Pseudo-Steady State Flow

Pseudo-Steady State Flow

• Pseudo-steady state (PSS) flow occurs during the late time region when the outer boundaries of the reservoir are all no flow boundaries.

• During the PSS flow regime, the reservoir behaves as a tank. The pressure throughout the reservoir decreases at the same, constant rate. PSS flow does not occur during build-up or falloff tests.

Flow in a reservoir is often characterized as being one of two types: transient or boundary-dominated.

Transient flow takes place during the early life of a well, when the reservoir boundaries have not been felt, and the reservoir is said to be infinite-acting.

• During this period, the size of the reservoir has no effect on the well performance, and reservoir size cannot be determined except to deduce minimum contacted volume.

• Since the boundary of the reservoir has not been contacted during the transient flow period, static pressure at the boundary remains constant.

• Pressure transient analysis/well testing theory relies heavily on the assumption that the well flows at a constant rate. Several terms are often used when describing flow from a well at constant rate:

• Transient Flow – Pressure transient migrates outward from the well without encountering any boundaries.

• Steady State Flow – Pressure transient has reached all of the boundaries but the static pressure at the boundary does not decline. This is often called “constant pressure boundary”.

• Pseudo-Steady State Flow – Pressure transient has reached all of the boundaries and the static pressure is declining at the boundary and uniformly throughout the reservoir.

• Boundary-Dominated Flow – Pressure transient has reached all of the boundaries and the static pressure is declining at the boundary, but not uniformly because the flow rate is not constant. This is also often called “tank-type flow”.

Boundary-Dominated Flow