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Abstract This paper presents the benefits of surveillance efforts adopted for monitoring a waterflood in a geologically complex reservoir that contains high-viscosity (90 cp) oil. The reservoir is part of a large brownfield in South Oman and has been on production for more than 25 years. The development of the reservoir has been carried out in phases and full-field water injection was started after approximately 18 years of primary depletion. The water-flood project is expected to increase recovery factor by about 10% over primary depletion. The current phase of development is focused more on well and reservoir management (WRM) to optimize production from the reservoir over a short-to-medium term without detrimentally affecting subsequent implementation of a polymer flood. The implementation of a surveillance program in adverse mobility scenario in presence of faults and fractures and the constraints of existing well completion called for innovation and adaptation of the conventional surveillance techniques to existing downhole environment. The paper presents field examples to illustrate benefits of tailor-made surveillance execution to optimize a high viscosity oil waterflood project. Introduction A methodical surveillance and monitoring process is critical to success of any water-flood project. In case of high viscosity oil water-floods, surveillance techniques used in normal water-flood reservoirs need to be adapted to yield meaningful information. Surveillance work in the discussed field posed a formidable challenge because of well completion that is not amenable to surveillance-data acquisition and subsequent remediation. All wells are completed with pumps (beam-pump, progressive cavity pumps or electrical submersible pump) and have some form of sand-control device (external/internal gravel pack or wire-wrapped screen) restricting direct logging access to specific production/injection zones. Routine surveillance logging such as production logs, cased hole resistivity, Pulsed Neutron logging for saturation etc. could not be run in these wells and even if managed the data obtained provided challenges to interpret results due to complexities arising from completion configuration and flow behind screen. This situation also restricted many conventional interventions like mechanical water shut-off, re-perforation or addition of perforations etc. The above mentioned completion types were in turn dictated by the objective of maximizing well productivity; given the formation properties, crude oil characteristics and reservoir pressures. Many options to overcome these restrictions were considered and a cost effective means implemented was to recomplete wells with dual strings with one string carrying lift pump and other used as observation string. This observation string enabled running of all necessary logs while well is producing. The observation string also enabled routine well intervention without the need for a workover unit and associated cost. As next improvement, all new wells are now completed with wire-wrapped screens using external swellable packers to achieve producing reservoir segmentation and intervention capability. |
