With oil and gas demand outstripping supply, new discoveries lagging behind production rates, and the growing geological complexity of many fields, operators are facing increased pressures to reduce risk in their exploration efforts, maximize production, and accelerate cash flow.

As a result, technologies such as 3-D and 4-D seismic, electromagnetics, velocity modeling, and deformation monitoring techniques like GPS have come to the fore as a means of monitoring and generating accurate information from complex reservoirs and previously intractable geologies.

This figure shows a schematic representation of the measurable ground points using radar satellites. (Figures courtesy of TRE)

There is another technology, however, that is generating interest among operators – the ability to monitor reservoirs and generate surface deformation measurements from space. This is achieved through Interferometric Synthetic Aperture Radar (InSAR), a remote sensing technique based on satellite radar systems circling the globe, capable of accurately measuring ground displacement. Operators that are using the technology already include BP, ConocoPhillips, Devon, Eni, PDO, and Shell.

How InSAR works
Satellite radar sensors broadcast signals toward Earth, and illuminated targets reflect back-scattered signals to the satellite where they are read and stored. As the radar satellites regularly retrace over the same orbit, a sequence of temporally spaced images can be acquired over the same area – images that are compared to detect changes in ground surface and displacement.

Since InSAR came to prominence in the early 1990s through the European Space Agency and its ERS 1 satellite, which started building a catalog of interferometric data covering the entire globe, the technology has continued to develop and generate ground displacement information to increased levels of accuracy.

Milestones include a new multi-image approach called PSInSAR, which was developed at the Politecnico di Milano in the late 1990s, and the recent SqueeSAR algorithm, which Tele- Rilevamento Europa (TRE) began commercializing in January 2010.

The PSInSAR algorithm and its multi-image datasets made it possible to identify stable reflectors (referred to as

This figure shows the east-west cross section (left) through a cumulative vertical displacement map (top right) and the evolution of displacement that illustrates discontinuities, and an east-west horizontal displacement map (bottom right).

permanent scatterers or PSs), which typically are manmade structures such as buildings, bridges, and pipelines. PSs provide stable signals to the satellite sensor, allowing ground displacement velocities to be measured with greater accuracy.

TRE’s algorithm SqueeSAR has taken this a step further with the identification of a new set of ground targets – distributed scatterers (DSs). DSs consist of an extensive area where the back-scattered energy is less strong but remains statistically homogeneous within that area and can correspond to rock outcrops, non-cultivated lands, or desert areas.

SqueeSAR enables the user to process this energy and detect ground displacement in DS areas where there are poor PS levels. The end result is more effective insight into ground deformation and associated surface movements.

Today InSAR is more sophisticated, handling multi-image datasets, identifying PS and DS groundpoints, and providing millimeter accuracy on ground displacement values.

Applications that use InSAR include the characterization of areas vulnerable to landslides, ground subsidence detection, the monitoring of faults and earthquakes, and infrastructure stability monitoring.

Since TRE was founded in 2000, it has, through many satellite missions, processed SAR data over an area of more than 463,440 sq miles (1.2 million sq km) – four times the size of Italy.

Accuracy and value
What are the implications for the oil and gas sector? How is this technology from space addressing some of the operator challenges identified?

The benefits of SAR data can be broken down into two main areas – the accuracy and value of the data, and the cost.

InSAR provides accurate vertical and east-west data on ground displacement and changes at the reservoir layer – often to the nearest millimeter. This provides the operator with a strong understanding of subsurface dynamics and the effects of reservoir depletion. The high density of ground points means more measurement points are generated than with other traditional survey techniques such as GPS. Regular updates also take place.

The locations of the known faults were reactivated due to the active working of the reservoir and align well with the satellite data. Red areas indicate rapid changes in surface deformation velocity and are likely to correspond to active faults or fractures in the overburden.

The result of this high-density data is that key geologies and volumetric changes in the reservoir can be identified, and valuable constraints can be placed on the dynamic behavior of the reservoir. If an operator is pumping oil or injecting gas, displacements are a matter of centimeters and sometimes millimeters. InSAR can deliver in such detail.

Observation of spatial patterns of vertical displacement through the data also affords the operator insight into the structural geology of the reservoir by highlighting the location of active faults or fractures. For example, SAR data can detect dip-slip and strike-slip faults, which help determine the reservoir’s integrity and boundaries. The constant monitoring of surface displacements also can detect and locate reactivated seismic faults caused by extraction activities with millimetric precision and at a fraction of the cost of seismic surveys.

In addition, recent studies appear to demonstrate that surface displacement measurements can become a valuable tool in constraining geomechanical models and allow permeability estimations to be calculated for the reservoir layer.

InSAR also is becoming an important tool in monitoring compacting reservoirs that induce surface subsidence. With subsidence often leading to reduced well life and potential wellbore instability during drilling, the ability to accurately measure the deformation and identify non-depleted compartments is crucial to operators when making reservoir management decisions.

And, with historical ground deformation data available over large areas of Western Europe, North Africa, and the US, historical analysis also can occur, enabling the operator to reconstruct the evolution of surface deformation and track trends over the last 15 to 20 years.

Finally, the algorithm comes with higher resolution X-band sensors and faster repeat acquisition rates. X-band sensors come with a shorter wavelength, resulting in increased sensitivity to movement, increased spatial density of measurements due to the higher resolution, and higher temporal sampling rates due to shorter satellite revisiting times.

Cost and access
The non-invasive nature of the technology allows for quick, accurate, and efficient exploration of vast regions. There are no access implications when monitoring environmentally fragile and culturally sensitive areas when compared to other surface deformation monitoring techniques such as GPS, optical leveling, and tiltmeters – techniques that require ground-based surveys. The light footprint also is attractive to operators.

The density of data generated means that InSAR also is a cost-effective technology. Unlike traditional surveying techniques, the algorithm generates hundreds of measurement points per square mile and offers high precision and low costs over long periods.

Other oil and gas applications include gas storage, where the radar satellite analysis shows a strong correlation between the volume of gas injected in and out of a reservoir and surface displacements resulting from such activities; and CO2 storage, where InSAR can detect surface deformation and pressure changes in the subsurface due to the injection of CO2.

Reservoir monitoring in the Middle East
Using satellite data acquired between 2004 and 2007, TRE conducted an InSAR survey over a stacked carbonate reservoir in the Middle East as a means of measuring reservoir depletion.

The top reservoir is a mature gas reservoir at a depth of approximately 2,460 ft (750 m), which is known to compact. The lower reservoir is an oil reservoir at 3,937 ft (1,200 m) that is produced by waterflood techniques. Both reservoirs are intersected by a major graben fault as well as numerous additional extensional faults.

Approximately 500 producer and injector wells have been drilled in the area, so it is important to mitigate large-scale well failure caused by depletion of the reservoir.

Analysis of two datasets from the RADARSAT and ENVISAT satellites, using the PSInSAR algorithm, allowed the company to monitor both horizontal (east-west) and vertical surface deformation.

A comparison of the PSInSAR data with the known faults at the reservoir level revealed a correlation between the known fault system and the gradient of the subsidence field. Further, the inverted dataset also appears to be bounded by known faults in some areas. A comparison of the inverted dataset with pressure change maps (from reservoir simulation) also demonstrated a strong correlation between the predicted and estimated pressure changes.

The PSInSAR survey provided useful datasets that can be used for enhanced recovery strategies.

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