It's likely not something you would want to bring up for cocktail party patter, yet it has more far-reaching impact than you might think.
We're talking about all those creaks and groans going on inside the earth, making the downhole world a fairly noisy place.
These many noises actually are used as seismic sources in the realm of passive, or sourceless, seismic technology. As such, they have been harnessed for a number of applications, including mine fracture monitoring, geothermal reservoir performance and more.
Passive seismic events also have been used for some time as a sort of fringe application in the E&P industry — with little fanfare for the most part.
One such project, for example, involved an exploratory program in Greece where surface rocks and topography hindered adequate seismic imaging. Instead, buried seismometers were used to monitor small-earthquake activity for almost a year, and a well was drilled based in part on the results obtained.
Although the well was abandoned a few feet above the reservoir because of high pressure, project participants noted the passive data were quite compatible with the well data
Microseismic events triggered by earthquakes currently are being tested in other E&P projects. But not all applications are earthquake-dependent.
"I neither predict nor study naturally-occurring earthquakes, but I utilize microseismic detection in passive reservoir monitoring and hydraulic fracture monitoring, where it's useful for fracture treatment diagnostics and optimization," said Mark Puckett, business development manager and principle geophysicist, data and consulting services at Schlumberger.
"In fact, hydraulic fracture monitoring using microseismic detection has shown itself to be a rising new star in the arena of reservoir characterization."
Ahead of the Game
One reason a company elects to record fracture treatment microseismicity is to determine where, in fact, the frac goes.
"The microseismic events occur as the rock breaks," Puckett said, "so we're getting measurements in time of the rock breaking down. This gives a pretty good idea of what the geometry is.
"You can tell geometrically where events are occurring," he noted, "and when you bring it all together, it gives a good picture of reservoir symmetry, co-planarity and medium homogeneity. This enables a more accurate assessment of the treatment methodology being used.
"This knowledge helps optimize treatments for economy and performance," he added, "resulting in a more profitable operation for the operator."
To observe and measure microseismic events during a hydraulic fracture application requires an observation well, which is instrumented with either a permanent or retrievable array of triaxial geophones no less than 400 feet long. Orientation of the phones is determined by recording the perforation of the treatment well, or by discharging an impulsive source in the treatment well prior to the hydraulic fracture operation.
Surface equipment is set up for continuous monitoring and recording, even though systems only record the buffered data when an event is detected. The data are processed on site and results transmitted to the system associated with the fracturing operation. The data are then sent to a processing center for additional processing and event interpretation.
For an example, Puckett cited a project in a sandstone reservoir in West Texas where the fracture program model called for 400,000 gallons of slurry, meaning the fracs would be pricey.
During the effort, the operator learned about the microseismic technique as a fracturing diagnostics tool and recorded the microseismicity on the next well.
"The microseismic data showed maximum fracture half-length (only one side of the well is modeled in frac design) was achieved with only 270,000 gallons of slurry, reducing treatment costs by 35 percent," Puckett said. "The data also showed the achieved half-length to be 20 percent longer than predicted by the model.
"To reduce treatment cost, it's common practice to reduce the amount of proppant in the slurry and still maintain wing-length (actual length of the frac on a given side of the wellbore)," Puckett noted, "but this can negatively impact production performance. The microseismic data let the operator reduce costs without sacrificing proppant or wing-length."
Where hydraulic fracturing is required to achieve well flow, hydraulic fracture monitoring using microseismic detection can be a highly effective tool in reservoir management, answering an array of concerns, including:
- How efficiently a reservoir is being drained.
- The volume of hydrocarbons left behind.
- If field rules should be changed to reduce well spacing.
Wells subjected to frac treatment typically have low matrix permeability, limiting their drainage area. Also, they suffer from an asymmetric drainage pattern because hydraulic fracturing has a preferred azimuth. The geometry of the treated zone is sensitive to rock heterogeneities, including reservoir stress changes as a result of structure, stratigraphy and fluid replacement.
"We had a case where a central Texas operator was drilling a naturally fractured reservoir using hydraulic fracturing with a modeled half-length of 300 meters," Puckett said. "Initial production rates were good, but he noticed about a third of the wells watered out sooner than expected. To understand the impact of the natural fractures, the operator used microseismic measurements to map the standard hydraulic treatment on a well."
Instead of a symmetrical linear frac of 600 meters, the treatment interacted with the natural fractures to produce three linear elements: one along the expected azimuth and the other two parallel to the central element, offset by 300 to 400 feet on either side of the well. Total wing length was approximately 600 meters as predicted, but its geometry was completely unexpected, according to Puckett.
"Using this pattern as a template for updating the well pattern, it was discovered that watered-out wells were victims of new frac treatments 'walking' on existing treated areas," he noted.
"Wells drilled on the new pattern have not watered out, resulting in significant production revenues."