Abstract
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.
