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total oil that can be withdrawn from an oil reservoir.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:
(1) fouling by very small particles (fines) that were released from the porous
material by the flow of brine through the material (internal fouling) and (2)
fouling 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.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
22 November 2005
- Revised manuscript received:
4 October 2008
- Manuscript approved:
9 October 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
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