Advanced Upscaling For Kashagan Reservoir Modelling

Abstract

The Kashagan field is a huge carbonate formation located 4.5 km below the North Capian sea bottom. The reservoir is saturated by over pressured 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, matrixlike inner platform and a highly fractured/karstified rim. The reservoir geology is modelled by means of massive geological grids consisting of tens of millions of cells, with vertical spacing of one meter or even less to account for high order depositional cycles. Fine scale geological grid cannot be used to run compositional simulations and much coarser grids, where hundreds of geological layers are lumped in few tens of dynamic layers, are used by reservoir engineers. To minimize errors due to the coarse scale, an average lateral spacing of 250 m × 250 m is used for both simulation and geological grid, but 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 methodology where the outcome of the upscaling are effective transmissibility (T*) instead than k*. T* upscaling has been proposed in the past as an alternative to k* upscaling, but it is neither part of commercial work-flows 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 reference. We considered single porosity simulations with geological grid populated by only matrix (first medium) and karst+fracture (second medium) properties and dual porosity/dual permeability simulations encompassing both media. Contrary to k* upscaling, T* based coarse simulations perfectly replicate fine scale field and wells injection/production potentials. Using T* upscaling as cornerstone for Company activities on Kashagan, we can run coarse scale full field simulations in 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 computationally expensive simulation grids together with the implementation of ad-hoc multi-phase upscaling.