Improving Fiber-Optic System Reliability in Permanent Installations

Abstract

Abstract Permanent installation of fiber-optic (FO) sensors, whether discrete or distributed, provides beneficial monitoring of a wide variety of parameters during every stage of a well's useful life—from drilling and completions, through the productive period, up to and including permanent abandonment—assuming that the sensors do not fail during installation or subsequent wellbore intervention operations. Long-term reliability is critical to obtain maximum benefit of FO based sensor installations and is crucial to increased adoption of this technology in the oil and gas industry. FO system reliability depends on several factors and requires diligent attention to numerous details, a responsibility shared by manufacturers, service companies, and oil and gas operating companies. Unexpected signal degradation of FO cables experienced in several instrumented wells required an engineered program approach to understand the failures and guide system improvement developments. Multiple-wavelength optical time domain reflectometer (OTDR) technology was used to diagnose symptoms of the fiber degradation. The importance and usefulness of OTDR technology was recognized, and a program to monitor fiber “health” with measurements recorded at regular intervals was developed to capture crucial historical records of the fiber/cable performance before system installation, during installation, and after installation. The historical progression of optical signal degradation proved to be invaluable in discerning root causes for this degradation. In the problematic FO cables, OTDR traces suggested various failure modes, including hydrogen darkening, fiber bending, and, in one well, a cable breach. An enabler to frequent OTDR data collection is the ability to remotely operate the FO interrogation units and OTDR equipment. Travel to remote well locations is costly in terms of travel expenses and personnel. It interferes with accomplishing other tasks and often requires the assistance or presence of field operations personnel for onsite access. The system requirements and components used to remotely operate the onsite FO electronic interrogators in the monitoring program are described in this paper. Based on the OTDR data, additional testing programs were initiated, new cables were designed and assembled, and quality assurance protocols were added. This paper describes the optical fiber “health” monitoring program, reviews case histories, emphasizes the importance of this diagnostic information, and summarizes developments to improve FO system reliability. Significantly improved reliability has been recognized in current installations and is expected to continue for future permanent installations based on the FO “health” program learnings.