Evergreen workflows that capture uncertainty – the benefits of an unlocked structure

Uncertainty in the reservoir’s geological structure is the reservoir modelling profession’s version of the ‘elephant in the room’. Its significance is well known to most practitioners, but often overlooked due to time constraints. Structural uncertainty is, however, one of the most important factors when determining both in-place and recoverable hydrocarbons, and it tends to remain substantial throughout the E&P lifecycle (Figure 1). One of the key challenges is to avoid locking the structural model early on in the reservoir modelling lifecycle. This is unfortunately a natural consequence if the available tools are not designed for revisiting these early assumptions. The locking of geological structure can be attributed to two key factors: 1) The fact that the logical process of build¬ing a reservoir model in a sequential workflow chain starts with defining the structure, and 2) the limitations in common reservoir simulation practices, where changing the geometry of the simulation grid is comparable to starting from scratch. In many applications of best practice, the revisiting of assumptions made early in the modelling process, such as the choice of a velocity model, is usually not prioritised because the traditional model update is manually-based, resource demanding, and takes too much calendar time. The same arguments apply for adding uncertainty to the structure at a later decision gate. Hence, after the structural modelling job is finished, exploring uncertainties in static and dynamic volumes tend to be limited to parameters that do not affect the grid layout. Changing faults and horizons are not among those. The higher risk of geological inconsistencies emerging in the simulation model(s) is a consequence of this sequential approach, where flow simulation is seen as ‘the last step’ and where there is a lack of iteration back to the starting point.