In-situ seismic-wave tool offers increased oil recovery

An in-situ seismic-wave stimulation tool that promises to produce trapped reserves and increase oil cuts in mature reservoirs is showing promising field results. In a recently completed 3-year field study in the Permian Basin, Applied Seismic Research's (ASR's) Hydro-Impact Technology reportedly reduced oil-production decline by 20%, using seismic waves that loosen trapped oil from reservoir walls.

The Hydro-Impact tool includes two modified tubing pumps consisting of two barrels that house two plungers connected by sucker rods. Several joints of specialized tubing connect the two barrels. The lower, larger plunger has a ball and seat, while the upper, smaller plunger is plugged and remains in the upper barrel to act as a seal.

On the upstroke, both pistons are lifted simultaneously. The lower plunger brings in more water than the upper plunger can displace, causing the liquid trapped inside to compress and build up pressure. This pressure is contained within the specialized tubing between the plungers and does not affect static wellbore pressure. At the top stroke of the pumping unit, the lower plunger quickly exits the top of the lower barrel, releasing the compressed fluids in a time span of less than 50 ms.

This rapid release generates a hydrodynamic shock wave into the formation. This subsurface shock wave mimics the primary waves of an earthquake, moving through perforations and propagating through the formation at speeds up to 1.5 miles/sec, with a power ranging from 2 to 10 MW and a pressure at the wave front greater than 3,500 psi.

The company reports that as this shock wave travels through the formation, it becomes a high-energy elastic wave. As this wave reaches oil-bearing strata, oil droplets dislodge from the pore walls and coalesce into larger droplets that migrate into flow streams. These flow streams then connect to existing flow streams such as fractures, where they flow to a nearby well for production.

When the pumping rate is set at 6 strokes/min, the tool generates new shock waves into the formation every 10 seconds; these are measured at a distance up to mile from the source in all directions. The company states that this is a significant improvement over previous sonic-stimulation devices, whose effects are recordable only 20 ft from the source. Over the 8-month life of the tool, approximately 1 million shock waves can be created.

The speed at which the shock wave propagates from the source prevents any damage to the wellbore casing and cement, according to the company. In addition, there is no damage to the formation, as the time is insufficient to fracture the rock. ASR has reportedly used this technique in the casing of injection, production, or abandoned wells, and in sandstone, carbonate, and diatomite formations in conjunction with waterfloods or natural waterdrive.

In determining if this tool would be a profitable introduction into a new field, ASR recommends that several criteria be met. First, reservoirs with gas/oil ratios greater than 2,200 scf/bbl and/or oil reserves with API gravities of less than 15 would not be suitable. Secondly, ASR does not recommend seismic stimulation in fields where at least a 10% increase in oil production and oil cut could not be achieved. Finally, the company suggests that in order to justify the economic expense, the operator should aim for a 5 to 10% increase in total oil production within a 1/2- to 3/4-mile radius from the seismic source.

To help operators decide on these criteria, ASR can run a computer program that provides a qualitative determination of a field's susceptibility to seismic stimulation. The model requires several inputs from the field, including cumulative oil and water production, monthly oil/water production over the previous 3-year period, core data analyses from at least three wells, the API gravity of the oil, and a field map highlighting well types and locations.

In the case of the recently completed Permian Basin study, the operator commented that, "the unit decline rate improved from 14.3% during the preproject stabilization period (January 2003-January 2005) to 11.9% since the start of the seismic-stimulation tools in February 2005 to date." ASR added that this stimulation work was performed in 90 wells across the field with only two in-situ seismic tools.

To learn more about ASR's in-situ seismic-stimulation tool and techniques, visit www.appliedseismicresearch.com.

Ted Moon is the Technology Editor of JPT Online. He brings information on emerging technologies, R&D successes, new field applications, updates from SPE papers about recent innovations, and more. If you have a question or suggestion for future article topics, email Ted at teched@spe.org.