Seismic used to shake up reservoir

Shockwaves Loosen Stuck Oil

American Association of Petroleum Geologists (AAPG)

Feeling a tad glum ’cause you’re saddled with an old field that’s on its last legs production-wise?

You just know there’s lots of grease still there, but you can’t afford the big bucks required for conventional EOR methods?

Maybe all you need to do is shake it up, baby – the reservoir, that is.

We’re talking “hydro-impact technology,” which uses seismic wave stimulation technology to shake loose the residual oil trapped in the reservoir around existing wells.

It’s simple to apply, and it’s easy on the checkbook.

The low cost, field-proven technology was developed by Plano, Texas-based Applied Seismic Research (ASR) to enhance oil recovery in mature oil fields by increasing the yield from producing wells, according to Bill Wooden, vice president of ASR.

The Texas Railroad Commission certified the technology in 2007 as an official EOR process – the first new EOR certification in more than 30 years.

The user of the technology receives a 50 percent reduction in severance tax on total production for a 10-year period. The operator is required to show only a positive response to the technology to qualify.

Russian scientists were the first to recognize that high-energy elastic waves could mobilize immobile oil – but the tool they designed for field application had significant drawbacks, according to Wooden.

He noted ASR decided to go straight to its own drawing board to develop their in-situ seismic stimulation technology.

All Shook Up

Here’s the way it works:

  • The seismic stimulation tool is installed similar to a tubing pump at the depth of the producing horizon in the wellbore.
  • The pumping unit provides power for the tool’s plungers to compress fluid drawn into a barrel.
  • The compressed fluid is released upon the upstroke, creating a high-energy seismic shock wave.

“These high energy elastic waves travel into the reservoir to dislodge and mobilize bypassed oil,” Wooden said. “Seismic stimulation appears to work primarily on a mechanical level, shaking droplets from an immobile state so they migrate in the flow stream.”

He noted the seismic waves encourage oil production via a couple of mechanisms:

  • The force of the wave pushes trapped oil droplets through constricted pore throats that blocked their movement through the reservoir.
  • Individual oil droplets combine into extended droplets that can more easily surmount the pore throat constrictions.

It’s long been recognized that earthquakes can stimulate field production that ordinarily trails off the day following the event. In comparison, the ASR tool pounds the reservoir continuously with earthquake-like events.

“It creates seismic waves every 10 seconds,” Wooden said. “These subsurface shockwaves mimic primary waves generated by earthquakes, and that’s why they shake the oil loose. The operating lifespan of the tool is eight to 12 months, after which it is pulled and replaced.

“The waves produce power in the range of one to 10 million watts, and pressure at the wave front can exceed 4,000 psi,” he said. “Los Alamos studies showed a pressure disturbance as low as 0.01 psi dislodged oil droplets.

“The seismic wave travels about one-and-a-half miles per second,” Wooden noted. “Because it travels through the wellbore casing and cement within microseconds, it doesn’t damage the wellbore or formation.

“Typically the area stimulated is three-quarters of a mile out to a one-mile radius (both horizontal and vertical),” he said. “It can extend beyond a mile in some cases, such as carbonates like the Permian Basin.

“Producers within the radius of stimulation on average increase oil production and oil cut by 10 to 20 percent,” Wooden added.

He noted the unique thing about the seismic wave is that it stimulates through fault blocks, through subsurface horizons and can dislodge oil in separate producing blocks. In fact, the tool can be placed in the middle of a highly faulted field and it won’t matter because the wave goes everywhere.

Because in-situ seismic stimulation ignores boundaries, multiple horizons can exhibit equal stimulation effects.

Wooden noted they have observed successful results in sandstone, carbonate and diatomite formations when the technology is used in combination with waterflood or natural waterdrive.

After stimulating myriad types of reservoirs, ASR concluded that fields with significant heterogeneity, API gravity above 15, and GOR less than 2,000 scf/Stb respond best to seismic stimulation.

Elk Hills Case Study

The highest profile field applications of the technology thus far are Oxy’s Elk Hills project near Bakersfield, Calif., and several other operators’ ongoing programs in the Permian Basin.

Oxy initiated the Elk Hills program in 2003, installing ASR’s downhole in-situ seismic stimulation tool in a sealed wellbore above a cement cap. At that time oil production from 73 pilot wells within a half-mile radius around this well was 1,556 barrels per day.

Close to a year after stimulation began, production had increased by 42 percent to 2,212 bopd – an increase of 60 percent based on the projected decline, according to Wooden. At the same time, oil cut increased from an average of 16.8 to 21.6 percent for an overall oil cut increase of 28 percent, or 47 percent based on the projected trend line.

The Elk Hills project continued for three years before a management turnover apparently led to a decision to operate the field differently from the past.

The success at Elk Hills convinced Oxy to implement the Wasson 72 field program in the Permian Basin. The ongoing project kicked off in 2005 with the installation of two of the ASR stimulation tools.

Of the field’s 333 million barrels of OOIP, only 75 million barrels, or 25 percent, had been produced between 1940 and 2005 despite the application of waterflooding, CO2 injection and considerable infill drilling.

Implementation of the seismic stimulation procedure reportedly slowed the field’s historical decline from 14.5 percent to 12 percent and resulted in incremental oil production of 100,000 barrels between January 2005 and September 2007.

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In Summary

Bill Wooden, vice president of Applied Seismic Research (ASR), succinctly summarized the key benefits of seismic stimulation technology:

  • Stimulation crosses fault blocks and horizons.
  • Effective in sandstones, carbonates and diatomites.
  • Simple to install.
  • No maintenance.
  • Eight to 12 months of continuous stimulation.
  • Low cost relative to all other EOR methods.
  • Dramatic improvements in recovery and improved production.

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