Summary
The aim of this paper is to investigate experimentally the fouling of a porous material by external particles and the optimal way to clean the porous material with high-frequency acoustic waves. In particular, we are interested in the fouling by mud particles of the near-wellbore region of an oil reservoir. In the experiments, therefore, we used natural sandstone as porous material and mud particles as fouling particles. To generate fouling, mud particles were flushed through a sandstone core. Next, the core was treated with very short bursts of ultrasound, and the change in permeability was measured after each burst. (Earlier papers report only the end result after applying the total amount of acoustic energy.)
Experiments were carried out under different acoustic-cleaning conditions to investigate the influence of the relevant parameters on the cleaning process. For instance, the amplitude of the acoustic waves, the duration of the bursts, and the time between the bursts were varied. During the ultrasonic-cleaning process, brine flowed through the core. The effect of this flow was studied by changing the flow rate. Also, the effect of the temperature, pressure, and initial core permeability on the cleaning process was investigated. The experimental results show that short bursts of acoustic energy are more efficient for cleaning than long bursts or continuous application of ultrasound (for the same total amount of acoustic energy). The overall conclusion is that the optimal method of ultrasonic cleaning is to apply many very short bursts of low-amplitude acoustic energy, with a short rest time between the bursts while keeping the liquid (brine) flow at a very low velocity. More acoustic energy is needed to clean a core with a high initial permeability than a core with a low initial permeability. At low pressure, cavitation occurs and prevents the generation of ultrasonic bursts. Cavitation can even have a negative effect on the cleaning process.
Introduction
Reduction of permeability in the near-wellbore region is a major problem for the oil industry. It causes a reduction in the oil-production rate and in the total oil that can be withdrawn from an oil reservoir. Several techniques have been developed to solve this problem, such as hydraulic fracturing and acid injection. These techniques have negative side effects: e.g., they are expensive, environmentally unfriendly, and require production shutoff. New techniques are being developed (Tambini 2003); among them, ultrasonic stimulation is promising. Field tests to investigate the applicability of ultrasonic cleaning were carried out in Russia during the 1980s and showed contradictory results: An increase in permeability was reported in 50% of the cases, while no improvement or deterioration was reported in the other 50% (Beresnev and Johnson 1994). No explanation for this is available; therefore, we think a more fundamental investigation of the technique is necessary. As it is often difficult to make a satisfactory interpretation of field tests, laboratory experiments are crucial. The first laboratory studies concerning the application of ultrasound to clean a porous material were performed by Venkitaraman et al. (1995) and Roberts et al. (2000). They investigated the cleaning of porous materials that were damaged by different fouling mechanisms:fouling by very small particles (fines) that were released from the porous material by the flow of brine through the material (internal fouling) andfouling by mud particles or polymers from the outside into the porous material (external fouling).
Poesio and Ooms (2004) and Poesio et al. (2004) performed detailed studies on the acoustic removal of fines from a porous material. Van der Bas et al. (2004a) performed experiments on oil-saturated rocks and gravel-pack completion. They reported positive effects after the application of ultrasound bursts. Van der Bas et al. (2004b) also reported the application of ultrasound by a prototype acoustic tool in a radial geometry. Preliminary results were so encouraging that a new prototype was planned. In this paper, we focus on removing particles caused by external fouling. We will pay particular attention to the effect of short acoustic pulses to find an optimal way to use the acoustic energy. Moreover, the influence of many relevant parameters is reported. In the next section, we discuss the experimental setup and experimental procedure taken throughout the investigation. The fouling process and the measurement results for the level of fouling and for the penetration depth are given in the section after that, followed by a discussion of the ultrasonic-stimulation results. Conclusions are drawn in the final section.
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