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 volume estimation. Perched water transition zones are commonly misinterpreted as lithological trends, while 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 deepwater 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 movable perched water is confirmed via direct extraction from core samples. The associated saturation profile is evaluated using logs and special core analysis (SCAL) measurements. The chemical composition of extracted water samples is compared with that of 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 movable water located high above the field water contact and offer useful analogs for perched water detection in deepwater 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 a limited volumetric footprint and do not result in large quantities of produced water; however, they still need to be accounted for as potential risks in major capital projects.
