During the production of oil from a reservoir, permeability decrease due to fouling particles in the near well-bore region leads to a significant decline in productivity. Several techniques (acid treatment, hydraulic fracturing) have been used to overcome this problem, but many drawbacks make them unattractive (they are expensive or environmental unfriendly, for instance). A relatively new technique has been proposed: the ultrasonic irradiation of the near well-bore region. This technique is not always successful and before applying it, a more fundamental investigation is needed. To date, only few experimental data are available and a more systematic investigation is required prior to using the techniques effectively. We focus our attention on removal of fine clay particles in Berea sandstone by high-frequency acoustic waves. We designed an experimental set-up where down-hole conditions (pressure of 180 bars and 80° C) are simulated and where we can control independently all the relevant parameters. Two acoustic horns are used: 20 kHz and 40 kHz, with a maximum power of 2 and 0.7 kW respectively. The influence of several parameters has been tested; an increase in temperature produces a better effect on the cleaning, since the particles structures become more instable as a consequence of the increase of electrical repulsion and, at the same time, the wave penetration increases. Pressure is noted to have no effect above the cavitation point: if the tool operate above 120 bars no effect of pressure is observed, on the contrary lower pressures induce cavitation and then most the acoustic energy is lost before reaching the core. The key parameter of an optimal use of the technique turned out to be the flow rate: as the flow rate decreases the cleaning effect increases: this effect can be explained by the fact that high flow rate keeps, more strongly, the fouling particles into the pore throats. The effect of the initial permeability is investigated together with the effect of the initial level of fouling. A study on the most effective use of a certain amount of energy is carried out. As conclusion, we now have a better understanding of the influence of the relevant parameters on the ultrasonic cleaning.
Permeability reduction of the near wellbore region is one of the major problems of oil industry since it reduces both the production rate and the total amount of oil that can be withdrawn from a reservoir. Several techniques to remove fouling particles from the near wellbore region have been proposed and used to restore the initial permeability: injection of acids, mechanical scrapers or hydraulic fracturing. These techniques have negative side effects. They are, for instance, expensive, environmental unfriendly or dangerous. New techniques have been proposed as replacement or auxiliary of the conventional ones, see for instance Tambini6. Among the novel techniques, ultrasonic cleaning of the near wellbore region seems to be the most promising. Field tests showed a success of this technique in the 50% of the cases (in these 50% an increase in permeability was achieved), while it failed in the other 50%, see Beresnev & Johnson1 for more details. We think that a more fundamental study is necessary to understand the phenomena involved and to be able to optimise the ultrasonic cleaning technique. The reduction in permeability by colloidal particles can be due to particles adhered to the pore walls or due to the formation of particle bridges in the throats of pores inside natural sandstones (see Fig. 1). Of course a combination of the two mechanisms is also possible.