Distributed sensors for oil logging using power cables and sensing optical cables for temperature measurement

Fiber optic temperature sensor, Intelligent monitoring system, Distributed fiber optic manufacturer in China

Distributed fiber optic sensors can measure various parameters such as temperature, stress, gas concentration, and multiphase flow in oil wells, but in reality, the most mature technologies are temperature and stress sensors. Especially temperature sensors, they are not only technologically mature but also widely used. In oil well detection, other parameters such as viscosity and pressure can be indirectly derived based on temperature data.

1. Sensing optical cable for distributed temperature measurement of power cables

The laying method of urban power cables is generally buried directly in soil or placed on cable trays in tunnels, with the latter being the most common method. The sensing optical cable is fixed to the surface of one phase of the cable circuit, which is generally the phase with higher temperature rise, by interval binding.

The temperature measurement system host measures the surface temperature of the cable through the Raman scattering effect of optical fibers, and can calculate the real-time temperature of the cable conductor through the cable’s thermal conductivity equation. The current technological level can achieve a temperature measurement accuracy of 1 ℃ and a position resolution of 1 meter within the cable length. Through this distributed temperature measurement system and analysis software, the fire, critical insulation breakdown, current conveying load, real-time current carrying capacity, and other states of the cable can be observed. It should be noted that the temperature on the surface of the cable is sensed through the optical fibers in the cable, while the surface in contact with the cable is the surface of the optical cable. Therefore, in practical applications, the temperature sensing system ignores the structure of the sensing optical cable, and the deviation of the test data is within a completely acceptable range.

The optical cable used for temperature sensing of power cables takes into account the simple, lightweight, flame retardant, flexible, and easy to move characteristics of the optical cable structure. The sensing optical cable structure used in temperature monitoring systems for highways, bridges, and tunnels is basically the same as that used for power cable monitoring,

2. Distributed Fiber Optic Sensing Fiber Optic Cable for Oil Logging

At present, the depth of oil wells being mined is generally between 3000 to 10000 meters. Previously, electric sensors were used to detect reservoir information, installed in continuously connected steel pipes, and buried in the oil wells. This method is not only extremely inconvenient for construction, but also due to the detection signal being an electrical signal, the amount of information collected is small and the accuracy is not high due to factors such as detection distance and distortion. Later, point type fiber optic sensors represented by FBG were developed, which were connected to FBG through optical cables at locations where detection was needed. This method took a big step forward in technology, but there was still a problem of incomplete detection of oil well data information, until the emergence of distributed sensors and their application in the field of oil logging. It is insensitive to electromagnetic interference and can withstand extreme conditions, including high temperature, high pressure (tens of megapascals or above), as well as strong impacts and vibrations. It can accurately measure environmental parameters of the wellbore and well site. At the same time, fiber optic sensors with distributed measurement capabilities can measure the spatial distribution of the measured area and provide profile information. Moreover, fiber optic sensors have a small cross-sectional area and occupy very little space in the wellbore.

The point type fiber optic sensor and distributed sensor used for oil logging both use a section of optical cable buried in the testing oil well. The difference is that the former sensor is an FBG connected to the end of the optical cable, while the latter is the optical cable itself. If we ignore these differences and only consider the requirements of logging technology for the optical cable itself, the two are the same, that is, we need to consider the impact of the optical cable’s resistance to high temperature, high pressure, high corrosion, high hydrogen environment, and large length vertical deployment. The structure of optical fiber composite overhead ground wire (OPGW) used in power systems provides inspiration for designing logging optical cables.

In the structure of logging optical cables, the use of sealed units welded with stainless steel strips provides favorable protection for the optical fibers. Considering the large length of vertical distribution and the high temperature environment of oil wells, it is no longer possible to use filling composites in the steel pipes. Oil paste fillers are no longer necessary in this structure and purpose of optical cables. Only the problem of fixing the position of the optical fibers in the pipes needs to be considered, and materials such as high-temperature resistant yarns can be used to assist in positioning. The galvanized steel wire in the optical cable structure not only provides necessary strength, but also has the characteristics of corrosion resistance and high temperature resistance (short-term 400 ℃).
Fiber optics in sensing optical cables
Currently, the optical fibers used for sensing optical cables are generally multimode fibers. The temperature sensing system for cables and highway tunnels typically operates at temperatures not exceeding 70 ℃ under normal operating conditions, and can use ordinary communication multimode optical fibers. The optical cable used for oil logging generally operates at a high temperature, with some oil wells reaching temperatures above 400 ℃. However, ordinary optical fibers coated with acrylic resin have a maximum operating temperature of no more than 100 ℃. Under these conditions, high-temperature resistant optical fibers must be used.