This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 199070, “Water-Shutoff Technique Extends the Productive Life Cycle of Cretaceous U Sandstone: The Iro Field Case in Ecuador,” by Luis Roberto Bailón, SPE, Ney Holger Orellana, SPE, and Santiago Villegas, Repsol, et al., prepared for the 2020 SPE Latin American and Caribbean Petroleum Engineering Conference, originally scheduled to be held in Bogota, Colombia, 17-19 March. The paper has not been peer reviewed.
The water-shutoff technique is used in some wells of the U reservoir in the Iro field of the Oriente Basin in Ecuador as a remediation plan to restore production after an early water breakthrough. The production historical data, workovers, and sand-body correlation of wells are compared to understand reservoir behavior, shale-baffle-sealing continuity, the existence of different sand units, and the effect on production.
The Iro field is in the south of Block 16. Production began in March of 1996. Iro is considered a mature field that produces heavy crude oil. The U sand-stone reservoir at Iro field is constituted by quartz grains subtransparent with fine grain sizes to medium, moderately classified, occasionally clay-like matrix. A thin limestone layer subdivides the U sandstone reservoir into two main stratigraphic units, Upper U and Lower U sandstone. Logging acquisition during the drilling campaign revealed heterogeneous sand-body deposition throughout the field. Depositional features of fluvial channels are developed from the base of the reservoirs, which are overlaid by sand bars. In addition, interbedded shale layers and baffles are present in the U reservoir, in some cases locally. However, the main shale layers are effective seals when they subdivide the Upper and Lower U sandstone units into two or more subunits. A good example is the shale layer that separates channels and bars in the Lower U sandstone unit. This identification was possible after the development of the well-drilling campaign, well correlation, and years of production behavior. Two subunits of the Lower U reservoir, Ui1 and Ui2, were classified as a result of the acquired data.
Given the maturity of the fields, during the last 2 years, a logging campaign of pulsed-neutron cased-hole logs has been performed. In the case of the Iro field, pulsed-neutron logs were run in six wells; three of these reached the Lower U reservoir. These three wells have a good correlation between the analog density and neutron curves of the cased-hole and the original openhole curves, providing certainty in the reading of the tool.
The logging program obtained data of chemical-element spectra in capture and inelastic modes. This information was processed and analyzed to derive hydrocarbon saturation. In this way, by-passed oil can be identified as well as reservoir zones already drained by production of the same wells or by neigh-boring wells. Data in the Lower U reservoir show fluid movement.