Fibre Optic Well Monitoring System (1995)

ABSTRACT The Fibre Optic Well Monitoring (FOWM) system is designed for permanent downhole installation and reliable pressure and temperature acquisition, also at high pressure, high temperature (HPHT) conditions. FOWM is based on an optical sensor system concept, integrating simple passive silicon resonator sensors with fibre optic communication. To gain experience, the FOWM system has been installed onshore in the Netherlands for Nederlandse Aardolie Maatschappij B.V. (NAM), and offshore in the Norwegian sector for BP Norway. The paper will review the basic technology of the FOWM system, give descriptions of the installations, and present results obtained with the FOWM system during well testing. INTRODUCTION On-line downhole pressure and temperature data is important input to increase productivity and to optimise hydrocarbon production. Permanently installed electrical downhole monitoring (DHM) systems are used more and more to providethis data in a cost efficient manner. However, in the past these electrical systems have shown an inadequate reliability. A target specification currently being considered as a practical aim is 90% probability to perform within the system specification over a period of 5 years. Data have been accumulated for surface and subsea installations of electrical systems in the NorthSea, being realised between 1987 and 1993, Figure 1. Some installations were made in reservoirs with temperatures up to 130°C, but most were at temperatures around 100°C. Approximately 15% out of 149 surface completed systems failedshortly after installation, and approximately 30% failed after two years. Results of 102 subsea installations indicate that 15% of the systems failed directly after the installation, whereas 30% to 40% failed within two years. These failure probabilities of electrical systems are high in relation to the current practices of planned intervention work, typically once every 5 years. It is important to point out that the reliability of electrical DHM systems has improved during the last couple of years; however, the target reliability is not yet within reach. Two additional aspects will affect the reliability requirement for permanently installed systems and reduce the applicability of electrical DHM systems for future applications:Extension of the period between interventions to up to 10 years, enlarging the discrepancy between reliability target and the current reliability performance of electrical systemsNew applications in deeper reservoirs will have higher pressure and higher temperature characteristics. The high temperatures in future applications impose a severe problem on the use of electrical DHM systems. Only special 'MILspec' components can be used for applications up to 120°C continuously and up to 150°C on short term bases. The reliability of permanently installed electrical DHM systems is insufficient for current applications and will be further reduced for future applications with high pressure and temperature characteristics and longer periods between interventions. In that light, a new and diverse technology approach to the problem is made to realise the high reliability targets FOWM The FOWM system is a new type of permanently installed DHM system [1]. In contrast to the currently available electrical systems, the FOWM system is based on an optical sensor system concept, integrating simple passive silicon resonator.