Improved Compaction Modeling in Reservoir Simulation and Coupled Rock Mechanics/Flow Simulation, With Examples From the Valhall Field
Authors: Pettersen Øystein , Kristiansen Tron Golder 
Published: April 2009

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.