COMMUNICATION BETWEEN OVERPRESSURED COMPARTMENTS

Abstract

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.