7/29/2019 Acustic

1/3

Written by; W.G. ( Bill ) Groves, President, Clean Well Tools Ltd.

Acoustic Stimulation has been used for over 30 years. Several acoustic tools have been used

over the years

pressure fluctuation tool, perfclean, sonic hammer, and pulsator, to namea few. The success of acoustic stimulation has varied in good wells until the development of

the Clean Well Tool. The Clean Well Tool (CWT) has proven very successful on good wells

with near wellbore damage. This is especially true on long horizontal wells. The greatest

advantage is the cost of operation versus the productivity increase.

First, let us define Acoustic Stimulation. Acoustic Stimulation relates to stimulation using

acoustic energy. Acoustic energy in the hearing range (sound) is sonic (20 to 20,000 Hz),

above the hearing range ultrasonic (20,000 Hz plus), below hearing range is infrasonic (1 to

20Hz). Acoustic (sonic, ultrasonic, and infrasonic) and seismic stimulation use the same

mechanisms to stimulate. Maurice A. Biot defined high frequency as "the frequency where

the wavelength is equal to the pore diameter". For most formations, high frequency isabove 2,000,000 Hz, this is above the range of all the present equipment and will not be

discussed in this writing.

The difference between acoustic stimulation and seismic stimulation, is (a) acoustic

stimulation has been applied on a temporary basis through downhole tools to remove near

wellbore damage, and (b) seismic stimulation is normally operated for relatively long

periods of time or on a continuous basis. These stimulation techniques use Primary waves

and Shear waves. The Primary ( P ) wave is a pressure or compression wave. The Shear (

S ) wave is also a secondary wave. An S wave (at ninety degrees to the P wave) is not

transmitted through liquid or gas. Acoustic stimulation relates to seismic stimulation in

that the same P waves and S waves are used to increase production or injection.

Measurement of the reflected P wave and S wave and the computations of this information

form the basis for Geophysics. A train going over a producing field and enhancing

production was the original observation of seismic stimulation. Seismic stimulation was

done on a trial basis using a thumper or vibroseis unit. The results varied and the cost of

running the unit was high for the extra production. It was determined that less energy

would be required if the vibration equipment were placed down the hole. Several tests are

being carried out using down hole tools. These systems are all permanent or semi-

permanent installations and are operated either continuously or for relatively long periods

of time.

A leading principle of acoustic stimulation is to physically agitate or elastically move the

formation immediately around the wellbore. The P wave from the tool is transmitted

through the stimulation liquid to the formation. It should be noted if the stimulation liquid

contains nitrogen, much of the energy in the P wave is attenuated before it reaches the

formation. Some of the P wave energy is transmitted to the formation, the remainder

7/29/2019 Acustic

2/3

continues through the liquid through the permeability in the formation. The P wave in the

formation liquid continues until the path is totally blocked, at this point the P wave is

reflected back through the liquid. This is the effect I named "The Toilet Plunger Effect"

the P wave hitting the blockage is like pushing on the toilet plunger, the reflection of the P

wave is like pulling on the toilet plunger, both actions assist the removal of the blockage.

The energy transmitted to the formation is split into a P wave and an S wave. The S wave

(at ninety degrees to the P wave) causes elastic shaking of the formation. This shaking of

the formations helps to dislodge near wellbore damage. The next P wave helps to flush the

dislodged damage out of the flow channel. If the wellbore is under balance the damage will

flow into the wellbore, if over balance the damage goes deeper into the formation. This

flushing of the flow channels allows for an improvement of inflow or injection of chemicals

into the formation.

The next item to define is near wellbore. The Darcy equation indicates that the majority of

flow restriction comes from the last three feet radially around the wellbore. This "three-

foot rule" adequately defines near wellbore. If the formation has good permeability, butthe near wellbore has been damaged, removing this damage can significantly improve

inflow or injection. The best way to have the damage removed is to flow or pump it out of

the well. The second best method is to move the damage as far away from the wellbore into

the formation as possible.

The next item to address is "How to Acoustically Stimulate a well?" How to Acoustically

Stimulate a well is related to the original formation permeability, bottom hole pressure,

damaging mechanism and the procedure and materials that created the damage. Using the

two-foot rule, damage removal should be done to reach out two feet. The acoustic tools,

such as CWT, should be selected for their ability to remove damage rapidly to a two-foot

radius. An additional item to consider is that the higher the frequency of the tool, the fasterthe cleanup. Wave studies show that effective cleaning will take place out to one-quarter

wavelength, but the speed of cleaning decreases rapidly after one-eighth wavelength. Using

the two feet required times eight gives a wavelength of 16 feet. A 16 foot wavelength is

created by a frequency of 260 Hz (cycles per second) if the velocity of sound is 4800 feet per

second in the stimulation fluid. (Velocity of sound in fresh water @ 200

C is 4860 feet per

second). From these rudimentary calculations, the optimum frequency is 260 Hz. The type

of damage and the volume of solids contained in the damaged zone determine the required

duration of operation and the energy levels. Considering that a higher frequency cleans

faster, a tool with a base 260 Hz frequency and dual or multiple higher frequencies is more

efficient than a single frequency tool. An improvement of inflow has been observed while

moving a tool, with multiple frequencies of 2x base, 4x base, 8x base Hz, at 10 metres perminute. However, more than one pass was required for a complete cleanup. These

improvements were observed on varying wells. The calculated energy per pulse ranged

from 10 to 20 joules for these jobs. Jobs using 5 to 10 joules per pulse of calculated energy

required additional cleanup time.

In addition to the acoustical properties, stimulation tools, used on horizontal wells, require

additional properties. The tools are often run into wells containing drill cuttings or other

7/29/2019 Acustic

3/3

solids. To break through these beds, requires a downward facing jet or jets. This jet or jets

does not require a large volume to allow the tool passage further into the well. Once the

tool is into the toe, it is essential that it can be brought out. To ensure this occurs, jets

facing upward at the top of the tool are required. The acoustic generator is required to

operate in the presence of these jets. The design of the jetting portion of an acoustic tool is

very critical

too much flow, will over power the well in under balance and reduce theenergy available for the acoustic pulses, too little flow means that the solids can not be

removed from the well bore in under balance or flush the solids deep enough into the

formation in over balance.