per allocation, while the alternative (production
logging) usually costs >$50,000 per well. The greater cost of
the production logging approach is due not only to the costs of
running the log, but also to the associated rig costs, and the
costs of lost production during logging. These costs are not
applicable to the geochemical approaches. Other advantages of the
geochemical techniques include:
- Detection of zone performance problems at any point during
the life of a well: The low cost of the geochemical techniques
for production allocation allows field engineers to monitor
production frequently over long periods (weekly, monthly,
quarterly). This ability to monitor continuously the relative
performance of discrete pay zones allows early identification of
zone performance problems. The much higher cost of production
logging limits that technique to infrequent use; therefore,
production logs typically provide only a "snap shot" of the
production origin at the time the log was run, and not a continuous
performance history.
- Applicability to vertical, deviated and horizontal
wells: Geochemical techniques are applicable not only to
vertical wells, but also to highly deviated and horizontal wells.
In contrast, production logging is problematic at high deviations,
and especially difficult at deviations greater than about 70
degrees.
- Applicability to pumping wells: Geochemical techniques
can be applied to all types of pumping wells (including those with
tubing-deployed electrical submersible pumps, and progressive
cavity pumps). In contrast, most pumping wells (except those with
unusual completion styles, such as Y-block completions) cannot
accommodate a production logging tool because the pumping apparatus
prevents access of the logging tool to the underlying perforated
interval.
- Ability to quantify uncertainty: Geochemical
techniques provide multiple, independent solutions to the
allocation problem, allowing one to quantify accurately the
uncertainty of an allocation result. In contrast, the uncertainty
associated with logging results is more difficult to quantify.
- No risk of sticking a logging tool: Because the
geochemical approach relies only on produced oil samples, there is
no risk of sticking a tool in the well.
Assessing the contribution of multiple fields to commingled
pipeline production streams
- Ability to allocate in the absence of flow meter data:
Geochemical techniques can allocate commingled production at points
in the production stream where flow meter data are
unavailable.
- Ability to identify problems with flow meter data:
Where flow meter data are available, geochemical data provide
complementary information for allocating production, because
geochemical techniques measure the relative contributions of oil
(instead of water + oil) to a production stream. Since geochemical
production allocation cannot be impacted by entrained water, the
geochemical techniques provide an independent check on allocation
data from flow meters.
Methods
Allocation of commingled oil: Methods for using oil
compositional differences to allocate commingled production from a
single well are detailed in Kaufman et al. (1987; 1990; 1997),
McCaffrey et al.(1996), and Nicolle and Boibien (1997). Similar
methods for allocating the contribution of multiple fields to
commingled pipeline production streams are discussed by Hwang et
al., (1999; 2000). These allocation techniques have been refined
further by OilTracers, LLC. In brief, production allocation is
achieved by identifying chemical differences between "end-member"
oils (samples of oil from each of the zones or production streams
being commingled). Parameters reflecting these compositional
differences are then measured in the end-member oils and in the
commingled oil. The data are then used to mathematically express
the composition of the commingled oil in terms of contributions
from the respective end-member oils.
Allocation of commingled gas: Schoell et al. (1993)
describe techniques for allocating gas production. Gas allocation
is conceptually similar to oil allocation; the techniques differ
primarily in the types of geochemical parameters measured, as
discussed on our GasChem.com
web page.
Background
The geochemical approach described here is based on the
well-established proposition that oils from separate reservoirs
tend to differ from one another in composition (e.g., Slentz, 1981;
Kaufman et al., 1990; Hwang and Baskin, 1994; Hwang et al., 1994).
Depending on the field, these compositional differences exist for
one or more of the following three reasons:
- Processes that affect oil composition after oil enters a
reservoir (e.g., processes such as biodegradation, water washing,
and evaporative fractionation) do not operate to exactly the same
extent in separate compartments.
- Oil which a source rock generates at a given time differs
slightly both from subsequently generated oil and previously
generated oil due to continuous, subtle changes in the maturity of
the source rock and changes in precisely which part of the source
rock is in the oil window. Since no two compartments are of
identical geometry, and since no two compartments have exactly the
same filling history, it is difficult to achieve precisely the same
homogenized composition in two separate compartments - even with
oil from the same source.
- More than one source rock may contribute oil to an
accumulation, and the oils from different sources differ in
composition. Since oils from different source rocks have different
times of generation and/or different migration paths, the presence
of more than one source may cause different compartments to fill
with different mixes of oil from the respective sources. For
example, Prudhoe Bay oil is known to be a mixture of petroleum from
three source rocks (Masterson et al., 1997 and 2001), and source
variations are therefore a significant cause of the compositional
differences in that field.
When oils from discrete zones are commingled, these chemical
differences between the oils can be used to assess the contribution
of each zone or each field to the commingled production.
Out-Dated (Obsolete) Method for Geochemically Allocating
Commingled Production from Two Zones:
As described in the figure below, using a simple mixing model, a
single geochemical difference between oils from two sands is
sufficient to allocate commingled production from those two units
(e.g., Kaufman et al., 1990):
Figure 1: Out-Dated (Obsolete) Geochemical
Allocation of Two-Zone Commingled Production
Figure 1: This figure shows a previously published, now
obsolete, method for quantitatively allocating 2 commingled zones.
Oil samples from each zone (the "end members") are mixed in the
laboratory. Analyses of these mixes and the pure end members allow
construction of mixing calibration curves, such as those shown
above. Each curve shows the values for one peak ratio (i.e., a
ratio of two compounds observed in the oil) in different mixes of
the oils. To assess the contribution of each zone to a commingled
oil, gas chromatography data for these peak ratios in the
commingled oil are simply plotted on the calibration curves (red
symbols in this figure). To allocate a sample derived from two
zones, only one calibration curve (one peak ratio) is needed.
However, by using data for several peak ratios, independent
solutions to the problem are derived, allowing the accuracy of the
allocation to be assessed. This method, while easy to understand,
actually has rather limited utility, because (1) it only can be
used to allocate contributions from two zones and (2) it requires
analysis of artificial mixes of the end member oils, since compound
ratios do not necessarily mix linearly. At OilTracers LLC, we
use a much more sophisticated method for geochemically allocating
commingled production: our method can "unmix" contributions
from an unlimited number of zones and can do this without making
artificial mixes of the end member oils
At OilTracers LLC, we allocate production from multiple zones
(two OR MORE zones) using OilUnmixer™ v 4.0, which is a proprietary
software package developed and owned by OilTracers LLC. Our
approach is completely different than that described in Figure 1
above. Instead of using compound ratios, our software is based on a
more sophisticated version of the linear algebra method first
published by McCaffrey et al (1996). That publication showed how
the commingled production from several sands (or several fields)
can be allocated to the discrete units using a linear algebra
manipulation of the concentrations (not ratios) of several
compounds in the end members and the commingled oils. Our software
package (OilUnmixer™ v 4.0) is more sophisticated than the
McCaffrey et al. (1996) approach in that our current software
package utilizes more advanced methods for:
- dealing with analytical uncertainty,
- assessing the validity of end member (zone specific)
calibration samples,
- finding and mathematically removing contamination in the end
members or commingled oils, and
- "testing" the validity of the allocation results (in a
graphical, easy to understand form).
This software package is available for licensing from OilTracers
LLC. Results of the
numerous blind tests of the OilUnmixer™ software are
available.
For more information on the technique described here, or to
discuss a specific project, e-mail us at info@oiltracers.com, or call
us at (214) 584-9169.
References
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