Summary
The Kashagan field is a huge carbonate formation located 4.5 km below the
bottom of the North Caspian sea. The reservoir is saturated by overpressured
light oil, and the development is based on first-contact-miscible gas
injection.
The reservoir is highly stratified, with a fine sequence of depositional
cycles and long-range lateral correlations. Three porosity systems (matrix,
karst, and fractures) can be organized in two main environments: a massive,
low-permeability, matrix-like inner platform and a highly fractured/karstified
rim.
The reservoir geology is modeled by means of detailed geological grids
consisting of tens of millions of cells, with vertical spacing of 1 m or even
less to account for high-order depositional cycles. Geological grid cannot be
used to run compositional simulations, and much-coarser grids, in which
hundreds of geological layers are lumped in few tens of dynamic layers, are
used by reservoir engineers. To minimize errors because of the coarse scale, an
average lateral spacing of 250x250 m is used for both simulation and geological
grid; nonetheless, upscaling remains a challenge. Traditional permeability
(k*) upscaling methods, including flow-based methods, overestimate
Kashagan field/wells production and injection potentials.
We implemented a method in which the outcome of the upscaling are effective
transmissibility (T*) instead of k*. T* upscaling has been
proposed in the past as an alternative to k* upscaling, but it is
neither part of commercial workflows nor widely accepted in the
reservoir-modeling community. In our T* upscaling, the solution of local
flow problems around coarse-cell interfaces is used to compute coarse
transmissibility. T* and k* upscaling were compared by simulating
both single-phase and gas-injection problems, including platform and rim, using
the results of fine-scale simulation as a reference. We considered (1)
single-porosity simulations with geological grid populated by only matrix
(first medium) and karst+fracture (second medium) properties and (2)
dual-porosity/dual-permeability simulations encompassing both media. Contrary
to k* upscaling, T*-based coarse simulations perfectly replicate
fine-scale field and well injection/production potentials.
Using T* upscaling as a cornerstone for company activities on
Kashagan, we can run coarse-scale full-field simulations in a few hours without
loss of consistency with the results provided by weeks-long, often unpractical,
fine-scale simulations. On the contrary, the inaccuracy of k* upscaling
would have required much finer and more computationally-expensive simulation
grids together with the implementation of ad hoc multiphase upscaling.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
30 June 2011
- Meeting paper published:
31 October 2011
- Revised manuscript received:
21 December 2011
- Manuscript approved:
16 January 2012
- Published online:
29 March 2012
- Version of record:
3 April 2012
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