Do Perched Water Contacts Exist?

Perched contacts do exist, but are extremely rare.

When a barrier separates two reservoir zones, capillary pressures vary across it, creating distinct water saturation (Sw) profiles or transition zones on each side. At the top of the barrier, this difference in Sw profiles results in a capillary imbalance, slowly moving water molecules from the higher to the lower Sw side. Over geological time, perched contacts in structural saddles will disappear.

Modelling suggests perched contacts can only exist in extremely high permeability reservoirs where the water saturation is reduced to a very low value. (Iulian N. Hulea SPWLA-2018-K).

The Free Water Level (FWL) is a horizontal surface with zero capillary pressure, while the Gas Water Contact (GWC) is a variable surface based on local porosity and permeability. Many fields in the Southern North Sea were originally misinterpreted as having perched contacts; in reality, they have a single FWL and multiple GWCs due to porosity-permeability variations.

When the base of the reservoir lies above the FWL, there may appear to be a perched contact. The water at a given height in a reservoir is determined by the balance between the capillary forces that hold the water up, and gravitational forces, which pull it down. This balance creates a Sw vs. Height Function (SWHF) for the field. High above the FWL, gravity acts vertically, but its effect reduces towards the base of the reservoir as the molecules are attracted in the direction of main reservoir. This shift, which follows the cosine of the angle, results in a SWHF that is less sharp but accurately describes how Sw changes with height. The Harding field is a good example of this. It is important to point out that all the water in this apparent perched contact is immobile.

NB. Borehole logging tools do not directly measure Sw. Rather, they measure the conductivity of the bulk volume of water (BVW). All electrical tools detect volumes, not percentages. Similarly, core analysis doesn’t measure Sw. Instead, it measures the BVW expelled from rock samples at different pressures. Consequently, SWH Functions based on BVW rather than Sw are more accurate for reservoir modelling.