Distributed Acoustic Sensing Optimizes Hydraulic Fracturing
As conditions in the oil and gas market remain uncertain, the industry is faced with the challenge of maximizing recovery rates and production efficiency. Fundamentally, operators need to substantially reduce the break-even point on any given well.
Data from September 2014 highlighted that in some unconventional plays, such as the Eagle Ford shale, a high percentage of wells were not producing from all of the stages, and ultimate recovery was as low as 5 to 15 percent.
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This was despite an increase in drilling efficiency.
Significantly, this data comes from before the price crashes that dominated late 2014 and the first half of 2015. Of course the decline in prices has shifted the playing field for operators around the world — when oil is below $50 a barrel, 5 percent recovery rates mean that many wells are simply not economically viable. In a climate where every barrel counts, improving production efficiency for unconventionals is more important than ever.
The key challenge for operators trying to improve efficiency is during the hydraulic fracturing process. Well engineers have limited visibility during this activity, so overcoming this challenge relies on gaining more real-time intelligence. Operations now include the adoption and deployment of the latest sensing technologies to plan and monitor completion and production activities.
Hydraulic fracturing is inherently difficult to monitor and there remains significant uncertainty regarding various subsurface parameters including: fracture propagation geometry, cluster contribution and interference remediation among others.
One of the key sensing technologies is the Distributed Acoustic Sensor (DAS), which offers new levels of insight, changing the production economics in unconventionals when used in combination with the traditional Distributed Temperature Sensors (DTS) and micro-seismic monitoring. These three technologies working together give operators a multi-dimensional and dynamic profile of well conditions. Ultimately, the additional information DAS delivers is vital in removing the guesswork from down-hole activities.
By providing accurate and actionable intelligence to well operators, DAS enables real-time adjustments in each stage, as well as providing insights over a longer series of operations, maximizing efficiency and safety.
Fundamentally, DAS will minimize the resource requirements for each fracturing operation, thereby reducing the production cost for each well and enabling more wells to remain economically viable, even in a depressed market.
Hydraulic fracturing is inherently difficult to monitor. Sensing technologies such as DAS, DTS and micro-seismic monitoring provides insight into subsurface parameters and removes the guesswork from down-hole activities.
What is DAS?
DAS is a ground-based interrogator unit of light backscatter. By sending 1- to 2-minute pulses of light along fiber-optic cable, normally many thousands of pulses a second, DAS continually receives a small amount of that light in the form of backscatter. When sound and vibrations disturb the casing of the fiber, the characteristics of that backscatter are changed. DAS interrogates the backscatter for those changes to identify and locate the disturbance.
In down-hole applications the fiber-optic cable is run along the entire length of the well, either permanently installed outside the casing in cement or deployed by an intervention method, such as wireline, slickline or coiled tubing. Here the acoustic disturbance at each point along the fiber allows engineers to visualize and record what is going on down-hole in real time. These acoustic disturbances can be caused by everything from complications in a completion process to changes in flow dynamics.
This enhanced visibility allows engineers to gain a far greater short and long-term understanding of completion, production performance and well integrity. This not only helps to focus time and effort on value-adding activity, but also increases the success rate of operations and improve the efficiency and safety of exploration.
DAS systems are capable of being deployed on new wells or retrofitted to old. If in-well fiber optics is already in place, then DAS only requires the deployment of new surface equipment.
DAS and Frac Optimization
By providing a real-time log of the operation, DAS provides a clear indication of fracture success in the exposed formation. DAS is collecting and interpreting acoustic data to help build an understanding of the entire operation.
During the initial phases of the operation, DAS can track the wireline logging equipment that goes into the well. So as the perforating guns, or the devices to set the packers, are put into the well, DAS can provide assurance that they are on depth, that the packer is set properly and that the guns fire. In a traditional cement and perf operation, the DAS also enables engineers to see that the fracture balls seat on the right packer.
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During the operation DAS provides far more sophisticated insights to help engineers fully optimize the frac. With an accuracy of 1 to 2 meters, DAS not only assures engineers that the process is being carried out successfully, but confirms if there are any down-hole events that need to be addressed.
For example, engineers will see where integrity failures are causing cross stage communication issues or opening up previously fracked stages to frac fluid and potentially wasting significant amounts of resource. Although the operational impact depends on the severity of the integrity issue, DAS shows issues as soon as they occur, alerting engineers to cease activity and adapt to re-establish an effective and efficient operation.
Well engineers also face significant challenges in deciding how pressure is introduced in the frac and optimizing pumping application. Pumping too hard, too soon risks destroying the cement rather than penetrating the formation as intended. Furthermore, errors in the pumping application risk driving the fractures out of the formation of interest altogether — again leading to the wasting of water, proppant and time. By enhancing the fracture process design to optimize how pressure is introduced, DAS helps operators cooperate with the demands of the frac, minimizing cement damage and keeping the frac within stage and conclude jobs far more efficiently in terms of time and resources, rather than fighting the frac.
Fotech’s Helios DAS system provides an in-depth report within 30 minutes of completing a frac stage. This data helps engineers to analyze each stage and enhance performance.
Post-Op Analysis
In addition to accurate and reliable real-time monitoring, operators also want to ensure that production at each well site is being maximized, both immediately for each fracture operation and over a series of operations.
For this post-process analysis, Fotech’s Helios DAS system, for example, provides an in-depth data and visualization report within 30 minutes of completing a frac stage. This data helps engineers to rapidly analyze each stage and significantly enhance performance of each stage in a well incrementally. For an entire well bore Helios DAS quickly highlights the stages that have taken the most sand and fluid.
By analyzing the DAS data from a larger sample of operations, engineers determine operational inefficiencies and assess what may need to be changed in future. Furthermore, correlating both distributed temperature and acoustic data over time under flowing conditions also allows for new insight into flow profiles and reservoir response to surface operations. Divergence from an idealized temperature and/or acoustic profile can quickly identify and precisely locate and diagnose operational problems.
Conclusion
The combination of real-time monitoring and post-operation processes means that DAS presents a powerful new well surveillance tool to enhance production and diagnose operating problems. The increased intelligence that DAS provides gives the fiber-enabled well huge advantages. DAS delivers valuable insight into production-enhancing real time decision making, improving overall efficiencies and increasing the safety of all hydraulic fracturing operations.
In this way DAS is crucial in bringing down the overall break-even point for wells, meaning that more wells can remain economically viable, even while oil and gas prices remain low.
Tags: December 2015 Print Issue, Fotech Solutions, GE Sondex, Haliburton Energy Services, NuTechStuart Large is product line director for oil and gas at Fotech Solutions. He has worked in the oil and gas industry for more than 16 years and has a wealth of industry experience and technical expertise. Large started his career as a field engineer at Haliburton Energy Services, before spending 10 years at GE Sondex and then joining Fotech from NuTech Energy Alliance.
