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Acoustic Well StimulationIn this project a new technique for the enhancement of oil production is investigated.

This new technique comprises the stimulation of wells by means of an acousticsourcedown in the wellbore. The advantage of acoustic well stimulation in contrast to

conventional well stimulation by means of chemicals, is the drastically reduced impacton the environment. In order to commercialize acoustic well stimulation the acousticsource needs to be optimized, in terms of for instance frequency and power. To thatend, the physical phenomenon responsible for the increase oil flow needs to beunderstood, as well as its dependence on the oil and reservoir properties.

Introduction:It has been observed in laboratory experiments as well as in the field that an acousticsource was able to stimulate the flow of oil through a porous medium. This observationhas triggered oil companies to develop a new acoustic tool for the stimulation of oil

wells. However, the mechanism responsible for the increase in flow rate is notunderstood. Therefore, various mechanisms, amongst which peristaltic transport, whichmight have induce additional flow have been investigated. By means of a mathematicalmodel the increase of flow rate has been predicted, as well as its dependence on thevarious parameters like reservoir properties, acoustic frequency and power.

Aim of Project: Determine whether the 'peristaltic transport mechanism' is responsible for the

enhancement of oil by acoustics.

Determine the dependence on reservoir properties, fluid properties, acoustic

frequency and power.

0

Pow.P

G freq. f

Fig. 1: Schematic of an acoustic tool in a borehole of an oil

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Model and Results:In the peristaltic transport mechanism a pore in the reservoir is modeled as a long

capillary with its wall deformed by means of a traveling wave induced by the acousticfield. By means of a perturbation analysis in terms of the ratio of the traveling waveamplitude to the pore radius it has been demonstrated that the traveling wave at thepore wall induces a net flow.

The net flow rate is analyzed in terms of the fluid viscosity and compressibility, theshear modulus and compressibility of the reservoir and the acoustic frequency. It isdemonstrated that the net flow rate is optimal under certain operating conditions.Nevertheless, the results also show that under borehole conditions the net flow inducedby peristaltic transport is small compared to a conventional Poiseuille flow.

z

Fig. 2: A schematic of a pore with its wall deformed in the shape of atraveling wave caused by an acoustic field.

R(t)=R0[1+ cos( (z-t))]

Optimum flow rate

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 4 8 12 16 20

Power P [kW]

max

[mm^3/s] soft sandstone

(G = 0.36 Gpa)

mediumsandstone (G =0.80 Gpa)

hard sandstone(G = 1.82 Gpa)

Fig. 3: The maximum achievable net flow rate as function of

applied acoustic power, for various types of sandstone.