Reservoir-Modeling Work Flow Introduces Structural Features Without Regridding
Authors: Wilson Adam 
Published: October 2018

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 188575, “A New Shortcut Work Flow in Flexible Reservoir Modeling: Introducing Structural Features Without Regridding,” by Alejandro Rodríguez Martínez and Stefano Frambati, Total, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed.

Current reservoir-modeling work flows are rigid, because modification to the understanding of the underlying structural model often requires a complete regeneration of the reservoir grid, which brings additional costs, delays, and incompatibilities with past calculations. This paper proposes a novel work flow for structural-features modeling that allows the introduction of faults and other structural and nonstructural features to any simulation grid without modification.

Introduction

The modeling of hydrocarbon reservoirs is a multiscale and multidisciplinary process that usually involves several months for advancement from seismic interpretation to reservoir simulation. One crucial moment in the life of a reservoir model is the creation of its reservoir-simulation grid. This involves the encoding of the so-called structural model (Fig. 1), a set of surfaces representing interfaces between different geological features. A given grid cell can belong only to one side of each of these surfaces, generating geometrical tension on the grid. Once the reservoir simulation grid is created, every aspect of the simulation depends on it.

Several weeks or even months are spent building and quality checking the reservoir-simulation model, which is completely based on the cell division of the space determined by the reservoir grid. Simulations are then run and compared with actual data, when it is available, or used for conceptual design of production methods and structures. Other times, the model already exists and new production data are acquired. These simulations and comparisons sometimes re-veal fundamental flaws in the simulation model, such as the existence of an unseen or underestimated fault that was not included or the need to model differently the flow across a fault because a fault relay was ignored. In such cases, the engineer can decide either to ignore the issue, leaving an inaccurate model in place for the rest of the field’s life but retaining compatibility with previous simulations, or to modify the reservoir properties manually to mimic the missing feature. These manual modifications, however, are extremely costly, and the result is often nongeological and possibly inconsistent with future evolutions of the model.

A final option is for the seismic-interpretation team to add the missing fault to the structural model. This may require many steps, ultimately changing the geometry and number of cells. Expected timelines will be delayed, the budget will grow, and some development decisions will have to wait for the new simulations. Because of this, in future projects, the engineer will take great care to ensure that every trace of a fault is included in the first version of the structural model. That way, transmissibility multipliers can always be set to unity and the faults can be removed de facto from the actual simulation if need be. This strategy, however, creates large, unwieldy models, often with more than 200 faults.