Fibre Optic Surveillance for Conformance Monitoring in a Polymer Pilot by Petroleum Development Oman

Abstract

Abstract Petroleum Development Oman (PDO) has been utilizing several Enhanced Oil Recovery (EOR) methods in the Sultanate of Oman to accelerate (polymer/steam floods) and increase (Alkaline Surfactant Polymer, or ASP, floods) recovery from fields with challenging rock and fluid properties. Polymer-flood is a mature EOR technique, and has been trialled and implemented successfully on full-field scale in a sandstone reservoir of excellent quality in the South of Oman. As part of this continuing effort, a currently ongoing polymer pilot focuses on testing the ability to sustain polymer injectivity and to prove the polymer efficiency in a significantly lower permeability sandstone reservoir. This lower permeability reservoir introduces injectivity and conformance concerns. Continuous surveillance to monitor and assess changes in areal sweep, vertical conformance and fracture initiation play therefore a pivotal role in delivering on the success criteria of the pilot. The pilot was designed with this surveillance scope in mind. Producers are equipped with either a dual well-head or a permanent down-hole gauge, and well-testing is conducted with a dedicated [mobile] test-unit. In the injector wells verifying vertical conformance and matrix injection is critical but conventional surveillance (Production Logging) is unreliable as the spinner is affected by the non-Newtonian behaviour of the polymer solution. The two pilot injectors were therefore equipped with a fibre optic (FO) cable, clamped and cemented on the out-side of the 7″ casing, enabling continuous monitoring of the injection profile and thus injection conformance through temperature (Distributed Temperature Sensing, DTS) and acoustic (Distributed Acoustic Sensing, DAS) measurements. The focus of this paper is to explain the decisions taken based on the information obtained from the FO data during the pilot period. The data was used for several key decisions; to select which pilot injector to use for polymer injection and which to keep as a back-up by assessing Out-of-Zone Injection, matrix versus fracture injection, and to prove conformance and injectivity improvement after acid stimulation jobs. The most valuable information inferred from the FO data allowed confirmation of the improvement in injection conformance by the polymer-flood over the water-flood. Fibre optic surveillance thereby has truly proven its surveillance value, for this polymer pilot, and any potential up-scaling of the project.