Monitoring Alternating CO2 and Water Injection and Its Effect on Production in a Carbonate Reservoir Using Permanent Fiber-Optic Distributed Temperature Systems

Abstract

Abstract This paper describes the use of fiber-optic distributed temperature systems to monitor the effect of water-alternating-gas (WAG) injection on production from several wells in a Texas carbonate reef reservoir. Cogdell is an old reservoir re-activated by using alternating injections of water and carbon dioxide (CO2) as the tertiary recovery mechanism. Complex geology makes it necessary to monitor both injection and production over time to determine the effectiveness of the sweep. Fiber-optic distributed temperature monitoring systems are installed on one injector and four producers, in a six-spot pattern, for continuous monitoring across the crest of the reservoir. Conventional production logging is not possible because electrical submersible pumps restrict access, and repeated injection logging required to monitor changes with the same resolution during several WAG cycles, would be impractical. Continuous distributed temperature monitoring of the injector enabled a quantitative analysis of the injection profile. This continuous profile monitoring process identified the zone taking the majority of the injected fluid and indicated that injection into the other zones varied with the WAG cycle. Producer monitoring allowed us to identify the flow contribution from each interval as well as changes over time. CO2 entry was also identified. This information confirmed sweep paths from the injector to the producers and provided an improved understanding of the operation of the WAG flood in the Cogdell reservoir. The results showed fiber-optic distributed temperature monitoring provided important information for well and pattern performance analysis in wells where our prior ability to monitor response was limited or non-existent. Introduction During 2003 Occidental installed fiber-optic distributed temperature monitoring (DTS) systems in 5 of their wells on the Cogdell field. One was in a water and CO2 (WAG) injector and the other four were part of a six spot pattern around the injector Fig. 1. The objective was to monitor both the injection and production from the reservoir and to identify the reservoir layers that were receiving pressure support and producing as a result of the WAG flood in this complex carbonate reef reservoir. The optical fiber was installed in 1/4 inch control line attached to the production tubing and supported across the reservoir interval by a stinger hung below the Electric Submersible Pump. The systems were installed during workovers that coincided with CO2 startup for this section of the Cogdell field. DTS temperatures from all 5 wells have been continuously recorded since May 2003 using a single DTS laser system acquiring data in sequence from each well. The DTS system acquires a temperature profile every meter along the length of the optical fiber by the analysis of backscattered Raman wavelength light from a pulsed laser source. This temperature profile can be interpreted to give information about the injection and production profiles in the well and how they change with time as a consequence of the WAG flood. During the period July to October 2003 the initial slug of CO2 was injected into the monitored injector. Other injectors around this pattern were also converted to CO2 injection at various times from April to July of 2003. In the monitored injector, after October 2003, water and CO2 were alternated between injection of 4,500bpd water for 10 days and 10MScfd CO2 for 20 days.