Fiber Optics in Well Logging - Coursework Example

Name University Course Tutor Date Fiber Optics in Well Logging Abstract Advancement in communication systems has led to development of more efficient transmission media. The development of fiber communication systems has increased data transmission rates as well as revolutionizing the information age. Fiber optic cables are made of glass, plastic or both. They utilize light waves in the visible part of the electromagnetic spectrum. Losses in the cables are minimal and multiplexing techniques allow coupling of several light signals into one cable. This increases the amount of information that can be transmitted in one cable. Fiber systems have several advantages over copper and coaxial lines. Attenuation in fiber cables is less and they require few repeaters for long distance transmission. Fiber cables are lighter and occupy smaller space. Introduction Fiber optic cables have three major parts; core cladding and jacket. The core is the inner most layer and is made of glass or plastic of high refractive index. The cladding is the second layer and is also made of glass or plastic but has a lesser refractive index than the core. The jacket is the outermost layer made of plastic or rubber which protects the core and cladding. Light waves travel in the core of the fiber[Agr02]. They utilize total internal reflection to transmit the light signals. The light is coupled into the fiber at an angle known as critical angle. This is the smallest angle at which a light signal will undergo total internal reflection at the core cladding interface. Monochromatic light is used to transmit data along the core. Multiplexing techniques such as wavelength dense multiplexing allow several light signals of different wavelengths to propagate in the cable at the same time (Agrawal, pg. 23). Well logging refers to data collection from boreholes and petroleum mines. Over the years acoustic imaging has been one of the ways of collecting data from the oil wells. Micro-electrical imaging techniques have gradually replaced acoustic imaging due to their larger dynamic range and higher sensitivity. Micro-resistivity imaging is also used, which has a higher resolution but coverage decreases with the borehole diameter. Several petroleum companies for example, British petroleum has switched to fiber optics for continuous monitoring of the wells and fast offshore communication[Koe11]. Monitoring involves the use of fiber optic sensors in the wells. These sensors provide continuous surveillance for down-hole temperatures and pressure. Fiber systems can withstand high temperatures in the wells compared to electric systems which fail at temperatures above 120 degrees. The fiber systems do not require subsea power supplies or electronic units and the cables transmit sensor data to the surface (Nina, pg .1). The applications of fiber optics in the field are varied. They range from temperature, pressure and strain measurements in wells and corrosion measurements in pipelines. This provides continuous and accurate monitoring of wells. A well logging cable contains a sensor and an enclosure surrounding the cable. The enclosure is adapted to maximum strain and can be elongated without deforming the cable. The enclosure is a corrugated tube surrounding the optical fiber. The tube is filled with hydraulic oil to prevent fluids from the well from entering the fiber optic when pressure is high[Goo11]. Corrosion detection Pipes used to transport fuel undergo corrosion due to reaction with oil and gas. This makes the joint weak and holes may develop on the surface of the pipes. Continuous assessment of internal corrosion is necessary and should be accurate. Tracking methods used include pigging, corrosion coupons and other periodic inspection forms. They provide the current conditions of the pipeline and require to be carried out frequently to determine corrosion levels. However, several limitations hamper their accuracy and efficiency. Pigging pipelines is very expensive when done over long distances. Coupons require flow exposure for several months to develop loss indications. These results are greatly influenced by position in the flow, temperature and other factors. Real time corrosion monitoring is necessary for effective corrosion control. Combinations of remote data telemetry and fiber optics loss sensors provide continuous monitoring of internal corrosion. Changes in the walls of the pipes are directly measured and non-invasive wall losses can be measured. The sensors are reliable due to their immunity to electromagnetic interference and data recorded is accurate (Don, pg.1). The flow of products is not disrupted, and frequent visits to the site are eliminated. The fiber cables ensure secure data transmission to the control centers. The sensing element is fabricated from single mode optical fibers. The small diameter of the cable allows flexibility of the sensor and can be configured to monitor both corrosion and pipe bending. Sensor displacements of over 15 mm can be measured. A battery powered demodulator is used to demodulate the optical signal and is used for periodic outdoor monitoring. An AC-powered demodulator is used for continuous monitoring. To determine wall thickness changes, a sensor is placed in the test area and another in an unaffected area. An optical temperature sensor is used to de-couple thermal and mechanical strains (Don, pg. 2). Reservoir Management Fiber optics are also used in well and reservoir management. Their primary focus is temperature profiling. However, continued research has provided a method for temperature, strain and acoustic sensing using fiber cables. Some of the down-hole sensing technologies include Distributed Temperature sensing (DTS), Distributed Strain Sensing (DSS) and Distributed Acoustic sensing (DAS). A combination of DAS and DTS provides accurate flow information from the wells. DSS allows monitoring of tiny well deformations. The optical fiber is wrapped helically around a casing, sand screen or well tubular to provide three-dimensional images of well deformations. This data is used to determine the deformation trends to prevent well failure. Production of reservoir fluids causes load stress from the overburden which makes the sediments to consolidate and compact. This leads to compression of the reservoir and extension in the overburden. The wells undergo axial strain, which makes them bend and buckle (Koelman, Lopez and Potters, pg.1). Faults and slip surfaces cause shearing in intersecting walls. Effective strain monitoring is achieved through DSS and provides operators with early warning regarding zones of well integrity problems. DAS is used in flow profiling. A fiber deployed in the well path acts as an array of microphones. This is important for hydraulic fracture optimization and geophysical monitoring. DAS is also applicable in inflow and injection profiling and monitoring gas lift. Permanent installations of fiber cables subject to interrogation by DAS provide geophysical monitoring. This provides down-hole seismic inspection. The use of DAS for down-hole acquisition is more convenient than using geophones in that an entire well is covered by a single seismic shot. DAS has a stable phase recording and strong reflections are observable. In slim-hole wells, DAS is the only method for geophysical monitoring (Koelman, Lopez and Potters, pg 3). Real-time logging Real time logging is required especially when mining occurs in shale environment. These are areas where oil and gas is trapped between shale layers. Real time logging helps evaluate the wells while drilling due to stability problems which arise when the hole is left uncased. Production logging in these reservoirs enhances production due to better zone targeting, hydraulic fracture design and zonal isolation procedures. DTS technology is applied to identify zonal gas production in the shale wells. The DTS contains and industrial laser, reference coil and signal analyzer. The laser fires continuous bursts of light through the reference coil down a fiber installed in the coiled tube. Back-scattered light that returns to the DTS box is fed to the signal analyzer. The speed of light in the fiber is constant but the amplitude of the wavelengths of the back-scatter varies with temperature (Menkhaus, p.g 1). Conclusion Increased oil and gas exploration requires faster and more accurate methods of data transfer and precise monitoring of changes occurring in the wells. Pipelines suffer from corrosion which requires early recognition to avoid spillage caused by pipe bursts. Fiber optics and sensors provide more accurate data, and their use is increasing in the mining industry. Several mining companies like BP have partnered with communication companies for research and development of technologies applying fiber optics in mining. Communication between onshore and offshore stations has been improved by use optical fibers due to high bandwidth and fast signal speeds. The applications of fiber optics in the industry remain diversified and researchers continuously develop new applications. Work(s) cited Agr02: , (Agrawal), Koe11: , (Koelman, Lopez and H), Goo11: , (Google), Read More