Case Study

VBM Algorithm Defines Complex Structures

Published: 05/19/2015

Case Study

VBM Algorithm Defines Complex Structures

Published: 05/19/2015

Woodside Energy Limited, the largest Australian-owned operator of oil and gas production, needed to accurately model and simulate field segments with a high degree of structural complexity. It required the ability to represent unusual structural features—especially for new frontiers with challenging reservoir styles—and incorporate stacked synthetic/antithetic fault truncations and crossing faults, in particular.

The company also found that corner-point gridding methods slowed down modeling when scarce input data was available, causing modeled horizons to display inconsistent geometry around complex fault compartments. Correcting this issue required manual editing of fault-horizon intersections, which proved time consuming.

The company turned to the VBM capabilities of the Petrel E&P software platform. The Petrel platform can handle complex geometries characterized by a wide range of structurally complex fault configurations and stratigraphic truncations in high detail, resulting in more representative reservoir models.

In capturing the complex field architecture, the VBM workflow also has the potential to highlight previously bypassed hydrocarbons, compartmentalized by the fault network, for infill drilling opportunities.

Volume-based modeling

The VBM algorithm creates horizons based on depositional sequence. Instead of considering horizons as discrete surfaces, this technique directly models volumes using a discretized and heterogeneous tetrahedral mesh encompassing the fault framework.

The result is a 3D discontinuous stratigraphic property from which a model horizon, that takes sequence boundaries into account, can be fully reconstructed. Three sets of input data with varying densities were used: full 3D interpretation, coarsely stepped inline/crossline interpretation, and horizon interpretation from wells. The VBM algorithm was instrumental in modeling all horizons sequentially and preventing horizons from crossing.

VBM Algorithm Defines Complex Structures

The VBM workflow, encompassing the fault framework model, through the isostratigraphic property, to the final zone model.

By initially modeling the reservoir as a volume, accurate fault relationships were maintained throughout the sequence—even with significant structural complexity from multiple stacked synthetic/antithetic faults, large scoop-shaped faults, faults with multiple terminations, and erosional stratigraphic truncations.

Zone models—and the output from the structural framework—were converted directly to simulation-ready structural grids, ready for property population. Because they are composed of stair-step faults, the structural grids have desirable fault geometries for simulation, minimizing occurrences of non-orthogonal cells that can cause issues during simulation. Woodside was able to control the grid resolution for horizontal and vertical directions, via layering settings, optimizing the number of cells for reservoir simulation.

Woodside structural model, showing high-resolution zonal division, erosional truncation, and significant faulting, creating thin fault block wedges and intricate compartmentalization.

The ability to directly model all existing faults, from interpretations alone, proved useful for history matching dynamic behavior. By incorporating greater detail in the fault network, it was possible to gain greater insight and understanding of fluid flow behavior within the reservoir, over time, especially between compartments.

Tetrahedral mesh zone model, stair-step fault network, and structural grid.

Improved field development planning

Using the tools within the Petrel platform, appropriate transmissibilites were assigned to faults in the grid model, yielding more representative modeling of fluid impedance around faults. The Petrel platform also enabled direct export to a dynamic simulator, allowing case results to be viewed and analyzed and uncertainty applied via Petrel Reservoir Engineering workflows.

Example of the complexity of the fault network with a large number of stacked faults truncating against a main scoop-shaped fault.

Woodside can now use this information in its field development planning and reservoir behavior prediction. It is able to create highly detailed reservoir models and produce accurate stratigraphic zonation more easily and efficiently.

The team can control very complex structural configurations—even when sparse or noisy data is used as input—as well as include data at varying levels of detail, due to the treatment of all horizons as a volume within a stratigraphically conformable sequence.

“The VBM capabilities in the Petrel platform have allowed a new level of intricacy to be modeled in complex and sparse data areas, while minimizing the level of seismic interpretation required, assisting our field development planning, production strategies, and economic predictions.”

Woodside