Fault block modelling — a material balance model for early production forecasting from strongly compartmentalised gas reservoirs

The fault block model (FBM) is an alternative simulation tool, developed with the purpose of forecasting well needs and reservoir performance in strongly faulted gas and gas–condensate reservoirs. This model is particularly designed to handle reservoirs where the uncertainty is related to gas communication between the various parts or compartment blocks of the field. The model can predict reservoir production behaviour with reference to the concept of a most likely reservoir, and to display the reservoir uncertainty with respect to business opportunities and risks (upside and downside forecast). Based on geological characterisation and fault seal analysis, reservoir compartmentalisation is performed. The model defines drainage volumes based on a statistical evaluation of the probability of communication between neighbouring compartment blocks. Uncertainty analysis is carried out using error propagation techniques, forming an optimistic and a pessimistic representation of the reservoir. Production is simulated by material balance calculation technique, producing the reservoir reserves through wells allocated in different drainage volumes. The FBM is intended to be an advantageous tool in early stage field development of gas and gas–condensate reservoirs, where the effect of well spacing, number of wells, and optimum well production sequence are important unknowns. Using an example from a North Sea gas–condensate reservoir, production profiles from different well location strategies are investigated and various tests involving uncertainty in inter-block communication and in block volume are presented. Uncertainty analysis also includes suggestions on how to reduce reservoir uncertainty and recommendation for an optimum well location strategy.