What's new in production

	Vol. 230 No. 6
DAVID MICHAEL COHEN, MANAGING EDITOR

Seismic stimulation shakes up the reservoir. 

As oil companies seek to squeeze the remaining oil from increasingly mature reservoirs, they are turning to more and more exotic methods of extraction. Among these is seismic stimulation, which generates acoustic waves to literally shake the oil out of otherwise depleted formations. Seismic stimulation potentially has significant economic and environmental advantages over established EOR methods such as gas injection, chemical injection and thermal techniques in that the operator doesn’t have to transport and pump downhole large quantities of valuable products.

Interest in harnessing seismic waves to produce oil began in the 1950s, when researchers observed that earthquakes and noise from railroad trains correlated with increased fluid levels and pressures in nearby water wells, and with increased production from oil wells. This observation led Soviet scientists to perform field trials using surface seismic sources in producing oil fields, with mixed results; production increased in some fields but actually declined in others.¹

These early efforts were followed by decades of research in the US, the Soviet Union/Russia and elsewhere to better understand the effects of seismic stimulation on fluid flow in porous media and to find more effective ways of exploiting those effects. Researchers have proposed a number of possible mechanisms for improved oil production in the presence of seismic stimulation, among them:

• Increase in relative permeability of the phases
• Increased rock permeability and porosity due to deformation of pores
• Mechanical vibration of pore walls “squeezing” fluids into adjacent pores
• Reduction of surface tension, density and viscosity as a result of heating by ultrasonic radiation.²

Recent research has proposed two broad categories of primary mechanisms by which seismic waves encourage oil production: 1) the seismic wave forces trapped oil droplets through pore throat constrictions that block their progress through the reservoir; and 2) the seismic wave causes individual oil droplets to coalesce into longer droplets that have a larger pressure drop along them, and therefore can more easily overcome the pore throat constrictions they encounter.³ For more on this, see the article “Mechanisms of seismic EOR” on page 49 of this issue.

Since 2001, a number of downhole tools have been developed and applied in the field for seismic EOR. One such device, developed by Eureka Oil ASA, a Norwegian company, applied seismic vibrations delivered by a hydraulically driven hammer and anvil along with AC electrical current into the reservoir. (See World Oil, November 2002, pp. 29–35.) Successful field trials of the tool were conducted from 1991 to 2000 by Petrobras in a Brazilian heavy oil field, by the manufacturer in Texas, and by PDVSA in the Orinoco heavy oil belt.

In 2003, a study funded by the US Department of Energy and Phillips Petroleum tested a downhole seismic stimulation tool developed by Seismic Recovery LLC. The tool used a unique orbital-vibrator design that generated backwards whirl to send strong vibrational forces at high frequency directly into the producing interval. (See World Oil, October 2001, pp. 57–66.) In a test run in Osage County, Oklahoma, the tool generated strong and clear seismic signals in wells more than 1,200 ft away from the well in which the tool was deployed. Unfortunately, the second phase of the field trial, to measure the improved recovery, was cut short after the tool became stuck in the well during a routine removal after only one week of operation. No improved oil recovery effect was measured, but in any case insufficient stimulation time had passed for an effect to be expected.⁴

The most consequential seismic stimulation trial to date has been Occidental’s pilot program at the declining Elk Hills Field near Bakersfield, California. Oxy initiated the program in October 2003 using Applied Seismic Research Corporation’s sucker-rod-operated downhole seismic device. The tool uses two plungers to compress gaseous fluid that is drawn into a barrel and then releases the fluid on the upstroke, creating a seismic shock wave. From initial production of 1,556 bpd in December 2003 for the 73 pilot wells within a ½-mi. radius of the stimulation well, output increased in about a year by 42% to 2,212 bpd (an increase of 60% based on the projected decline rate). During the same time period oil cut increased from 16.8% to 21.6%, for an overall increase of 28% (47% based on projected decline).⁵

The project’s success convinced Oxy to expand the program to its mature Wasson 72 carbonate field, located in West Texas on the northwest shelf of the Permian Basin. Only 75 million bbl (23%) had cumulatively been produced of the field’s 333 million bbl originally in place from 1940 to 2005, though waterflooding, CO2 injection and multiple phases of infill drilling had been applied. Operation of two downhole seismic devices there between January 2005 and September 2007 slowed the field’s historical decline from 14.5% to 12%, resulting in incremental oil production of 100,000 bbl. More importantly, the Oxy projects’ success led the Texas Railroad Commission in September 2007 to certify seismic stimulation as the first new EOR process in decades, giving operators who use it a 50% reduction in severance taxes for the first 10 years of production.3

There are still many mysteries to solve in seismic EOR, like why sometimes there is no increase in production, or an increase in watercut instead of oil. Much must depend on geology, and perhaps further research will discover a formula to determine the exact frequency and amplitude of stimulation to maximize oil production rate or EUR from any given type of reservoir. In the meantime, look for increased use of this up-and-coming EOR method in the next few years. 

LITERATURE CITED

¹  Jackson, S., Roberts, P. and E. Majer, “Advances in seismic stimulation technologies,” Petroleum Technology Transfer Council website, 2001, http://www.pttc.org/technology_summaries/statev7no2.htm.
²  Hamida, T. and T. Babadagli, “Effects of ultrasonic waves on immiscible and miscible displacement in porous media,” SPE 95327 presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, Oct. 9–12, 2005.
³  Kostrov, S. and W. Wooden, “Possible mechanisms and case studies for enhancement of oil recovery and production using in-situ seismic stimulation,” SPE 114025 presented at the SPE/DOE Improved Oil Recovery Symposium, Tulsa, Oklahoma, April 19–23, 2008.
⁴  Westermark, R., “Enhanced oil recovery with downhole vibration stimulation in Osage County, Oklahoma: Final report,” November 2003.
⁵ “Technology update: Value of in-situ seismic waves—Regain lost reserves, increase oil cut,” Journal of Petroleum Technology, April 2005, pp. 24–25.