Water-Encroachment Monitoring Applied to Offshore Oil Field

Abstract

A methodology for tracing the advancement of water encroachment from an aquifer by extensive use of behind-casing saturation evaluation logs is introduced. This approach relies on a large number of behind-casing saturation evaluation logs with a large coverage in space and time. A visualization of water saturation, populated through a smooth interpolator in a 3D grid at prescribed timestep, is obtained. Water movement can be visualized in cross sections and maps per reservoir layer. These results aid in the understanding of the path of water invasion and can be used for history-matching a reservoir simulation model at a later stage.Data required are behind-casing saturation logs (sigma) as well as interpreted elemental volumes from openhole logs, production logs, production history, and perforated intervals. The time-lapsed saturation is cross-checked against waterbreakthrough evidence, such as water-production history and production-log runs. The perforated intervals aid in the verification of the depths of entry of water. Unreliable evaluations are screened out. Time-lapsed saturation is interpolated in time for standard timesteps on a well-by-well basis. A form of production history transformed into a form of time-lapsed well log format is used to supplement the water saturation. The saturation logs are filtered for nonreservoir rock and then used to populate the 3D grid. This methodology was used in evaluating water advancement in a reservoir in the Middle East. Petrophysical evaluations were performed to review the openhole logs for consistency and determine the time-lapsed water saturation. Using the water population in the 3D grid, the advancement of the aquifer was evaluated. A series of well fences and average saturation maps per reservoir subzones were created to show the water advancement evolution through time in high resolution. Maps of reservoir potential by subzone were created, and areas of interest for infill drilling were identified.The methodology proved capable of revealing the water paths in the reservoir where the density of data is large. Upward movement of bottom-aquifer rise, water invaded upper intervals, can be visualized. The results, where applicable, provide a cross-check and aid history matching of reservoir simulation models.