Optical fiber sensing technology is a new type of sensing technology that has been rapidly developed in the 1970s with the development of optical fiber communication technology. Some developed countries have achieved fruitful results in the application of optical fiber sensing technology, and many optical fiber sensing systems have been practical. Become a commodity replacing traditional sensors. This article mainly introduces the application cases of optical fiber sensors in the petroleum industry to readers.
I. Introduction
Optical fiber sensing technology is a new type of sensing technology that has been rapidly developed in the 1970s with the development of optical fiber communication technology. Some developed countries have achieved fruitful results in the application of optical fiber sensing technology, and many optical fiber sensing systems have been practical. Become a commodity replacing traditional sensors.
In the development of oil fields, people need to know the detailed information about the persistence and status of the fluid in the well during fluid production or water injection. This requires the use of petroleum logging. Its reliability and accuracy are crucial. However, traditional electronic-based sensors cannot work under harsh underground environments such as high temperature, high pressure, corrosion, geomagnetic and geoelectric interference. Fiber optic sensors can overcome these difficulties. They are insensitive to electromagnetic interference and can withstand extreme conditions, including high temperature, high pressure (above tens of megapascals) and strong shocks and vibrations. They can measure wellbore and wellsite environmental parameters with high accuracy, while Optical fiber sensor has distributed measurement capability, can measure the spatial distribution of the measurement, and give profile information. Moreover, the fiber optic sensor has a small cross-sectional area and a short profile, and takes up very little space in the wellbore.
Fiber optic sensors have made considerable progress in the field of geophysical well logging. Major oil production companies, well logging service companies, and various fiber optic sensor R & D institutions and enterprises around the world have participated in the research and development process. In order to explore the application field of optical fiber sensors, this paper reviews the research and development of optical fiber sensors in the field of geophysical well logging, and hopes that its research can contribute to further improve the level of petroleum development.
2. Research progress of optical fiber sensors in logging
1. Monitoring of reservoir parameters
(1) Pressure monitoring
Due to the need of development plan, the management of the reservoir pressure needs to be particularly cautious. The purpose of this is to reduce the loss of crude oil caused by the exploitation below the bubble point pressure and reduce the overpressure of the reservoir during gas injection. Crude oil loss caused by squeezing crude oil into aquifers. The sensors used in traditional downhole pressure monitoring mainly include strain gauges and quartz crystal gauges. Strain gauges are affected by temperature and hysteresis, while quartz gauges are affected by rapid changes in temperature and pressure. During pressure monitoring, these sensors also involve problems such as installation difficulties and poor long-term stability. Downhole optical fiber sensors do not have the advantages of downhole electronic circuits, easy installation, small size, and strong anti-interference ability, which are necessary for downhole monitoring.
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. The developed sensor can work up to 175oC, 200oC and slightly higher temperature products are being developed, 250oC is the next goal of research and development. The pressure measurement error at different temperatures and pressures is within ± 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 150oC), working temperature The range is 25oC ~ 175oC. In 1999, the company conducted a pressure monitoring system test in the Baker field in California. The results showed that the system has very high accuracy and has been delivered to commercial sales. In 2001, the company's pressure sensor was installed in several wells of British BP company to monitor the stress changes, and the results showed that it had sufficient reliability.
TsutomuYamate and others from Schlumberger Oilfield Service Company Doll Research Center conducted a long-term study on the use of Bragg fiber grating sensors for downhole monitoring. They developed a temperature-insensitive side-hole Bragg fiber grating sensor, The maximum working temperature is 300oC, and the maximum measuring pressure is 82MPa. At the maximum measuring pressure, the sensitivity to temperature is extremely small, which can be applied to the pressure monitoring in the well.
(2) Temperature monitoring
Distributed optical fiber temperature sensors have the potential to provide a new way to monitor production and reservoir by continuously collecting temperature data along the entire completion length. Because changes in the well's temperature profile can be compared with other surface data collected (flow rate, water cut, wellhead pressure, etc.) and open-hole logging curves, thereby providing operators with qualitative and quantitative information about changes occurring downhole. Traditional temperature measurement tools can only measure the temperature of a certain point in any given time. To test the full range of temperature, point sensors can only be realized by moving back and forth in the well, which inevitably affects the balance of the environment 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 state in the well is not affected. And because the optical fiber is placed in the capillary tube, the distributed fiber temperature sensor can be tested wherever the capillary tube can be reached.
One of the most widely used optical fiber sensors for downhole monitoring applications is the Raman backscatter distributed temperature detector. This method has been widely used in measuring wellbore temperature profiles (especially in steam flooding wells). The distributed temperature sensor should consider the measured points and connector attenuation comprehensively. The problems encountered and solutions are:
a. The problem of signal attenuation by optical fibers and connectors, the solution is to minimize the number of connectors, use Bragg fiber grating sensors, and improve the performance of the connectors;
b. It is easy to damage when installed underground. The solution is to equip skilled workers, fiber optic sensors need an external protective layer, and reduce stress (including perforation and temperature-induced stress). For the optical fiber distributed temperature sensor system, the British Sensa company has always been in a leading position in technology, a series of products come out, and in cooperation with major oil companies, actively explore the application of optical fiber distributed temperature sensors in oil wells. CiDRA has also been researching optical fiber temperature sensors. At present, the company's temperature sensor technical indicators are: measuring range 0 ℃ ~ 175 ℃, accuracy ± 1 ℃, resolution 0.1 ℃, long-term stability ± 1 ℃ / yr (150 ℃ Under continuous use).
One of the most important shortcomings of current optical fiber temperature and pressure sensors is the temperature and pressure cross-sensitivity. How to eliminate or use this cross-sensitivity is a hot research topic.
(3) Multiphase flow monitoring
In order to do well in reservoir monitoring and oil field management, the most critical link is to obtain a stable and reliable total flow profile of production wells and water injection wells and the holding rate of fluids in all phases. 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 measuring and analyzing the production status of the oil well using conventional production logging equipment. The friction of the liquid in the tubing and the injection from the reservoir into the wellbore make the differential pressure density instrument unable to accurately measure, and the electronic probe cannot detect the small oil bubbles in the liquid.
There are two methods for measuring multiphase flow through optical fiber. The first one is Schlumberger's gas holding rate optical fiber sensor, which can directly measure the gas holding rate in multiphase flow. Its four optical fiber 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 light signal reflected by the probe is used to determine the gas holding rate and the amount of foam (Both are related to gas flow). In addition, the measurement value of each probe is used to establish an image of the gas flow in the well. These image data are particularly suitable for inclined wells and horizontal wells, which can better understand the flow pattern of multiphase flow and explain these flow patterns under inclined conditions. Inherent phase separation. Recently, this instrument has successfully conducted logging experiments around the world. The information provided by it can directly measure and quantify the gas and liquid in the multi-phase mixture, can accurately diagnose the borehole problem, and help the production adjustment. The instrument passed the field test of 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, also suitable for multi-phase mixed flow system. The volume fraction of each phase fluid is determined according to the sound velocity of the mixed fluid, that is, the volume fraction of each single phase flowing through the flowmeter is measured (that is, the holding ratio measurement). Whether a fluid phase hold ratio is equal to the flow volume fraction of the phase depends on whether the phase has severe slippage relative to other phases. For the oil-water two-phase mixed flow system without severe slippage, a uniform flow model can be used for analysis; for the flow state with severe slippage, a more complete slippage model must be applied to interpret the data measured by the flowmeter to accurately determine Flow of each phase. The flow cycle experiment shows that for oil-water mixed fluids, the long-wavelength sound velocity measurement of the flowmeter can determine the volume fraction of each phase (ie, hold-up ratio), and is not affected by flow heterogeneity (such as laminar flow).
CiDRA has tapped the inherent advantages of fiber optic sensors and developed a downhole optical phase multiphase flow sensor. Current samples are limited to measuring quasi-uniform fluids: such as two phases of oil and water or three phases of oil, water and gas (the volume fraction of the gas phase is less than 20%). In order to investigate the performance of this new type of optical fiber multiphase flow sensor for measuring oil / water / gas three-phase in production wells, CiDRA recently conducted experiments in a test well. Oil, water and gas are mixed in the test well. The mixture includes oil with viscosity 32API, water with 7% salinity 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 holding capacity of the crude oil and brine mixture, and its error is within ± 5% in the full range of water holding capacity to meet the production requirements. In addition to being able to measure the water holding capacity, the instrument also tested the volumetric content of the gas in the three phases, only the ratio of oil to water in the test is known. The results show that the instrument can find the percentage of gas volume in the liquid in the form of foam flow outflow.
2. Sound wave measurement
Compared with the past, exploration and development companies now face greater risks and more complex drilling environments, so it is of great significance to obtain accurate formation structure maps and reservoir mechanisms. Currently used seismic measurement methods, such as towed floating cable geophone sets, temporary seabed deployment geophones, and underground cable deployment geophones, etc., can provide measurement of the target oil production area, but these methods have relatively high operations The cost is that it cannot be run into the well or is restricted by environmental conditions. Moreover, the images provided are not comprehensive and discontinuous, and the resolution is not very high, so it is difficult to achieve continuous real-time reservoir dynamic monitoring.
An optical fiber-based downhole geophone system can solve these problems. It can provide permanent high-resolution four-dimensional reservoir images throughout the life of the well, which greatly facilitates reservoir management. Such downhole seismic acceleration geophones can receive seismic waves and process them into images of formation and fluid fronts.
Permanent downhole optical fiber 3-component seismic measurement has high sensitivity and directivity, and can produce high-precision spatial images. It can not only provide images near the borehole, but also provide formation images around the borehole. In some cases, the measurement range can reach thousands. foot. It runs 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. It is encapsulated in a 2.5cm diameter protective shell and can withstand strong The impact 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. Its signal bandwidth is 3Hz to 800Hz, which can record the equivalent response from extremely low to extremely high frequency.
3. Laser optical 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 more popular abroad, and there are a large number of relevant research papers in the United States, Germany, Russia and Belgium.
Laser fiber optic nuclear sensors are produced on the basis of fiber optic communications and fiber optic sensors. It utilizes physical effects such as photo-induced loss and photoluminescence. It has more advantages than conventional nuclear detectors and is a typical interdisciplinary subject. Optical fiber nuclear logging technology is actually nuclear detection technology in a specific environment, and its typical advantages are:
(1) Sensitive probes in this range can be developed for different nuclear detection energy levels.
(2) Because of the application of the photoluminescence effect, the probe can be located down to a kilometer downhole, and the photomultiplier tube is connected to the well by the transmission optical cable, away from the harsh downhole environment (high temperature and high pressure), thereby extending its service life.
(3) The optical fiber has high-speed, large-capacity transmission capability, and can also carry other downhole instrument signals.
However, laser fiber nuclear detectors also have shortcomings, which are mainly manifested in high temperature and high pressure resistant protective coatings, the mechanical strength of the transmission cable and the low attenuation loss of the radiation resistant transmission cable.
3. Conclusion and Prospect
It can be seen from the analysis in this article that the optical fiber sensor with its unique advantages can be widely used in the monitoring of reservoir parameters (including temperature, pressure and multiphase flow), acoustic wave detection and laser optical fiber nuclear logging in oil and gas wells. It has greatly enriched the oil and gas company's understanding of the reservoir and facilitated the optimization of oil and gas field exploitation and maintenance. It is worth mentioning that the system can obtain the injection pressure and temperature of injection and production in time to determine whether the pressure exceeds the standard, thereby preventing the casing damage caused by the pressure exceeding the standard. This is a brand new field. Report and introduction.
So far, major oil production and service companies all over the world have invested huge amounts of money to research and develop the application of optical fiber sensors in reservoir evaluation. There are also quite a few optical fiber sensor R & D institutions dedicated to this emerging field of work . It is conceivable that after overcoming its own shortcomings and disadvantages, the next-generation optical fiber sensor will be promoted in a large area, which can more effectively help to understand the level of oil and gas production dynamics in real time. Major oilfield companies can make full use of these favorable information to achieve and maintain the optimal production of oilfields, so as to achieve the highest oil recovery in the reservoir. 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 allows the production and service companies related to the oil industry to more effectively analyze and evaluate assets around the world.
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