Summary
Traditionally, the evaluation of CO2-flooding processes is performed with
finite-difference compositional-simulation models. However, compositional
simulation is impractical for modeling large-scale CO2 floods because of
computational run-time restrictions. In cases in which reservoir heterogeneity
and fluid mobility dominate the reservoir recovery mechanism, streamline
simulation offers a viable alternative to compositional simulation. The
“reduced” physics in streamline simulation allows field-scale CO2-flood
modeling to be feasible, as long as the streamline pressure/volume/ temperature
(PVT) model can be calibrated so that the streamline model will produce
accurate results for CO2-injection processes. Using streamline simulation
allows for the evaluation of multiple full-field development scenarios that
otherwise would not be possible with compositional simulation.
The objective of the study was to provide CO2-flood performance forecasts
under various full-field development scenarios for the Midale field. This paper
focuses on the methodology and results from the 1,000-well,
>400,000-gridblock, 45+-year streamline simulation of the Midale field. In
particular, it discusses the construction and history match of the full-field
model, the calibration of the streamline model with the compositional model,
and the development of the full-field CO2 forecasts.
Introduction
The Midale field, in southeast Saskatchewan, Canada (Fig. 1), was discovered
in 1953 and subsequently delineated on 80-acre spacing. The field produced
under competitive drainage until unitization in late 1962, after which an
inverted nine-spot waterflood scheme was implemented. During the mid-1980s, an
extensive vertical infill program was used to modify the waterflood patterns.
Horizontal wells in the late 1980s and multilegged perpendicular horizontals in
the mid-1990s were used to further improve waterflood conformance. To date, the
unit has recovered more than 125 million STB of oil (primarily from waterflood
operations), representing approximately 24% original oil in place (OOIP).
Recognizing the large volume of oil that would not be recovered by
waterflooding operations, a CO2-flood pilot project was initiated in 1984. This
project involved the drilling of 10 closely spaced wells in an area 4.4 acres
in size and generated an enormous amount of reservoir and geological
information. Results from the CO2 pilot project were used to justify the
larger-scale Midale CO2-flood demonstration project, a six-pattern CO2 flood
located in the southwestern part of the unit that began operations in 1992.
Positive results from the Midale CO2 demonstration project were instrumental in
justifying the neighboring Weyburn CO2-flood project, which began operations in
2000, and they were also key to the technical justification of a full-field
Midale CO2 flood.
Apache’s acquisition of the field in 2000 was followed by an aggressive
campaign to increase the recovery through infill drilling with horizontal wells
at 20- to 40-acre spacing, increased injection and throughput (by a factor of
three), and a review of the feasibility of a full-field CO2 flood.
The objective of this study was to assess the commercial CO2-flood potential
of the Midale unit within a 5-month study period. Traditionally, a
compositional simulator is used to accurately model the pressure-dependent
phase behavior of CO2. However, despite advances in computing power and
software, compositional simulation is impractical for field-level simulations
of large fields such as the Midale unit. An alternative to compositional
modeling is streamline simulation. Recent advances in streamline simulation
show that in cases in which reservoir heterogeneity and the
production/injection coupling dominate, first-order approximations offered by
streamline simulation are sufficient for full-field development decisions.
Invariably, the development plan is modified as the field is depleted and more
information becomes available. Also, full-field streamline simulation allows
for optimization of water-alternating-gas (WAG) cycles and pattern-injection
timing that would be difficult to evaluate with compositional simulation. The
difficulty with streamline simulation is that it lacks the direct PVT model to
accurately describe the interaction between the oil and the CO2 at various
pressures and temperatures. Detailed compositional models are required when
drastic changes in fluid properties occur, such as near the critical point or
condensate dropout in retrograde gas reservoirs. When the problem is one of
modeling a relatively smooth transition between miscibility and immiscibility
at a certain pressure, the modification of a black-oil model by Todd and
Longstaff is quite often used because of its significantly faster computational
speed.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
22 June 2004
- Revised manuscript received:
11 July 2005
- Manuscript approved:
8 August 2005
- Version of record:
15 October 2005
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