A comparison of technologies for monitoring, reporting and verification in wellbore sites for oil and gas production and geologic carbon sequestration. Guest Post by Jim McMahon
Monitoring, Reporting and Verification
Monitoring, reporting and verification (MRV) associated with oil and gas drill sites, and CO2 geologic sequestration (GS) sites, is an important component to provide critical data about the downhole well environment and monitor well conditions.
MRV informs engineers and scientists about the distribution of physical changes within and near the external wellbore environment. It ensures production performance enhancement and safety at the well site, optimizing production from oil and gas wells. Similarly, it ensures the integrity of CO2 geologic sequestration in the targeted formations to prevent movement into underground sources of drinking water (USDW), while confirming that injection zone pressure changes follow predictions.
MRV mandates address the following: a) Site characterization with an assessment of the geologic, hydrogeologic, geochemical and geomechanical properties of the proposed site to ensure that wells are located in suitable formations; b) Well construction using materials that can withstand contact with drilling fluids, or CO2, over the life of the site; c) Computational modeling that accounts for the physical and chemical properties of the injected drilling fluids, or CO2 , based on available site characterization, monitoring and operational data; and d) Periodic re-evaluation to incorporate monitoring and operational data, and verify that the drilling fluids and oil, or CO2 plume, and the associated area of elevated pressure, are moving as predicted within the subsurface.
Oil and gas drilling sites, and GS sites, are required to develop and implement a site-specific MRV plan approved by the U.S. Environmental Protection Agency (EPA).
Robust oversight must be maintained of the oil and gas streams, and CO2 streams, injection pressures, integrity of the injection well, groundwater quality and geochemistry. Additionally, comprehensive monitoring following cessation of drilling or CO2 injection is required to demonstrate that neither poses an endangerment to USDWs.
The EPA recognizes that monitoring and testing technologies used at oil and gas, and GS sites will vary and be project-specific, influenced by both geologic conditions and project characteristics. At certain sites additional monitoring may be needed.
Well-Based Monitoring Technologies for Oil & Gas Production, and Geologic Carbon Storage
Monitoring techniques are key tools to monitor geomechanical changes, such as formation, deformation and failure resulting from oil and gas drilling, and geologic CO2 sequestration.
Various geophysical techniques have been used to monitor subsurface wellbore conditions, these techniques include:
- Geodetic Monitoring
- Subsurface Micro-Seismic Monitoring
- Gravity Monitoring
- Electrical Resistivity Tomography (ERT)
- Geochemical Sampling
- Distributed Fiber Optic Sensing (DFOS)
Geodetic Monitoring
Geodetic monitoring, including global positioning system (GPS) monitoring, and Interferometric Synthetic Aperture Radar (InSAR), measure displacements and strains, both on the surface and within the interior of the Earth. Space-based InSAR is a geodetic technique for remote monitoring of land-based oil and gas production, and CO2 storage sites.
Oil and gas drilling, and CO2 injection may cause the Earth’s surface to deform, and geodetic monitoring is an approach to monitor reservoir integrity and detect possible CO2 leakage. The technique involves the repeated measurement of the deformation of Earth’s surface.
Subsurface Micro-Seismic Monitoring
Seismic monitoring can use active seismic surveys or micro-seismic events induced by oil and gas production, and CO2 injection and migration. Micro-seismic monitoring uses sensors/geophones deployed on the surface covering the monitoring region, or sensors/geophones in one or more boreholes to monitor induced micro-seismic events smaller than what surface seismic arrays can detect.
Active seismic monitoring uses time-lapse seismic reflection/transmission data. The underlining physical principle of this method is based on the effects of injecting fluids into oil and gas wellbores, and injecting supercritical carbon dioxide into subsurface elastic parameters. Drilling fluids and CO2 injection and migration alter elastic parameters such as compressional and shear velocities, density, and seismic attenuations in geologic formations.
Time-lapse 3D or 4D seismic monitoring are considered effective tools for 3D subsurface monitoring of oil and gas wellbores, and CO2 injection and migration. However, timelapse 3D seismic surveys and data processing are costly and time-consuming.
Gravity Monitoring
Oil and gas drilling, and CO2 storage sites, may change the mechanical state (effective stress) of the underground, causing subsurface mass redistribution. Lower density CO2 displaces higher density brine, for example, which results in reduction of bulk formation density. Time-lapse gravity monitoring is sensitive to bulk density changes. Gravity sensors can be deployed on the ground surface or in a borehole.
Because oil and gas wells, and CO2 storage reservoirs are often located at a large depth, and spatial resolution of gravity monitoring decreases with depth, there are limited applications where gravity monitoring can be applied.
Electrical Resistivity Tomography (ERT)
The injection of fluids for oil recovery, and liquified CO2 for storage, results in increased resistivity which may be detected by electrical and electro-magnetic (EM) imaging techniques: such as electrical resistivity tomography; magnetometric resistivity; and complex resistivity.
With downhole electrodes close to the target of interest, ERT can characterize the temporal and spatial resistivity changes effectively.
Geochemical Sampling
Geochemical sampling is used to assess the results from the injection of recovery fluids into oil and gas wells, and CO2 rock-water interaction, to understand the integrity of the well, and with CO2 storage, the condition of reservoir seals.
Numerous methods have been devised to obtain representative downhole samples while maintaining reservoir pressure conditions.
Distributed Fiber Optic Sensing
More recent oil and gas wells, and CO2 sequestration projects have implemented significantly more sophisticated strings of multi-function deployed sensors, aimed at increasing the amount and quality of information available from boreholes. This, to more fully understand oil and gas drilling, and the movement and distribution of CO2.
The deployment of fully distributed fiber optic sensors into deep wells to monitor acoustic vibrations, mechanical strain, reservoir temperature and reservoir pressure distribution, in support of oil and gas down hole applications and CO2 injection, has advanced considerably over the last decade.
Distributed fiber optic sensing (DFOS) is a technology that enables continuous, real-time measurements along the entire length of a fiber optic cable at very fine spatial intervals. Unlike conventional sensor systems that rely on discrete sensors measuring at pre-determined points, distributed sensing does not rely upon manufactured, discrete sensors, but in contrast utilizes the optical fiber itself as both the sensing device and as the two-way transmitter of the signal (light). The optical fiber is the sensing element without any additional transducers in the optical path.
Surface instruments called interrogator units (IU) send a series of laser light pulses into the fiber and record the return of the naturally occurring back-scattered light signal as a function of time. In doing this, the distributed sensing system measures at all points along the fiber which are at a pre-determined clock-time interval over periods of well operational time.
Because a fiber optic cable can be installed in harsh environments for long periods of time, the technology holds promise for environmental monitoring of sensitive subsurface operations. Many geofluid systems, including oil and gas wells, and GS, require dynamic acoustic, temperature, strain and pressure monitoring at great pressure, depth and temperature. Sensing systems that employ downhole fiber optic cables serve particularly well for long-term well monitoring and well-integrity monitoring.
Distributed fiber optic sensing provides the critical capability of measuring multiple physical phenomena along the entire length of an internal borehole, as well as monitoring the conditions of the near-well bore region, outside of pipe subsurface rock formations, supporting verification and accounting of oil and gas production sites, and geologic carbon sequestration projects.
DFOS technologies that support GS include:
- Distributed Acoustic Sensing (DAS)
- Distributed Temperature Sensing (DTS)
- Distributed Pressure Sensing (DPS)
- Distributed Temperature and Strain Sensing (DTSS)
- Distributed Strain Sensing Rayleigh Frequency Shift (DSS-RFS)
- Distributed Acoustic Sensing (DAS)
DAS is mainly used to listen to hydraulic fracturing related signals, fluid and gas flow signals, or to sense seismic source response, such as in a Vertical Seismic Profile (VSP). DAS senses the changes in very small physical acoustic vibrations along a glass fiber optic strand that is encased in a cable to measure vibrations. There are thousands of detection points along the fiber in the subsurface fiber optic cable.
DAS technology permits tens-of-thousands of points down the well to be measured simultaneously every 2 meters. The continuous glass fiber strand inside the cable can sense very small acoustic vibrations at a large range of frequencies. These vibrations are most often related to injected fracturing fluid dynamics and fracture propagation, and growth associated with hydraulic fracturing physics. These measurements are very valuable to engineers who use the data to sense what is occurring deep in the well where they cannot see.
2. Distributed Temperature Sensing (DTS)
DTS measures temperatures by means of optical fibers functioning as linear sensors. Temperatures are recorded along the optical sensor cable, not at points, but as a continuous profile. A high accuracy of temperature determination is achieved over great distances compared to other methodologies. Typically, DTS systems can locate the temperature to a spatial resolution of 1m with accuracy to within ±1° C.
3. Distributed Pressure Sensing (DPS)
The measurement of pressure by using distributed optical fiber sensors has represented a challenge for many years. While single-point optical fiber pressure sensors have reached a solid level of technology maturity, showing to be very good candidates in replacing conventional electrical sensors, distributed sensors are still a matter of an intense research activity aimed at determining the most proper and robust pressure-sensitivity enhancement mechanism.
4. Distributed Temperature and Strain Sensing (DTSS)
Distributed Temperature and Strain Sensing (DTSS) technology augments DPS and DAS. The combination of all of these sub-surface measurements produce simultaneous and independent measurements that inform engineers and scientists about the distribution of physical changes in or near the external well bore environment, at centimeter-order to meter-order spatial resolution.
DTSS provides not only temperature, but also the absolute, differential and dynamic strain deformation profiles along the full length of optical fiber, over distances reaching up to tens of kilometers. And the spatial resolution of the DTSS measurements are typically an order of magnitude better than DAS. Spatial resolution is a measurement to determine how small an object should be in order for an imaging system to detect it. It is measured in line pairs per centimeter (lp/cm).
5. Distributed Strain Sensing Rayleigh Frequency Shift (DSS-RFS)
The latest generation of fiber optic sensing systems employed to monitor deep well conditions – Distributed Strain Sensing Rayleigh Frequency Shift (DSS-RFS) – is a truly transformative technology for augmenting operational performance at wellbore sites for oil and gas production and geologic sequestration (GS). Providing critical data about the downhole well environment from distributed fiber optic sensing, DSS-RFS improves the ability of engineers and scientists to more efficiently and effectively understand strain and temperature dynamics of the subsurface and support engineering operational, monitoring, reporting and verification activities and goals that support oil and gas production and GS.
DSS-RFS uses Rayleigh Wavelength, optically sourced backscatter in a nonengineered single mode silica (glass) fiber to measure strain and temperature changes along the fiber. Advanced through research, development and field application by Neubrex Ltd, Kobe, Japan, distributed fiber optic-based strain and temperature sensing measurements are made based on the frequency shift of the Rayleigh optical scattering spectrum, which is linearly dependent on strain and temperature changes applied to the sensing fiber. Strain changes along the wellbore are continuously measured at a fine spatial scale during operations of the well.
The principle of the DSS-RFS method can be explained accordingly: When an optical fiber is manufactured, random inhomogeneities of the glass density are created in the fiber core. The random density heterogeneities manifest as a variation of refractive index along the fiber. For a certain laser frequency, the constructive and destructive interferences between the Rayleigh backscatters cause irregular but unique amplitude fluctuations in the coherent optical time-domain reflectometer along the fiber length. For each discrete fiber segment, a unique Rayleigh scattering spectrum (like a fingerprint) is obtained by scanning the fiber with a coherent optical time-domain reflectometer with a range of laser frequencies using a tunable-wavelength laser system. This unique Rayleigh scattering spectrum shifts in frequency space if the temperature and/or mechanical strain on the fiber section changes, which causes the spacing and optical delay to vary between the scatterers. This change is detectable and measurable with Neubrex technology.
DSS-RFS technology permits tens-of-thousands of points down a fiber that is attached to a tubing string or casing string to be measured very quickly every 20 centimeters along the entire fiber length deployed in or along the wellbore. The continuous glass fiber strand inside the cable can sense very small physical length changes at a large range of frequencies. These measurements of thermally- or mechanically-driven strain change, as a function of time and depth, are valuable to engineers who use the data to gain an understanding of what is occurring deep down in the well. No other technology provides such insight.
Changes in temperature (degrees), strain (micro-strain unit), acoustics (dB, noise) and pressure (psi) can be made in real-time while oil drilling or CO2 injection is occurring. This helps field engineers understand what is happening in these deep wells much better than with previous discrete sensor-position technologies. Data driven changes or adjustments to operational plans or maintenance plans can then be made when warranted, to optimize the GS operation and make wells with better long-term drilling or sequestration performance, efficiency and efficacy.
DSS-RFS Application in Oil & Gas Drilling and CO2 Sequestration
DSS-RFS is employed in application by Neubrex Energy Services, the U.S. division of Neubrex Ltd. The company’s DTSS product line is known as Neubrescope®. It is actively deployed in North America in different operational settings, such as oil and gas, CCS and geothermal operations.
“Neubrescope DSS-RFS is well designed for monitoring wellbores for oil and gas production and geologic sequestration operations,” said Dana Jurick, Executive Vice President and General Manager of Neubrex Energy Services. “Nevertheless, companies involved in oil and gas, and GS, are still in the learning, testing, qualification and acceptance phase of using fiber optics and how they can be reliably, safely and economically installed, and used in a well and long term well operations.”
“Once installed in a well, operators are learning what measurements can be made, and how it differs from competing technologies,” added Jurick. “The value proposition of this technology application is actively being explored by many oil and gas, and GS companies, both domestically in the U.S. and internationally in numerous projects.”
About Neubrex Energy Services
Neubrex Energy Services (US) serves the oil and gas, and renewable energy industries with advanced field-proven fiber optics-based measurements systems. From unique fiber optic cables to surface-based optical interrogator units to advanced data processing systems, the Neubrex fiber optics measurement and results-delivery platform provides innovative and differential data products that are proven to be of value to its customers. Neubrex offers superior and fully-distributed measurements of acoustics, temperature, strain and pressure (in development). Our deliverables generate a clear understanding of subsurface dynamic physical processes and physical conditions, and provide key measurements that end users can rely on to make key-point, real-time and forward decisions.
For more information, contact Dana M. Jurick, Executive Vice President and General Manager, Neubrex Energy Services (US), LLC; 11125 HWY 159 W, Bellville, Texas 77418:
Phone 713-899-1545; email [email protected]; www.neubrex.com.
About the Author:
Jim McMahon writes on industrial, manufacturing and technology issues. His features have appeared in more than 4,000 business and trade publications worldwide.
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