Permanent Downhole Sensors in Today’s Petroleum Industy

Omotosho, Risi Jadesola
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This report is a literature survey on permanent downhole sensors in the Petroleum Industry. It takes a comprehensive look at the various kinds of sensors used for acquisition of downhole data. Based on this study, it becomes obvious that a great deal of research still needs to be done in the area of sensor technology for oilfield applications. The sensors available today do not adequately meet the demands of the oil industry and the new generations of sensors are not much better than past generations in terms of accuracy. Both the fiber optic sensor and the electronic gauge exhibit accuracies of about 0.001 psi. The average life of an electronic quartz gauge is five years. It is still too early to determine the average life of the fiber optic sensor but the problem of drifting in fiber optic sensors raises doubts on their reliability. The increasing need for real-time downhole data related to modern production practices has resulted in the growing utilization of permanent well monitoring systems for both offshore and land-based applications. Development of these technologies has furthered the ability of production and reservoir engineers to make more pro-active decisions. This report takes a look at the various principles and designs behind three different families of permanent sensors, namely: electronic gauges, fiber optic sensors and the sensors from tool technology. The latter refers to sensors that appear to be permanent downhole versions of logging tools such as permanent electrode arrays and acoustic sensors. A closer look at how these sensors work and are used to acquire real-time data is undertaken in the form of case studies. Each case study gives insight to how the sensors were able to acquire data that would have either been much more expensive or impossible than other conventional forms of data acquisition such as wireline gauges. Permanent downhole sensors in many of the case studies proved to be a cheaper and quicker alternative to acquire the data. The report also establishes that permanent sensors could sometimes be the only form of data acquisition available because of wellbore inclination, size, or location. Whatever the reason, permanent sensors provide reliable and accurate data in spite of non-ideal wellbore conditions. Problems with downhole sensors are also addressed in this report. These include: (a) Study of failures in downhole permanent gauges from the Villa Fortuna Field in Italy, (b) problem of drifting in fiber optic cables, and (c) the biggest issue regarding failure in electronic gauges. From field-studies, it has been observed that such gauges are not properly designed for downhole applications. Presence of downhole electronics is responsible for only about 20% of failures while problems with cables accounted for more than 40% of the failures. On the other hand, regarding the fiber-optic cable, effective coating techniques are required to prevent drifting and to improve the accuracy of the fiber. The cause of drift in fiber-optic cables is explained while types of coating available in the industry today are also mentioned in the report.