Monitoring of underground condition is becoming increasingly important for optimization of well bores
Real-time monitoring of underground well conditions is becoming increasingly important in the oil and gas industry. To gain the most accurate comprehensive picture of underground conditions, operators are using downhole fiber-optic sensors to accurately measure pressure and temperature in real time along the length of the wellbore. A wealth of useful information about well condition and operation can be determined from the sensor data to help fine tune and maximize well production. The collection and analysis of fiber optic sensor data greatly reduces uncertainty, enabling better operational decisions that can increase well production and significantly reduce environmental impact.
There are many challenges to successful well operation that can be overcome using fiber-optic well monitoring. The Neobrara shale formation is a good example of how fiber-optic sensors can help with well development and operation. Here operators need to locate areas of high natural fracture density that will be easier to tap and will likely produce higher yields. To be economically viable expensive well operations like hydraulic fracturing need to be limited to the wells with the highest potential for success. Data from fiber-optic well sensors can used to identify the best opportunities and optimize hydraulic fracturing processes to maximize production while minimizing water use and environmental impact. New and improved monitoring techniques are being deployed worldwide to overcome challenges like these and to increase the production from new and existing reservoirs.
Well Monitoring with Fiber-Optic Sensors
Accurate downhole monitoring, data collection and analysis are critical to optimizing production and minimizing environmental impact over the life of the well. Optical fibers are well suited for hazardous well conditions since they can withstand very high temperatures and pressures better than conventional electronics; and they do not need electrical power to operate, making them safer for use in explosive downhole environments. For permanent downhole installations, optical fibers are protected inside hermetically welded metal tubes that are typically filled with specialty buffer gels.
Currently, fiber-optic sensors are commercially deployed primarily for measuring real-time temperature and pressure data down the wellbore. By examining changes in these temperature and pressure measurements over time and at various depths, operators can determine key well production parameters such as changes in liquid levels, pressures and flow rates inside the well.
Distributed Temperature Sensors (DTS) are currently the most widely deployed fiber-optic well sensors. DTS offers a cost-effective way for obtaining thousands of accurate, high-resolution temperature measurements over time and along the depth of the well. This DTS method measures temperature using a multimode optical fiber instead of an array of thermocouples or thermistors, as was done in the past. DTS measurement methods are based on Optical Time-Domain Reflectometery (OTDR) and Optical Frequency-Domain Reflectometery (OFDR) techniques derived from telecommunication cable testing. Often the DTS sensor fibers are looped back up to the surface so measurements can be made from both ends of the fiber to improve accuracy.
There are also many ways to measure pressure down the wellbore using optical fibers. One of the more popular techniques uses single mode fiber with reflective fiber Bragg gratings written periodically along the length of the fiber. The pressure along the fiber is measured by injecting a wideband optical source down the fiber and looking at changes in the wavelength reflected off the Fiber Bragg Gratings at various locations along the fiber. A mechanical assembly is often placed on the outside of the fiber surrounding the Bragg gratings to transfer the pressure from the outermost casing to the fiber core and to enhance pressure sensitivity. Operators sometimes place several of these fibers down the same well with the gratings from different fibers staggered to get more pressure measurements and finer distance resolution. Since these types of measurements are typically temperature dependent, they are often used along with temperature DTS systems to improve the measurement accuracy.
Even with the most robust packaging and hermetic sealing, the fiber assemblies are still susceptible to physical damage in the harsh downhole environment and some damage and degradation over time is inevitable. For all kinds of downhole fiber optical measurements, operators are finding that they can improve measurement quality by compensating for the fiber aging and degradation over time. To do this they need to switch the well sensor fibers between multiple equipment test sets.
Many routine operations, such as hydraulic fracturing processes and steam injection, add enormous stress that can damage the sensor’s protective outer casing. Exposed optical fibers are sensitive to hydrogen and darken in the presence of water and other chemicals containing hydrogen in the well environment. Exposure to hydrogen dramatically increases the fiber loss locally around the affected areas. High well temperatures also accelerate fiber aging and darkening processes. Specialized OTDRs, adapted for use in oil and gas applications, are used to measure downhole fiber degradations over time. This allows operators to compensate for aging, improve the accuracy of measurements and extend the useful life of the fiber-optic sensors.
Matrix Optical Switching for Well Monitoring Benefits
Monitoring using fiber-optic sensors is now commonplace in extraction and production of crude oil and natural gas worldwide. But, the monitoring of multiple fiber-optic cables down the wellbore, combined with an increase in the number of wells being monitored, requires a more efficient way to interconnect the downhole fiber-optics with a variety of surface monitoring equipment. They can save significant capital expense, and improve measurement quality, by using all-optical matrix switches to interconnect, and share, surface test equipment with multiple downhole fiber-optic cables, whether in the same or different wells.
Key All-Optical Switch Requirements for Well Monitoring
To make accurate downwell measurements, an optical switch with true instrument grade performance is required. The key optical switch parameters for oil and gas well monitoring include low optical loss, high connection stability and reproducibility along with minimal reflections. Switches need to be format independent to pass any type of measurement signal. They also need to support dark fiber connections to work with intermitted test signals. All-optical matrix switches are the superior option as they have the best performance available in industry and are ideally suited to oil and gas well monitoring applications.
Ultra-Low Optical Loss
The loss performance of all-optical matrix switches is the lowest available in the industry today. Low optical loss is essential for fiber optic oil and gas well monitoring applications since the test signals are reflective and pass through the switch twice, once on their way down the fiber sensor and again when they are reflected back from fiber to the sensor.
Stability and Reproducibility
Truly instrument grade all-optical switches, with the superior stability and reproducibly, are critical for achieving accurate measurements. Stability determines how accurately the switch holds connections over time and reproducibility determines the variations in loss when making and breaking connections.
Optical Return Loss
Optical back reflections from the switch can interfere with reflection based OTDR and OFDR
measurement techniques that are used with fiber-optic sensors and degrade measurement performance. The high return loss of the instrument grade all-optical switching provides a level of performance that is not achieved by other switching technologies.
Dark Fiber Switching
Dark fiber switching is required for oil and gas well monitoring applications. Dark fiber switching allows the switch to make and hold connections without light being present on the fiber. All-optical switches are true dark fiber switches where switching occurs completely independently of the power level, color or direction of light on the path. With true transparency independent of wavelength, data rate or protocol, all-optical switches will work with all types of intermittent or constant optical test signals with minimal impact on system budgets.
Typical All-Optical Switch Field Deployment
A typical matrix optical switching field deployment for oil and gas well monitoring is shown in figure 1. Here optical switching is used to interconnect temperature and pressure downhole optical fiber-optic sensors from two wells to surface test equipment. In this example, two all-optical switches are used to interconnect both single mode and multimode fibers. In this application a single Optical Time Doman Reflectometery (OTDR) is shared among all the downhole fiber-optic sensors to monitor and compensate for fiber degradations and damage over time. Reflectometery (OTDR) is shared among all the downhole fiber-optic sensors to monitor and compensate for fiber degradations and damage over time.
Figure 1: A typical all-optical switch application for Oil and Gas well monitoring.
Benefits of All-Optical Switching
- Reduced equipment costs by allowing sharing of expensive test equipment
- Automates interconnections of downhole fiber-optic sensors and above ground test equipment
- Allows for 24/7 remote real-time monitoring of multiple oil and gas wells
- Improved flexibility allowing operators to make multiple types of measurements on the same fiber-optic sensors to maximize results
- Allows for remote troubleshooting of both fiber-optic sensors and test equipment
- Eliminates the need to manually make and break optical fiber connections
- Allow for fast recovery from equipment failures
- Simplifies integration of new test equipment and fiber-optic sensors
Operators are increasingly using downhole optical fiber instruments to provide a more accurate picture of the underground conditions. Improved monitoring of underground well conditions is a key part of the push to improve production from existing fields and to open up new areas where gas and oil was previously considered inaccessible. Operators are finding that they can reduce costs by using optical switching to create a higher level of interconnection flexibility between surface test equipment and downhole fiber-optic sensors. This flexibility enables remote operation and automation of testing that reduces costs by increasing equipment utilization and allowing for sharing of expensive test equipment between fibers and wells.