Summary
In traditional flow simulation, compaction is modeled as a function of fluid
pressure, whereas in reality, it is dependent on effective stress (e.g., mean
effective and shear stress). Therefore, although compaction computed by a flow
simulator may be correct on a regional average basis, the true variation
throughout the reservoir (both spatial and temporal) cannot be accounted for by
a traditional approach. A stress simulator (i.e., geomechanics model) honoring
material properties, rock mechanical boundary conditions, and
material-to-material interaction is needed to achieve this compaction.
Especially for sands, chalk, and other weak materials, which in general, have a
compaction-dependent permeability, the spatial variation of compaction may have
a significant impact on the flow pattern. The industry standard approach for
computing true compaction is by either doing a fully coupled simulation or by
using partial coupling with pore-volume iterations, both typically being
expensive in terms of computer processor time. For this reason, the simplified
compaction calculations are often used in practice thus disregarding actual
physics in the reservoir simulation.
In this paper, we describe a procedure whereby a modified (pseudo) material
definition is constructed and used to improve compaction calculations by the
flow simulator. The construction is based on results from a simplified, coupled
flow–stress simulation, typically consisting of three to six explicit stress
steps.
The resulting compaction field is comparable to the true one and represents
a significant improvement over the traditional approach. This compaction state
is the optimal input to the stress simulator in a coupled scheme and,
therefore, assures the rock mechanics calculations can be performed with
maximum efficiency. By using our suggested procedure, the pore-volume
iterations in a coupled scheme are eliminated or significantly reduced, and the
simulated reservoir state is accurate at all times--not only when stress
simulations are performed. Our main goal is to reduce the total computer time
in iterative-coupled simulations without loss of accuracy, especially focusing
on two mechanistic models from the Valhall field, which is a highly compacting
chalk reservoir in the North Sea. We also demonstrate benefits of using the
procedure in a simplified form to increase accuracy in reservoir simulation for
reservoirs in which coupled simulation is traditionally not seen as needed
because of either a perceived lack of complexity or the computing costs.
In this paper, we demonstrate that the developed construction methodology is
general in use. Further, the maximum permitted difference between
flow-simulator calculated compaction and true compaction (i.e., computed from
strain using a geomechanics simulator) is user-controlled, such that by proper
definition of this parameter, the coupled simulation in most cases can be
guaranteed to converge at the first pore-volume iteration.
© 2009. Society of Petroleum Engineers
View full textPDF
(
2,741 KB
)
History
- Original manuscript received:
28 August 2007
- Revised manuscript received:
29 July 2008
- Manuscript approved:
31 July 2008
- Published online:
15 April 2009
- Version of record:
15 April 2009
View Cart
