Abstract
This paper discusses the successful, fully integrated petrophysical and geological modeling of the Hollin formation in the Sacha field of Ecuador. The goal of the modeling project was to unravel aquifer heterogeneities and hydrodynamic effects; it focuses on suitable petrophysical rock typing, supporting the establishment of a tilted water/oil contact (WOC) that follows a particular architecture across the field.
Documenting WOC changes is extremely important during reservoir modeling. Changes in fluid levels can be related to, among other things, structural complexities, rock heterogeneities, and hydraulic movement effects. However, when dealing with a mature field characterization that has undergone an accelerated production, flow capacity differences and remaining reserves become critical points to delineate aquifer heterogeneities. The identification of aquifer heterogeneities and possible hydrodynamic flow were considered separately to address this problem, using available core data, a log-derived water saturation (Sw) model, and dynamic performance integrated with a rock typing approach.
Such factors were considered when studying the Cretaceous Lower Hollin formation in the Sacha field. The Sacha field is a mature field in Oriente basin, Ecuador, and the reservoir is characterized by a sequence of amalgamated sandstones deposited primarily by a braided stream system. A strong aquifer underlies this sequence of amalgamated sandstones and maintains a stable pressure. As a result, various WOCs are displayed at different levels in the wells with a variation of up to 70 ft across the field. Aquifer heterogeneities were identified as facies changes, faults, and capillarity effects that perhaps triggered these WOC irregularities. Although these anomalies do not prevent vertical fluid communication within the reservoir, they represent an important step for proper electrical rock property interpretation; thus, providing a more realistic reservoir Sw model. The construction of rock-fluid surfaces helps to identify the aquifer heterogeneities related to steady state WOC depths. The textural features of the rock in conjunction with an identified water recharge point created an WOC surface that vary the inclination rates from 2 ft/km northward, 13 ft/km eastward, and southward across the field, creating a predictable WOC architecture that affected water production.
This improved understanding of key petrophysical aspects and methods arising from work undertaken to solve various problems related to the tilted WOC have revealed new opportunities to the field edges. These opportunities have been tested with newly drilled wells with excellent results. The results obtained from these tests are having a significant effect on future drilling programs.
