Fault uncertainty modelling

The existence, position, shape, size and displacement of faults play an important role in several aspects of reservoir modelling including flow simulations, volume calculations and well planning. To model the faults, seismic inputs with both interpretation and migration uncertainty and well observations with uncertainty in log interpretations are applied. Uncertainty in the interpretation of the horizons causes uncertainty in the fault displacement. All these different sources of uncertainty should be included in the fault modelling.
The relationship between faults and their size and shape defines the compartments of the reservoir and influences the simulated flow paths. The modelling of the fault displacement can have great impact on whether the fluids flow through the fault or the fault acts as a barrier in the model. The joint uncertainty in the position and size of boundary faults plays a role in the volume estimation, and this uncertainty should be accounted for also in well planning to avoid drilling outside the desired area or directly through a fault.
We will present a fault modelling tool where different realizations of the structural model related to the above mentioned uncertainties can be generated. This is done by using a flexible representation of the faults which lets us update the fault geometry, fault displacement and fault size both in deterministic and stochastic ways.

We will show how uncertainty envelopes around faults can be generated. These envelopes are based on seismic uncertainty and define the space for the fault to reside in. Both stochastic and deterministic change of position and shape of the fault can be performed within the envelope. The envelope and fault position can be conditioned to well data. The displacement and the length of the faults can be modified by deterministic or stochastic processes. The displacement is modelled by elliptic trends combined with input from interpreted seismic horizons. The deterministic modification implies either a scaling of the throw, leaving the length of the fault unchanged or increasing/decreasing the throw by a constant which means that a new fault tip line is estimated. In both the case of position change and displacement modification, the deterministic workflows are typically used to establish base and min/max scenarios while stochastic realizations are applied in a Monte-Carlo setting to calculate statistical properties.
Faults below seismic resolution can be modelled by a stochastic process that distributes smaller faults in the reservoir either through global trends or as secondary faults linked to seismic visible primary faults.

The modelling tool allows for setting up different workflows to analyse the sources of uncertainty one at the time or combined to give a total effect. Examples from a synthetic case based on a real reservoir will be given to show different aspects of the uncertainty modelling.