Summary
Understanding reservoir behavior is the key to reservoir management. This
study shows how energy modeling with rapid material-balance techniques,
followed by numerical simulations with streamlines and finite-difference
methods, aided understanding of reservoir-flow behavior. South Rumaila’s long
and elongated Zubair reservoir experiences uneven aquifer support from the
western and eastern flanks. This uneven pressure support prompted injection in
the weaker eastern flank to boost reservoir energy.
We learned that aquifer influx provided nearly 95% of the reservoir’s energy
in its 50-year producing life, with water injection contributing less than 5%
of the total energy supply. The west-to-east aquifer energy support is
approximately 29:1, indicating the dominance of aquifer support in the west.
Streamline simulations with a 663,000-cell model corroborated many of the
findings learned during the material-balance phase of this study. Cursory
adjustments to aquifer properties led to acceptable match with pulse-neutron
capture or PNC-derived-time-lapse oil/water contact (OWC) surfaces. This
global-matching approach speeded up the history-matching exercise in that
performance of most wells was reproduced, without resorting to local
adjustments of the cell properties. The history-matched model showed that the
top layers contained the attic oil owing to lack of perforations.
Lessons learned from this study include the idea that the material-balance
work should precede any numerical flow-simulation study because it provides
invaluable insights into reservoir-drive mechanisms and integrity of various
input data, besides giving a rapid assessment of the reservoir’s flow behavior.
Credible material-balance work leaves very little room for adjustment of
original hydrocarbons in place, which constitutes an excellent starting point
for numerical models.
Introduction
Before the advent of widespread use of computers and numeric simulators,
material-balance (MB) studies were the norm for reservoir management. In this
context, Stewart et al. (1954), Irby et al. (1962), and McEwen (1962) presented
useful studies. Most popular MB methods include those of Havlena and Odeh
(1963), Campbell and Campbell (1978), and Tehrani (1985), among others.
Pletcher (2002) provides a comprehensive review of the available MB
techniques.
In the modern era, classical MB studies seldom precede a full-field numeric
modeling, presumably because MB is implicit in this approach. Nonetheless, we
think valuable lessons can be learned from analytic MB studies at a fraction of
time needed for detailed numeric modeling, preceded by geologic modeling. Of
course, the value and amount of information derived from a multicell numeric
model cannot be compared to a single-cell MB model. But, an analytic MB study
can be an excellent precursor to any detailed 3D modeling effort. Although this
point has been made by others (Dake 1994; Pletcher 2002), practice has,
however, lagged conventional wisdom.
In this paper, we attempt to show the value of a zero-dimensional MB study
prior to doing detailed 3D numeric modeling, using both streamline and
finite-difference methods. Streamline simulations speeded up the
history-matching effort by a factor of three. However, we used the
finite-difference approach in prediction runs for its greater flexibility in
invoking various producing rules.
Initially, the MB study provided key learnings about gross reservoir
behavior very rapidly. In particular, energy contributions made by different
drive mechanisms, such as uneven natural water influx and water injection, were
of great interest for ongoing reservoir-management activities. Estimating
in-place hydrocarbon volume and relative strength of the aquifer in the western
and eastern flanks constituted key objectives of this study segment.
Following the MB segment of the study, we pursued full-field match of
historical data (pressure and OWC) with a streamline flow simulator to take
advantage of rapid turnaround time. Thereafter, prediction runs were made with
the finite-difference model to answer the ongoing water-injection question in
the eastern flank of the reservoir. We learned that water injection should be
turned off for improved sweep, leading to increased ultimate oil recovery. In
addition, the numeric models identified the presence of remaining oil in the
attic for future exploitation.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
10 November 2006
- Meeting paper published:
14 March 2007
- Revised manuscript received:
25 October 2007
- Manuscript approved:
31 December 2007
- Version of record:
20 August 2008
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