Communication between overpressured compartments

Overpressure build‐up in compartments, and communication between overpressured compartments across faults are studied with simple analytical and numerical models. It is shown that the excess pressure in a (vertical) one‐dimensional, one‐compartment model can be written as the sum of the excess pressure generated in the seal above the compartment, and a second part, which is due to the expulsion of fluid from the compartment and the rocks below. The one‐compartment model is generalized to a two‐compartment model, which accounts for the fluid communication between the compartments through a fault zone. The volume rates of flow through the seals and the fault zone are shown to be the weighted mean of the volume rates of the one‐dimensional, one‐compartment model. The normalized weights are given by dimensionless numbers, called fault–seal numbers, which control the communication between the compartments. A fault–seal number much less than unity implies that the fault is a stronger barrier for the fluid flow than the seal. A fault–seal number larger than unity implies the opposite: that the seal is a stronger barrier than the fault. The conditions for isolated compartments and other regimes are identified in terms of the fault–seal numbers. It is discussed how the compartment fault–seal numbers can be computed when the permeability is given in the fault zone. The results given by the analytical compartment models are demonstrated and validated with two‐dimensional numerical (finite element) simulations.