Oil & Gas - Maximising Recovery Programme (formerly SHARP) IOR Views

Some variations in oil production were noticed in Kern County, California, during the earthquake of 1952 [1]. This incident and others prompted research into weak elastic wave stimulations of oil reservoirs in the late 1950s. Activity peaked in the 1970s and 1980s in the US and in the Soviet Union. However, in recent years there has been a resurgence of interest and research.

Two different types of waves are usually distinguished: high power frequency (around 20 kHz) and low power frequency waves (e.g. 40 Hz). High power frequency waves have a local effect in the reservoir and are mainly applied for wellbore stimulation. Low frequency acoustic wave effects can cover a larger region and are consequently used for reservoir, rather than well stimulation. They are generally applied by using surface vibrators, but there are also examples of downhole application.

This technology is believed to have a number of enhancing effects on oil recovery through:

• decreasing the cohesive and adhesive bonding and capillary forces
• poroelastic motion giving rise to changes in pore volume
• heating the oil, causing its viscosity to decrease
• releasing trapped gas.

The application of acoustic waves for reservoir stimulation would appear to be at an intermediate development stage. Many laboratory studies and some onshore field trials have been performed, but there are still unanswered questions, particularly related to the relative effect of different mechanisms and offshore applications. Laboratory and field trials have provided some positive and encouraging results, particularly for sustained rate enhancement. The effect on ultimate recovery has not yet been addressed sufficiently.

Laboratory studies have showed that the acoustic wave effects depend on factors such as application time and applied frequency. Oil rate increases tend to be higher at lower application frequencies. The following changes have been observed:

• increases in absolute permeability
• decreases in oil viscosity
• decreases in oil-water mobility ratio giving higher oil relative permeabilities and lower water production at a given water saturation
• increased oil/water segregation rates
• decreases in surface tension.

Onshore field trials have demonstrated increased oil rates and reduced watercuts for a few months to two years, in areas up to two km from the source and at depths down to 1500 m. An increased fraction of light components in the oil samples has also been observed, indicating degassing.

Extraneous losses in wave energy to the porous medium can be reduced by placing the vibration source downhole and using a waveguide to target the formation. This technique has been used in some recent field trials and is also more suitable for offshore application than placing sources on the seabed. The energy loss with depth is estimated to be 5% to 10% per km.

The technique may be applicable to a number of UK field types but offshore UKCS application presents the following challenges:

• The technique has not been applied offshore, equipment and procedures will require adapting and there will be greater financial exposure.
• Reservoirs may be deeper than hitherto and there is an uncertainty in wave attenuation with depth.
• There is also an uncertainty in lateral wave attenuation due to reservoir heterogeneities.

Reference

• I.A.Beresnev et al, “Elastic-wave stimulation of oil production: a review of method and results”, Geophysics, Vol. 59, 1994