Chapter 1 - Introduction to Distributed Fiber-Optic Sensing in Upstream Oil & Gas

1.01 Introduction and Science of DAS and DTS (7 min.) Sample Lesson Quiz: 1.01 Introduction and Science of DAS and DTS
1.02 Downhole Applications (6 min.) Quiz: 1.02 Downhole Applications
1.03 Fiber-Optic Cable Types (4 min.) Quiz: 1.03 Fiber-Optic Cable Types
1.04 Deployment of Fiber-Optic Cables (8 min.) Quiz: 1.04 Deployment of Fiber-Optic Cables
1.05 DFOS Data Acquisition (13 min.) Quiz: 1.05 DFOS Data Acquisition
1.06 Introduction to DFOS Data (9 min.)
1.07 DAS Strain Introduction (4 min.)
1.08 Introduction to DFOS Applications (7 min.)

Chapter 2 - Hydraulic Fracture Stimulation (Treatment Well)

2.01 Project Design & Setup (12 min.) Quiz: 2.01 Project Design & Setup
2.02 Principles of Distributed Fiber-Optic Sensing - Part 1 (19 min.) Quiz: 2.02 Principles of Distributed Fiber-Optic Sensing - Part 1
2.03 Principles of Distributed Fiber-Optic Sensing - Part 2 (9 min.) Quiz: 2.03 Principles of Distributed Fiber-Optic Sensing - Part 2
2.04 Principles of Distributed Fiber-Optic Sensing - Part 3 (11 min.)
2.05 Case Studies (4 min.)

Chapter 3 - Cross-Well Strain and Frac Hit Detection of an Offset Well

3.01 Project Design & Setup (11 min.) Quiz: 3.01 Project Design & Setup
3.02 Principles of Measurement (13 min.) Quiz: 3.02 Principles of Measurement
3.03 Advanced Interpretation (15 min.) Quiz: 3.03 Advanced Interpretation

Distributed Fiber-Optic Sensing in Upstream Oil and Gas / Chapter 1 - Introduction to Distributed Fiber-Optic Sensing in Upstream Oil & Gas

Lesson 1.01 Introduction and Science of DAS and DTS

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Transcript

01. Lesson 1.01: Introduction and Science of DAS and DTS 02. Outline 03. Distributed Fiber-Optic Sensing 04. Distributed Measurements in a Well 05. Optical Time Domain Reflectometer (OTDR)06. The Science Behind DAS 07. The Science Behind DTS

01. Lesson 1.01: Introduction and Science of DAS and DTS

Hello, my name is Kevin Boone and this course is on Applications of Distributed Fiber-Optic Sensing in the Upstream Oil and Gas Industry. I bring 10 years experience in fiber-optic services with focus on unconventional reservoirs and heavy oil. At the end of this course, you will have the knowledge and skills to setup a fiber-optic sensing project, understand the data collected, and how to use the data to make important design changes to your wells in completions and production.

02. Outline

Here's the outline for this course. Let's begin with an introduction to distributed fiber-optic sensing in upstream oil and gas.

03. Distributed Fiber-Optic Sensing

Distributed fiber-optic sensing takes a standard fiber-optic cable, as used in the telecommunications industry for example, and turns it into an array of sensors that are distributed across the length of cable. An instrument called an interrogator unit sends pulses of light into the fiber andsamples the backscattered light in the same instrument. Various technologies allow the data to be processed into acoustic, strain, or temperature data. Distributed temperature sensing technology has been used in oil and gas applications commercially since the 90s while distributed acoustic sensing has been used starting in the early 2010s. Both technologies are complementary and are often used together to improve data analysis and interpretation for many applications.

04. Distributed Measurements in a Well

In the history of downhole measurements and through to today, single point sensors have been the primary method of acquiring well data. You will all be familiar with moving a wireline tool up and down a well or installing a pressure gauge with a control line, as examples. Other methods to gather more simultaneous measurements involve deploying an array of multiple sensors. An example may be a geophone string or point fiber-optic sensor, such as Bragg Gratings. But a true distributed sensor consists of a continuous spatial array of sensors that enable full coverage of a well. At any and all depths that the fiber-optic cable covers, valuable sensing data can be acquired and interpreted for a better understanding of the subsurface.

05. Optical Time Domain Reflectometer (OTDR)

An OTDR device is an optical test instrument used to detect light loss in a fiber by injecting laser pulses into the core and then measuring the subsequent Rayleigh backscatter at all points along the fiber. It is used to characterize a fiber by determining the loss profile, identify step losses and reflections, fiber breaks, and connector problems. It shares some measurement principles with both DAS and DTS in that it measures Rayleigh backscatter like DAS, but also averages the measurement over time intervals like DTS to reduce the noise. They're available as handheld battery powered instruments that allow quick and easy testing of both single and multi-mode fibers. And they're also built into interrogators so fiber health can be checked via remote access to the interrogator or can be built into the acquisition program to take OTDR measurements periodically as defined by the user.
Here are some drawings of typical OTDR events along a fiber and how they look in the OTDR measurement data. The overall slope of the OTDR trace can be used to assess the loss profile that may change due to effects such as hydrogen darkening, where hydrogen atoms migrate into the glass and absorb light. Reflections are shown as peaks which are undesired for distributed sensing, but are there by design in the case of Bragg Gratings, and step losses or gains as near vertical shifts in the loss profile. In general, it's important that the combined loss due to attenuation and step losses does not exceed the optical budget of the instrument to be used. The optical budget is a performance specification of an interrogator. It is the maximum tolerable total loss of a fiber before data becomes inaccurate or unreliable. Interrogators with high optical budgets can be particularly important for very long fibers, fibers with large unavoidable step losses and fibers experiencing degradation. An OTDR is also a great way to assess the fiber health in a time-lapse manner to predict longevity of the fiber in the current downhole conditions.
An ideal fiber profile will have a constant slope matching the fiber specifications, for example 0.2 dB / km, with no significant step losses or reflections anywhere along the fiber. The fiber end will either have a sharp drop at the end of the fiber signifying no additional backscattered light, or a reflection followed by a sharp drop. Optical power beyond this point represents the noise floor of the OTDR instrument. Note that some older generation fiber-optic sensing systems require specific fiber terminations to remove the end reflection for optimal data quality right up to the end of the fiber.

06. The Science Behind DAS

Now, we'll talk about how the basic principles of DAS actually work. We interrogate the fiber by pulsing coherent laser light through the entire length of the fiber. As each pulse travels down the fiber, a very small fraction of the light is scattered as it interacts with the molecular imperfections in the glass and is reflected back towards the interrogator. For a given narrow incident wavelength shown here by the dotted black line, a distribution of wavelengths are produced in the scattering process. For DAS, we are interested in the Rayleigh scattering, the same wavelength of the incident light pulse. Other peaks are used for different sensing technologies. When the backscatter light returns to the interrogator, it is amplified before reaching the photodetector where a voltage is measured that is proportional to the backscatter intensity. A backscatter profile of the fiber of intensity vs. time (or distance, knowing the speed of light) is obtained and stays constant until a strain or temperature event occurs along the fiber. If an acoustic signal is introduced somewhere along the fiber, the backscatter will change locally based on the input signal. The fibers pulse 1,000s of times per second, allowing us to calculate the properties of the input signal such as frequency and amplitude. This is a simplified explanation and the processing is more complex but these are the basic principles. There are also newer technologies available that measure phase instead of intensity only for improved data in certain applications.

07. The Science Behind DTS

Now, we'll look at DTS technology. The fiber is interrogated in a similar way to DAS. This time we're interested in the Raman backscatter. This time we're interested in the Raman bands of the backscatter spectrum. The Raman bands are orders of magnitude weaker than the Rayleigh backscatter and are a result of atomic and molecular vibrations. The Brilluoin bands are associated with lattice vibrations and are very close to the Rayleigh band. Raman bands are shifted about 40 nm above and below the incident wavelength or Rayleigh backscatter peak and are referred to Raman Stokes and Anti-Stokes bands. A phenomenon is present where only one side of the Raman bands, the Anti-Stokes component, is temperature dependent. A relationship between the Stokes and Anti-Stokes bands has been developed as shown, which takes the ratio of these 2 peaks, along with the other references and constants, to calculate absolute temperature at each point along the fiber. Due to the fact that the Raman bands are very weak compared to the other backscattered signals, longer measurement times are required in order to reduce noise and increase the temperature resolution of the output. Measurement times may range from 10 sec to 10s of min. The shorter the measurement time, the noisier the temperature output but the faster the update rate. This trade off should be considered depending on the application of the data.