Optical fiber sensing technology is a new type of sensing technology developed rapidly with the development of optical fiber communication technology in the 1970s. Some foreign developed countries have achieved fruitful results in the application of optical fiber sensing technology, and many optical fiber sensing systems have been put into practical use. become a commodity that replaces traditional sensors.
In the process of oil field development, people need to know the detailed information about the persistence and state of the fluid in the well during the process of liquid production or water injection, which requires the use of oil well logging, whose reliability and accuracy are crucial. However, traditional electronic-based sensors cannot work in harsh downhole environments such as high temperature, high pressure, corrosion, and geomagnetic and geoelectric interference. Optical fiber sensors can overcome these difficulties. They are not sensitive to electromagnetic interference and can withstand extreme conditions, including high temperature, high pressure (above tens of megapascals) and strong shock and vibration. They can measure wellbore and wellsite environmental parameters with high precision. , the fiber optic sensor has distributed measurement capabilities, can measure the spatial distribution of the measured, and give profile information. Furthermore, the fiber optic sensor has a small cross-sectional area and a short profile, occupying very little space in the wellbore.
Optical fiber sensors have made great progress in the field of geophysical well logging. Major oil production companies, well logging service companies and various optical fiber sensor research and development institutions and enterprises all over the world have participated in the research and development process. In order to develop the application field of fiber optic sensors, this paper summarizes the research and progress of fiber optic sensors in the field of geophysical logging, hoping that the research can make contributions to further improving the level of oil development.
2. Research progress of optical fiber sensor in well logging
1. Monitoring of reservoir parameters
(1) Pressure monitoring
Due to the needs of the development plan, the management of reservoir pressure needs to be particularly cautious. The purpose of this is to reduce the loss of crude oil caused by the production at a pressure lower than the bubble point, and reduce the overpressure of the reservoir during the gas injection process. The loss of crude oil by squeezing it into an aquifer. The sensors used in traditional downhole pressure monitoring mainly include strain gauges and quartz crystal pressure gauges. Strain gauges are affected by temperature and hysteresis, while quartz pressure gauges are affected by sharp changes in temperature and pressure. These sensors also involve difficult installation and poor long-term stability when it comes to pressure monitoring. Downhole fiber optic sensors have no downhole electronic circuits, easy installation, small size, strong anti-interference ability, etc., which are necessary for downhole monitoring.
The American CiDRA company is at the forefront of fiber optic pressure monitoring research, and their researchers have discovered the linear response of Bragg fiber grating sensors to pressure. Sensors have been developed that can work up to 175oC, 200oC and slightly higher temperature products are being developed, and 250oC is the next target of research and development. The pressure measurement error under different temperatures and pressures is less than ±6.89kPa within the test range (0MPa~34.5MPa), which is equivalent to the best level of the electronic measurement system. At present, the indicators of CIDRA’s optical fiber pressure sensor are: measuring range 0~103MPa, overpressure limit 129MPa, accuracy ±41.3kPa, resolution 2.06kPa, long-term stability ±34.5kPa/yr (continuously maintained at 150oC), working temperature Range 25oC~175oC. In 1999, the company conducted a test of the pressure monitoring system in the Baker oil field in California, and the results showed that the system has very high accuracy, and it has been delivered for commercial sale. In 2001, the company’s pressure sensors were installed in several wells of BP in the UK to monitor stress changes, and the results showed that they were reliable enough.
TsutomuYamate and others from the Doll Research Center of Schlumberger Oilfield Service Company in the United States have conducted long-term research on downhole monitoring with Bragg fiber grating sensors. They have developed a side hole Bragg fiber grating sensor that is not sensitive to temperature. The maximum working temperature is 300oC, and the maximum measurement pressure is 82MPa. Under the maximum measurement pressure, the sensitivity to temperature is very small, and it can be used for downhole pressure monitoring.
(2) Temperature monitoring
Distributed fiber optic temperature sensors have the potential to provide a new way to monitor production and reservoirs by continuously collecting temperature data along the entire completion length. Because changes in the well’s temperature profile can be compared with other surface-acquired data (flow rate, water cut, wellhead pressure, etc.) and open-hole logs, this provides the operator with qualitative and quantitative information about changes occurring downhole. Traditional temperature measurement tools can only measure the temperature at a certain point at any given time. To test a full range of temperatures, point sensors can only be realized by moving back and forth in the well, which will inevitably affect the environmental balance in the well. The advantage of the optical fiber distributed temperature sensor is that the optical fiber does not need to move back and forth in the detection area, which can ensure that the temperature balance in the well is not affected. And because the optical fiber is placed in the capillary steel pipe, the optical fiber distributed temperature sensor test can be carried out wherever the capillary steel pipe can reach.
One of the most widely used fiber optic sensors for downhole monitoring applications is the Raman backscattering distributed temperature detector. This method has been widely used in measuring wellbore temperature profiles (especially in steam flooding wells). Distributed temperature sensors should comprehensively consider the number of measured points and connector attenuation. The problems and solutions encountered are:
a. The attenuation of the signal by optical fibers and connectors can be solved by reducing the number of connectors as much as possible, using Bragg fiber grating sensors and improving the performance of connectors;
b. It is easy to be damaged during underground installation. The solution is to equip skilled workers, optical fiber sensors need an external protective layer, and reduce stress (including stress caused by perforation and temperature). For the optical fiber distributed temperature sensor system, the British Sensa company has been in a leading position in technology, and has launched a series of products, and cooperates with major oil companies to actively explore the application of optical fiber distributed temperature sensors in oil wells. CiDRA has also been researching fiber optic temperature sensors. At present, the technical indicators of the company’s temperature sensors are: measurement range 0°C~175°C, accuracy ±1°C, resolution 0.1°C, long-term stability ±1°C/yr (150°C continuous use).
One of the most important shortcomings of current optical fiber temperature and pressure sensors is the temperature-pressure cross-sensitivity characteristic. How to eliminate or utilize this cross-sensitivity characteristic is a research hotspot.
(3) Multiphase flow monitoring
In order to do a good job in reservoir monitoring and oilfield management, the most critical link is to obtain a stable and reliable total flow profile and holdup of each phase fluid for production wells and water injection wells. However, most oil wells are produced in layers, each layer has different water content, and sometimes the flow rate is relatively large, which brings great difficulties to the measurement and analysis of the production status of oil wells with conventional production logging equipment. The frictional resistance of the liquid in the tubing and the injection from the reservoir into the wellbore make it impossible for the pressure differential density instrument to measure accurately, and the electronic probe cannot detect the small oil bubbles in the liquid.
There are two methods of optical fiber measurement of multiphase flow. The first is the gas holdup optical fiber sensor of Schlumberger in the United States, which can directly measure the gas holdup in multiphase flow. Its four fiber optic probes are evenly distributed in the cross-section of the wellbore, and its spatial orientation can be accurately measured by an integrated relative orientation sensor. In the gas-liquid mixture, the gas holdup rate and the number of foams can be determined through the optical signal reflected by the probes (both are associated with gas flow). In addition, the measurements from each probe are used to create a picture of the gas flow in the well. These images are especially suitable for deviated and horizontal wells, which can better understand the flow patterns of multiphase flow and interpret these flow patterns under inclined conditions. inherent phase separation. Recently, this tool has been successfully used in well logging experiments around the world. It provides data that directly measures and quantifies gases and liquids in multiphase mixtures, enables accurate diagnosis of wellbore problems, and facilitates production adjustments. The instrument passed field tests in three wells.
The second is to determine the phase composition of the two-phase mixed flow by measuring the sound velocity, because the sound velocity of the mixed fluid has a correlation with the sound velocity and density of each single-phase fluid, and this correlation generally exists in two-phase gas/liquid and liquid / liquid mixed fluid system, but also suitable for multiphase mixed flow system. To determine the volume fraction of each phase fluid according to the sound velocity of the mixed fluid is to measure the volume fraction of each single phase flowing through the flowmeter (ie holdup measurement). Whether a certain fluid holdup is equal to the flow volume fraction of the phase depends on whether there is serious slippage of the phase relative to other phases. For the oil-water two-phase mixed flow system without severe slippage, the uniform flow model can be used for analysis; for the flow state with severe slippage, a more complete slippage model must be applied to explain the data measured by the flowmeter in order to accurately determine flow in each phase. The flow cycle experiments show that: for oil-water mixed fluid, the long-wavelength sound velocity measurement of the flowmeter can determine the volume fraction of each phase (ie holdup) without being affected by flow heterogeneity (such as laminar flow).
CiDRA tapped the inherent advantages of fiber optic sensors and developed a downhole optical phase multiphase flow sensor. The current samples are only limited to the measurement of quasi-uniform fluids: such as oil, water two-phase or oil, water, gas three-phase (gas phase volume fraction is less than 20%). In order to investigate the performance of this new type of fiber optic multiphase flow sensor in measuring oil/water/gas three-phase in production wells, CiDRA recently conducted an experiment in a test well. In the test well, oil, water and gas are mixed. The mixture includes oil with a viscosity of 32API, water with a salinity of 7% and mine natural gas (methane). The test temperature is 100oF and the pressure is <2.75MPa. In the range of 0%~100% moisture content, the measurement error of the instrument is less than ±5%, and the accuracy meets the requirements. The flowmeter can determine the water holdup in the mixture of crude oil and brine, and its error is within ±5% in the full range of the water holdup, which meets the production requirements. And in addition to being able to measure water holdup, the instrument also tests the volume content of gas in three phases, but the ratio of oil to water in the test is known. The results show that the instrument can calculate the gas volume percentage in the liquid which appears in the form of froth flow.
2. Acoustic measurement
Compared with the past, exploration and development companies are now facing greater risks and more complex drilling environments, so it is of great significance to obtain accurate stratigraphic structure maps and reservoir mechanisms. Currently used seismic measurement methods, such as towed isobunt cable geophones, temporary seabed deployment geophones and downhole cable deployment geophones, etc., can provide the measurement of the target oil producing area, but these methods have relatively high operating costs. It is difficult to realize continuous real-time dynamic monitoring of reservoir because of the high cost, the inability to go into the well or the limitation of environmental conditions, etc., and the images provided are not comprehensive, discontinuous, and the resolution is not very high.
The fiber optic-based downhole geophone system can solve these problems, and it can provide permanent high-resolution 4D reservoir images throughout the life of the oil well, which greatly facilitates reservoir management. The downhole seismic accelerometers receive seismic waves and process them into images of formations and fluid fronts.
The permanent downhole optical fiber 3-component seismic measurement has high sensitivity and directivity, and can produce high-precision spatial images, not only near the borehole image, but also the formation image around the borehole, and the measurement range can reach several thousand in some cases foot. It operates during the entire life of the oil well, can withstand harsh environmental conditions (temperature up to 175°C, pressure up to 100MPa), and has no moving parts and downhole electronic devices. Shock and vibration, can be installed in complex completion strings and small spaces. In addition, the system also has the characteristics of large dynamic range and signal frequency bandwidth. The signal frequency bandwidth is 3Hz to 800Hz, which can record the equivalent response from extremely low to extremely high frequency.
3. Laser fiber nuclear logging technology
Laser technology and fiber optic technology can be used to develop downhole sensors for logging in wells filled with opaque fluids such as crude oil and mud. Research on laser fiber optic nuclear sensors is relatively popular abroad, and countries such as the United States, Germany, Russia and Belgium have a large number of related research papers.
The laser optical fiber nuclear sensor is produced on the basis of optical fiber communication and optical fiber sensor. It utilizes physical effects such as photo-induced loss and photoluminescence, and has more advantages than conventional nuclear detectors. It is a typical cross-discipline. Optical fiber nuclear logging technology is actually a nuclear detection technology in a specific environment, and its typical advantages are:
(1) According to the energy level range of different nuclear detection, sensitive probes in this range can be developed.
(2) Due to the application of the photoluminescence effect, the probe can be located at a distance of one kilometer downhole, while the photomultiplier tube is connected by a transmission optical cable and placed on the well, away from the harsh downhole environment (high temperature and high pressure), thus prolonging its service life.
(3) Optical fibers have high-speed, large-capacity transmission capabilities, and can also carry signals from other downhole instruments.
However, laser fiber optic nuclear detectors also have disadvantages, mainly in the protective coating that can withstand high temperature and high pressure, the mechanical strength of the transmission cable, and the low attenuation loss of the radiation-resistant transmission cable.
3. Conclusion and prospect
From the analysis in this paper, it can be seen that with its unique advantages, fiber optic sensors can be widely used in oil and gas downhole reservoir parameter monitoring (including temperature, pressure and multiphase flow), acoustic detection and laser fiber nuclear logging. Significantly enriches oil and gas companies’ understanding of reservoirs to optimize oil and gas field production and maintenance. It is worth mentioning that the system can obtain the injection water pressure and temperature in time, so as to judge whether the pressure exceeds the standard, so as to prevent casing damage caused by the pressure exceeding the standard. This is a brand new field, and there is no domestic and foreign research on this aspect. reports and presentations.
So far, major oil production and service companies around the world have invested heavily in the research and development of the application of optical fiber sensors in reservoir evaluation, and quite a few optical fiber sensor research and development institutions are also committed to this emerging field. . It can be imagined that after overcoming its own shortcomings and disadvantages, the next-generation optical fiber sensor will be promoted on a large scale, which can more effectively help to understand the dynamic level of oil and gas production in real time. Major oilfield companies can make full use of these favorable information to realize and maintain the optimal production of the oilfield, so that the oil reservoir can achieve the highest recovery rate. At the same time, due to the rapid development of the Internet, the well condition parameters monitored by optical fiber can be transmitted in time, which enables the production and service companies related to the oil industry to analyze and evaluate assets around the world more effectively.