ABSTRACT

Perched water is occasionally encountered above the main gas- or oil-water contact in stratigraphically and/or structurally complex fields. It is a consequence of localized water entrapment associated with relatively small-scale structural or stratigraphic traps during the migration process. Observations of perched water intervals and their associated transition zones in exploration, appraisal, or production wells can challenge subsurface characterization workflows and often lead to inaccuracies in in-place volumes estimation. Perched water transitions zones are commonly misinterpreted as lithological trends, whilst local perched contacts found at the bases of sands could be interpreted as segment or field-wide contacts. It is therefore important to recognize the presence of perched water and adequately characterize its impact on reservoir volumetrics and production volumes.

We present examples of perched water intervals in two Miocene deep-water fields. In the first example, a clear transition zone associated with perched water is observed in a fully cored gas-bearing sand located several hundred feet above field gas-water contact. The presence of moveable perched water is confirmed via direct extraction from core samples. The associated saturation profile is evaluated using logs and special core analysis measurements. The chemical composition of extracted water samples is compared with the aquifer waters. In the second example, we present a case of perched water in a reservoir located hundreds of feet above the regional oil-water contact. Perched water was detected by comparing resistivity- and capillary pressure-derived saturation profiles and confirmed with water sample analysis and production data.

The field cases provide examples of moveable water located high above the field water contact and offer useful analogues for perched water detection in deep-water sandstones. Perched water transition zone saturations are shown to follow normal drainage capillary pressure profiles and their chemistry differs from the aquifer waters. Perched water pools appear to have limited volumetric footprint and do not result in large quantities of produced water; however they still need to be accounted for as potential risk in deep-water projects.

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