Implementing Selective Water Injection Strategy in Thin Reservoirs of a Mature Field in Ecuador

Abstract

Abstract Cononaco is a mature oilfield located south of Block 61 in the Oriente Basin, Ecuador. The reservoir, Napo U, comprises two thin layers: U Superior and U Inferior. The objective of this study was to enhance recovery in U Superior by revitalizing the waterflooding patterns using four injector wells, while simultaneously pressurizing U Inferior. To accomplish this, a decision was made to employ a selective water injector well for both U Superior and U Inferior, utilizing a peripheral injection strategy to improve the displacement of trapped oil accumulations. Consequently, the chosen candidate well for this purpose was CNOF-064WIW, which satisfied the requirements and possessed the necessary resources. These requirements included minimal investment in existing facilities, good sand connectivity, and optimal rock properties for both reservoirs, as well as a peripheral location and favorable mechanical conditions to convert it into a selective water injector well. The collaboration among various technical disciplines such as subsurface, production engineering, and surface facilities has resulted in the development and implementation of a selective injection redesign. To ensure an adequate water supply for the expansion project in U Inferior and U Superior, as well as the addition of a selective injector well in the area, a novel redesign of the surface facilities was proposed. This proposal involved the reuse of the same water source well, CNOF-054WDF (aquifer water from the Hollin Formation, meeting all quality standards). This approach aimed to reduce costs associated with constructing a new injection plant, including the processing and water treatment facilities. Consequently, the well CNOF-054WDF (dump flooding – one string for water production and one string for water injection) was selected as the water source to serve both injector wells (CNOF-054WDF and the implemented selective injector, CNOF-064WIW). This was achieved through the redesign of a single pipeline to implement the selective injection system, thereby avoiding additional and costly expenses. The average injection rate for the selective injector is 2,000 barrels per day (BWIPD), with 40% allocated for U Superior and 60% for U Inferior. These allocations were based on measurements obtained from injection logging tools. Since the implementation of the water injection project in U Inferior through CNOF-064WIW in January 2022, the developed reserves through secondary recovery has been estimated in 0.154 MMBo up to the present date. Similarly, since the implementation of the water injection project in U Superior through the four injectors CNO-0021REWIW, CNO-007REWIW, CNOF-054WDF, and CNOF-064WIW in June 2019, the developed reserves through secondary recovery are estimated to be 1.68 MMBo up to the present date. Furthermore, the project resulted in a reduction of water cut from 90% to 70%, effectively displacing newly contacted oil banks in U Superior. In U Inferior, the reservoir pressure increased by around 300-600 psi, reaching a large area after water injection. Additionally, the selective injection equalized the reservoir pressure in both reservoirs, ranging from 2,000-2,200 psi, which has greatly benefited the commingled production in the area. The results achieved with this selective injector well have unlocked the opportunity for expansion into the southern portion of the initial study area. This project marks the first successful selective water injection project implemented in thin reservoirs (5-15 ft) in Ecuador. Its significance lies in the low investment required for its execution, as it only necessitated the redesign and reengineering of available resources to assemble a functional injection system for two independent reservoirs. The CNOF-064WIW selective water injector well effectively pressurizes and displaces oil from the east flank using a peripheral injection approach. This well contributes to the Cononaco field development plan and enhances oil production by achieving an adequate injection-production balance. The thin reservoir's nature benefits the waterflooding project's fast response, allowing for optimization and exploitation opportunities.