Table of Contents

Vol. 7, No. 2
2nd Quarter 2001

Advances in Seismic Stimulation Technologies
by Susan Jackson, RMC, Inc. for the National Petroleum Technology Office; Peter Roberts, Los Alamos National Laboratory; and Ernie Majer, Lawrence Berkeley National Laboratory

Seismic wave stimulation technology has the potential for being a relatively low-cost procedure for enhancing oil recovery in depleted fields, or returning some shut-in wells to production. Tests indicate that potential is greatest in fields with high water-cut and large amounts of immobile oil, making mature domestic reservoirs a prime target. Field tests of the technology, however, have yielded promising but sometimes mixed or inconclusive results.

Interest in seismic stimulation started in the 1950s with observed correlations between water well level and seismic excitation produced from railroad trains and earthquakes. Similar effects were observed in producing oil fields where distant earthquakes caused increases in production, and wells close to operating machinery, highways, or railroads appeared to produce more oil than wells in quieter areas. Early research in Russia explored surface Vibroseis stimulation tests. Research and interest have increased and, although much remains to be learned, knowledge is at a critical stage where continued R&D will have a high probability of answering key questions regarding when, where and how it works, so it can then be more reliably used.

Mechanisms of Increased Recovery

Mechanisms responsible for improved recovery are not well understood and remain the subject for further research. The following mechanisms have been proposed to explain the changes in fluid flow characteristics resulting from seismic stimulation.

• Changes in wettability. Some laboratory work indicates that the wettability of a core saturated with oil can be made more water wet, resulting in increased oil recovery rate by waterflooding in conjunction with seismic stimulation.
• Coalescence and/or dispersion of oil drops. It is thought that attractive forces acting between oscillating droplets of one liquid in another (Bjerknes forces) induce the coalescence of oil drops, enabling continuous streams of oil to flow.
• Reduced viscosity. Laboratory work has indicated that, immediately after a 30- to 60-minute long exposure to acoustic energy, oil viscosity dropped by 20-25%, then gradually returned to pretreatment level over a 120-hr period. 
• Surface tension. Under some theories, it is suggested that the fundamental source of the increased permeability is the reduction in surface tension caused by the differential velocity between the rock matrix and the pore fluid.
• Increased permeability. It has been speculated that seismic waves can disrupt immobile fluid boundary layers on pore walls, which would increase the effective cross section of pores. It has also been demonstrated that stress cycling of core samples at 50 Hz (cycles per second) can mobilize in-situ particulates that are plugging pore throats.

DOE-Funded R&D in Acoustic Stimulation Technology Development

Two projects have been funded by DOE's National Petroleum Technology Office in Tulsa. The first project with Los Alamos National Laboratory involves laboratory, modeling, and field experiments. The second project with Oil & Gas Consultants International, Inc. (OGCI) involves field testing a vibration stimulation device in Osage County, Oklahoma.

Los Alamos National Laboratory and Lawrence Berkeley National Laboratory

Los Alamos National Laboratory (LANL), Lawrence Berkeley National Laboratory (LBNL) and industry partners are investigating the physical conditions or mechanisms by which low-frequency (1-500 Hz) stress (seismic or acoustic) waves enhance oil production rates. 

Laboratory Experiments

Both steady-state constant flow and non-steady-state displacement tests were performed in Berea Sandstone using oil/brine and decane/brine systems. During steady-state experiments, the drop in pressure increased in the oil/brine system, while it decreased with the decane/brine system. Two possible explanations for the pressure increase in the oil/brine system are 1) additional oil and/or brine became trapped or 2) additional oil began flowing through a fraction of the pore space previously occupied by mobile brine. The pressure decrease observed during decane/brine flow could be explained by 1) previously trapped decane and/or brine becoming mobilized or 2) decane with its lower viscosity replaces flowing brine.

Non-steady-state displacement (flooding) tests using both oil/brine and decane/brine systems indicated that stress stimulation enhances brine production during drainage (oil flood) and decreases the oil displaced during imbibition (brine flood). These observations could be explained by the fluid-trapping mechanism mentioned above. Stimulation may cause the highly water-wet Berea sandstone to become temporarily more oil wet, causing oil to become trapped during stimulation. With the decane/brine system, stimulation had little or no effect on net fluid production during either drainage or imbibition. During imbibition, however, the rock reached residual decane saturation faster. Altered wettability cannot account for this observation. 

Modeling

Modeling work strives to describe how each individual component of oil and water should respond to a pressure pulse in a porous medium. The ultimate goal of the work is to be able to model performance in different geologies and formations as a function of porosity, fluid and matrix properties. 

Field Tests

LBNL and industry partners have conducted several field tests. The tests involve lowering three-component geophones and hydrophones into wells at distances varying from 200 feet to 2,300 feet from the source. To date, three different sources in different formations have been tested-a sandstone in oil fields just north of Loveland, Colorado, a sandstone in Central California, and a shale formation, also in Central California. Bandwidth ranged from 2 to 2,000 Hz (cycles/second). In all of these tests, no seismic energy above background was observed. It is thought that monitoring equipment was too far away. Future tests will place sensors much closer (within a 100 feet). 

For additional information, visit http://www.ees4.lanl.gov/stimulation or contact Peter Roberts, Los Alamos National Laboratory, phone: 505-667-1199, Email proberts@lanl.gov or Ernie Majer, Lawrence Berkeley National Laboratory, phone 510-486-6709, Email elmajer@lbnl.gov.

Oil & Gas Consultants International, Inc. (OGCI) is partnering with the Osage Tribe, Calumet Oil Company, the field operator, and Phillips Petroleum Company (Phillips) to test vibration stimulation in the North Burbank Unit, a mature waterflood field located on Osage tribal lands. Discovered in the 1920s, the North Burbank Unit still has more than 200 million barrels of movable oil in place, but currently produces only 1,200 bbls/day at 99% WOR. The Burbank sand is at about 2800 ft with permeability ranging from 50 md to 1 Darcy. Seismic Recovery LLC, a subsidiary of OGCI, will design, build, and test a new version of a downhole vibration tool based on their patented whirling orbital vibrator.

The tool developed by OGCI uses a backward whirling motion to create both compression and shear seismic waves from 5 to more than 500 Hz, and is capable of generating controllable force levels up to many tens of thousands of pounds. The direct mechanical contact with the formation allows the device to be used in reservoirs with a gas cap, a situation that would dampen a fluid pressure pulse technique. 

The downhole tool and surface power source will be tested in a newly drilled well that will be cored, logged and completed with 7-inch production casing. Phillips will conduct laboratory tests using their proprietary sonic core apparatus to determine fluid flow response to a range of vibration frequencies. Results will guide final adjustments to the frequency generation mechanisms of the downhole vibration tool. Drilling is imminent, as soon as mid-July 2001. Once baseline data are gathered, vibration stimulation will begin.

One or more offset wells, adjacent to the vibration test well, will be equipped with downhole geophones to determine strength of signal and if the producing formation has a dominant frequency response. 

Seismic Recovery LLC is actively seeking other opportunities to field test its tool for vibration-enhanced recovery, ground water remediation, and condensate dropout mitigation.

For more information contact: Bob Westermark, Seismic Recovery LLC, Phone: 918-828-2543, Email: bwestermark@ogci.com.

British Petroleum is looking into the use of pressure pulsing for enhanced oil recovery, well-bore cleanup, pipeline problems and drilling. In preparing for field trials, they have screened reservoirs looking for onshore, low gas content, and "softer" rocks to allow fluid-rock coupling. Five candidate reservoirs have been selected from about 300 reservoirs. 

Chevron is currently applying in-situ seismic stimulation technology in their Lost Hills Field to lower their stimulation costs. Observation wells are monitoring the results of the stimulation. The results of the field test will be published.

AERA Energy has tried seismic stimulation (in cooperation with LANL) in their Lost Hills, San Ardo, and Belridge fields, and two or three additional field tests are planned. Lost Hills and Belridge fields, among others, are being considered for the test. 

Marathon conducted a test in the Tensleep Formation in Wyoming, a high viscosity oil reservoir. Tool failure and lack of resources prevented completion of the test. The stimulation resulted in reduced injectivity that was attributed to mobilization of high viscosity oil. Marathon is considering another test of the technology.

Commercially Available Acoustic Stimulation Tools

Service companies with commercially available acoustic stimulation tools were invited to describe their tools, and three - Applied Seismic Research, PerfClean, and Sonic Production Systems -responded. Some other known suppliers were not able to respond. 

Applied Seismic Research

Applied Seismic Research (ASR) uses an in-situ seismic stimulation (ISS) technique that creates high-energy (up to 10 million watts of power) low-frequency shock waves that enhance oil mobility. The tool, similar in design to a tubing pump, is powered by a conventional pumping unit. On each cycle of the pumping unit, the tool releases highly compressed wellbore fluids creating seismic (hydrodynamic) shock waves. The shock waves transform into localized, high-frequency wave fields that act to dislodge oil droplets and/or coalesce thin oil films into mobile oil droplets. 

ISS has been effective in sandstone, dolomite, and diatomite reservoirs with permeabilities ranging from 0.0001 to 1 Darcy, API gravity ranging from 17 to 38 degrees, and on both fractured and non-fractured systems. Typically, ISS is most effective in reservoirs having a relatively low (field-wide) gas-oil ratio, less than 2,000 scf/stb, API gravities greater than 20-22 degrees, and heterogeneous reservoirs containing areas of by-passed or trapped oil. Reservoir rock type, depth, and wettability do not significantly affect performance. The tool can be installed in active injection wells that remain active during the stimulation and in production wells provided the tool can be set at or below the bottom set of perforations. 

A recent field test conducted in conjunction with LBNL, LANL and Chevron in Chevron's Lost Hills field illustrated the ability of the ASR ISS tool to enhance oil production and recovery. The ISS tool was placed in an active injection well at a depth of approximately 800 feet. The seismic waves created each 8.5 seconds by the Slimhole version of the tool had an average power of 1.5 megawatts (million watts). The seismic shock waves traveled down the wellbore to connect hydraulically with the formation through the perforations at 2,200 to 3,600 feet.

After a 38-day test performed during October-November 2000, a control group of 26 wells that had not been disturbed by stimulation procedures showed a 29% increase in oil cut and a 26% increase in oil production (see Figure 1). Since oil production and oil cut curves both return to original trendlines, continuous application of ISS in the Lost Hills reservoir is required.

For further information about the ISS Tool, contact Bill Wooden, phone: 972-381-4236, Email wow@zowi.to.

Although the PerfClean© Tool System is used primarily for near wellbore clean up, a field test to demonstrate applicability for enhancing recovery is planned in a South Texas field. The system uses a patented fluidic oscillator that creates pulsating pressure waves within the wellbore and formation fluids. These pressure waves break up any type of near wellbore damage and restore and enhance the permeability of the perforations and near wellbore area. The PerfClean© Tool System is run into the well via coiled tubing, conventional tubing, wash pipe or drill pipe. The desired treatment fluid (acid, water, seawater, diesel, nitrified fluids, etc.) is pumped down the tubing through the PerfClean© Tool.

The PerfClean© oscillators are true fluidic oscillators. There are no moving parts. They do not rely on cavitation to create pressure waves. There are no packer elements to fail. Unlike mechanical tools, which suffer from high-energy losses, the PerfClean© oscillator maximizes the energy potential of the pumped fluid. Theory and experience show that incompressible fluids provide the best coupling of the pressure pulse generated by the tool to the formation.

PerfClean tools have been used to treat over 2,500 wells in 12 states and 7 international areas, at depths ranging from 90 to 27,000 feet deep. The treatment design and execution varies according to well conditions and the goals of the operator. Applications of the technology are best in formations that can support a full column of fluid. This allows for easy manipulation of the wellbore pressures during the treatment. 

For further information, contact David Facteau, phone 915-686-7432, Email PerfClean@hotmail.com.

Sonic Production Systems, a business unit of Etrema, offers the PowerWave technology for enhancing production. The technology consists of two magnetostrictive (an alloy that changes shape and produces a powerful force in the presence of a magnetic field) actuators opposing an acoustic element. The set up of the tool is similar to that of an electric submersible pump. The PowerWave tool is a solid-state source with only 3 moving parts consisting of two output shafts and one acoustic element. The tool runs at 250-400 Hz continuously. 

Field tests are ongoing, with applications in Alabama, Oklahoma, and Indonesia. A short test was run in San Ardo field, California, where the tool operated for a period of 6 days in a cyclical steam flood. Production before installation was about 450 barrels of oil per day. Six days of stimulation resulted in a production increase to about 885 barrels per day. Pre-treatment water cut was above 90% in both cases, and even when production nearly doubled, water production did not increase.

Screening criteria include high water cut, high flow, consolidated formations (the harder the better), good production history, 5 1/2-inch or larger casing, and 440-480 volt 3-phase power available. Well temperature needs to be less than 180° F. Formations at depths less than 5,000 feet are preferable. 

For further information, contact Tim Drake, phone 816-246-0566, Email tim.drake@sonicproduction.com.