Summary
This paper presents a gridding study relating to reservoir simulation of a
giant, complex, low-permeability carbonate reservoir developed with 75 ultra
long horizontal wells in a densely spaced alternating injector/producer
pattern.
The lateral magnitude of the Al Shaheen field in Qatar and the radial layout
of the multiple ultra long horizontal wells in the field posed a challenge in
modeling of individual well performance using a manageable grid size with an
acceptable run time for history matching. Reservoir modeling was complicated
further by the complex reservoir characteristics with a tilting free-water
level (FWL), separate gas caps, large lateral variations in oil properties, and
wettability-dependent flow characteristics. These features had to be
incorporated into the initialization and dynamic modeling of the reservoir,
which added further to the memory requirements of the simulation model.
This paper describes the process of selecting a suitable simulation grid for
history matching the performance of this reservoir on a full-field basis.
Conventional Cartesian gridding techniques, including the use of local grid
refinements (LGRs) in areas of interest, were pursued initially but were shown
to be inadequate for full-field modeling of this complex reservoir. The
gridding problem was solved by the use of 2.5D perpendicular-bisector (PEBI)
grids around each of the horizontal wells in the field. This allowed for
sufficient resolution between wells and also aligned the grid with the well
paths, thereby avoiding grid nonorthogonality issues.
The efficiency of the PEBI model was also demonstrated by the comparison of
CPU performances. Run times for the full-field PEBI model were equivalent to
that of a conventional Cartesian model with suitable local grids covering only
20% of the wells. Both models had approximately 700,000 active cells and
required 3–4 GB of memory. A full-field model relying on conventional LGRs
around all wells was not built because it would involve significantly more grid
cells and, therefore, would become considerably slower and require more
memory.
Introduction
The Kharaib B reservoir is a thin, widespread, low-permeability reservoir
with an aspect ratio of approximately 1:1,000. The reservoir is a major
carbonate reservoir in the Al Shaheen field (Thomasen et al. 2005) in Block 5
offshore Qatar, covering an area of approximately 514,000 acres (2,080 km2).
The Kharaib formation forms part of the Lower Cretaceous Thamama group that is
widely present in the southern Arabian Gulf.
At the time of this study, the field was developed with 75 very long
horizontal wells in an irregular well pattern designed for waterflooding, which
relies on close well spacings because of the low permeability of the
carbonate.
Continuing evaluation of reservoir performance and assessment of further
development potential of the field prompted the need for detailed history
matching on a full-field basis. The modeling issues discussed in this paper
were part of a history-matching study completed in 2004 that formed the basis
to develop the reservoir further with 89 additional wells to infill existing
well patterns and to develop the thinner oil column toward the flank of the
reservoir, which is currently ongoing.
The complex nonequilibrium conditions prevailing in the low-permeability
reservoir are first described, together with the wettability-dependent flow
characteristics. The changes in fluid properties, for example, along the long
horizontal wells are substantial in some instances. The described complexities
also add to the memory requirements of a simulation model. The practical
implementation in the reservoir-simulation model is described in Appendices A,
B, and C.
The paper then illustrates how conventional Cartesian gridding techniques
were first used to model the reservoir, giving examples of the inadequacies
associated with this gridding technique. Further refinement of the grid using
LGR around wells to compensate for the lack of grid resolution between the
densely spaced wells was also attempted, but the required model size became
impractically large. It is then shown how PEBI gridding along the long
horizontal wells is fit for modeling this reservoir. It facilitates accurate
representation of interwell distances and provides sufficient resolution
between water injectors and producers, while ensuring realistic modeling of
water movement away from a water-injection well.
Finally, the comparison of CPU performances for conventional- and PEBI-grid
models supports the selection of the PEBI technique as an efficient solution
for the full-field gridding requirements for the giant Kharaib B field.
The subsequent history-matching process was facilitated dramatically by use
of PEBI grids, and significant time savings were realized.
© 2008. Society of Petroleum Engineers
View full textPDF
(
1 KB
)
History
- Original manuscript received:
31 July 2007
- Meeting paper published:
4 December 2007
- Revised manuscript received:
14 April 2008
- Manuscript approved:
20 April 2008
- Version of record:
29 December 2008
View Cart
