Øystein Pettersen

The impact of fault envelope structure on fluid flow: A screening study using fault facies

Structural elements of deformation-band fault zones are implemented as volumetrically expressed building blocks, that is, fault facies, in a series of synthetic reservoir geomodels and simulation models. The models are designed and built to reproduce a predefined range of fault system configuration, sedimentary facies configuration, and fault zone architecture. Using petrophysical properties derived from published field studies, the geomodel realizations are run in a reservoir simulator to monitor reservoir responses to variations in modeling factors. The modeled fault zones act as dual barrier-conduit systems, resulting in simulation models that can capture contrasting waterfront velocities, changes in waterfront geometries, and flow channelizing and bifurcation in the fault envelopes. The simulation models also show the development and sweep efficiency of bypassed oil and poorly swept regions because of the presence of the fault zones. Statistical analysis reveals that the fault facies modeling factors can be ranked according to impact on reservoir responses in the following descending order: fault core thickness, the type of displacement function, sedimentary facies configuration, the fraction of total fault throw accommodated by fault core and damage zones, fault system configuration, and maximum damage zone width. Fault core thickness is the most important factor because it governs the space available for fluid flow in the fault-dip direction. Other modeling factors affect the reservoir responses by controlling the geometry and continuity of fluid flow paths in the modeled fault zones.

CoLeMo: A collaborative learning environment for UML modelling

This paper presents the design, implementation, and evaluation of a distributed collaborative UML modelling environment, CoLeMo. CoLeMo is designed for students studying UML modelling. It can also be used as a platform for collaborative design of software. We conducted formative evaluations and a summative evaluation to improve the environment and discover how users collaborate through the environment. We received positive feedback from the evaluations. The findings indicate that this environment is both informative and helpful, especially for less experienced UML modelling users. We also found that less experienced users can learn from their more experienced collaborators.

Sandstone compaction, grain packing and Critical State Theory

Based on the physics of grain packing in a granular material, this paper demonstrates that sands or sandstones are modelled most correctly by Critical State Theory, which can be used to define a consistent compaction relationship for use in rock mechanics or reservoir simulation. The theoretical model is compared with experimental data for volume and permeability variation during loading or unloading.

Using relations between stress and fluid pressure for improved compaction modelling in flow simulation and increased efficiency in coupled rock mechanics simulation

ABSTRACT The conventional compaction model used in reservoir simulators defines compaction as a function of fluid pressure, whereas, in reality, it is a function of effective stress. The interrelationship between fluid pressure, effective stress and reservoir parameters (materials distribution, geometry, production scheme) is investigated. By modifying the conventional concept of flow simulator compaction a predictor is constructed for the rock mechanics computations in a coupled flow–rock mechanics simulation. This predictor reduces the time to converge the stress computations by reducing or eliminating the number of pore volume iterations in the coupling scheme. Overall computing time is thereby reduced considerably, while maintaining accuracy in the stress computations. Additionally, the compaction state in the flow simulator will be more accurate than in a conventional iterative coupling scheme.

Horizontal simulation grids as alternative to structure-based grids for thin oil-zone problems: a comparison study on a Troll segment

The layering in reservoir simulation grids is often based on the geology, e.g., structure tops. In this paper we investigate the alternative of using horizontal layers, where the link to the geology model is by the representation of the petrophysics alone. The obvious drawback is the failure to honor the structure in the grid geometry. On the other hand, a horizontal grid will honor the initial fluid contacts perfectly, and horizontal wells can also be accurately represented. Both these issues are vital in thin oil-zone problems, where horizontal grids may hence be a viable alternative. To investigate this question, a number of equivalent simulation models were built for a segment of the Troll Field, both geology-based and horizontal, and various combinations of these. In the paper, it is demonstrated that the horizontal grid was able to capture the essentials of fluid flow with the same degree of accuracy as the geology-based grid, and near-well flow was considerably more accurate. For grids of comparable resolution, more reliable results were obtained by a horizontal grid than a geo-grid. A geo-grid with local grid refinement and a horizontal grid produced almost identical results, but the ratio of computing times was almost 20 in favor of the horizontal grid. In the one-phase regions of the reservoir, relatively coarse cells can be used without significant loss of accuracy.

Horizontal Simulation Grids as Alternative to Structure-based Grids for Thin Oil-zone Problems

As a general rule, the layering in reservoir simulation grids is based on the geology, e.g. structure tops. In this paper we investigate the alternative of using horizontal layers, where the link to the geology model is by the representation of the petrophysics alone. The obvious drawback is the failure to honor the structure in the grid geometry. On the other hand a horizontal grid will honor the initial fluid contacts perfectly, and horizontal wells can also be accurately represented. Both these issues are vital in thin oil-zone problems, where horizontal grids may hence be a viable alternative. To investigate this question, a number of equivalent simulation models were built for a segment of the Troll Field, both geology-based and horizontal, and various combinations of these. In the paper it is demonstrated that the horizontal grid is able to capture the essentials of fluid flow with the same degree of accuracy as the geology-based grid, and near-well flow is considerably more accurate. For grids of comparable resolution, more reliable results were obtained by a horizontal grid than a geo-grid. A geo-grid with local grid refinement and a horizontal grid produced almost identical results, but the ratio of computing times was more than 20 in favor of the horizontal grid. In the one-phase regions of the reservoir, relatively coarse cells can be used without significant loss of accuracy.