Monitoring - Hydraulic Impedance Testing

detect hydraulic (or natural) fractures; intersecting a wellbore,

determine the fracture closure pressure (indicative of the total minimum in-situ stress); and,

to estimate fracture dimensions.

The test is performed by sending a pressure pulse down the wellbore (from the wellhead), and then analyzing the frequency response of the consequent wave train. HIT has been successfully used to determine fractures in numerous injection wells (Paige, et al., 1992)¹ and the method is reported to work in both poorly consolidated and more consolidated formations (Paige et al., 1993).²

Key Issues

Applications How It Works Procedures Pros and Cons References

Key Issue 1: Applications

Recommendation 1.1:	Consider using HIT to determine:
Whether a fracture is present. Whether the residual aperture is small - is the fracture nominally closed. Fracture height can be estimated from the magnitude of the reflection at the fracture mouth. Fracture length can be determined from the time that the pulse takes to traverse the fracture.

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Key Issue 2: How It Works

Recommendation 2.1:

You can explain HIT to someone by saying:

If the wellbore is not fractured, the pulse will reflect off the bottom of the wellbore.
If the wellbore is fractured, the fracture acts as a soft reflector. The reflected signal from the mouth of the fracture (where it intersects the wellbore) can be discerned. When a fracture is created, the reflection from the vicinity of the well bottom is reduced and energy is transmitted into the fracture.
The length of the fracture is calculated by using the wave-speed in the fracture, and the time difference between the signal reflected from the fracture mouth and the signal reflected from the fracture tip. The fracture length and width are calculated by an expression involving the excess pressure, the mechanical properties of the reservoir, and fracture height. However, the signal reflected from the fracture tip is often severely attenuated, and may be difficult to detect.
The fracture closure pressure may be determined from several HIT traces run during a pressure falloff test. The minimum in-situ pressure is estimated at where the reflected fracture signal disappears, or stays constant with a further reduction in pressure. Figure 1 is an example. It shows reflection coefficients (these can be calculated)at the fracture mouth for a series of HITs made during a pressure falloff. The reflection coefficient increases as pressure is reduced, until the wellhead pressure reaches about 450 psi. This is about the same pressure as that at which the gradient changes on the step rate test and is taken to be the fracture closure pressure.

Fracture mouth reflection coefficient.
The fracture impedance is used in calculations of the fracture height using the properties of the formation, and the excess pressure in the fracture.

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Key Issue 3: Procedures

Recommendation 3.1:

There are some operational considerations.

The pressure pulse is typically initiated by rapidly opening and closing a valve connected to the wellhead. In its simplest form, the pulse generator may be a ball valve connected to the wellhead and exhausting to atmosphere, which can be rapidly opened and closed by hand. Alternatively, mechanically controlled devices may be used which generate shorter, reproducible pulses. The pressure transducer may be attached to the wellhead, avoiding the use of any downhole equipment. Future refinements should consider varying the frequency of the pulses.
The equipment used to take the measurements consists of a device for introducing a pressure profile into the well and a high frequency pressure transducer connected to suitable recording equipment.
Baseline conditions must be first considered. A numerical model needs to be used to predict pressure oscillations without a fracture, including the influence of the annulus, internal tubing, and rathole below perforations.
Additional information on field equipment is available at Pinnacle Technology's web site in Delft at http://home.wxs.nl/~pinndelf/hit.htm.

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Key Issue 4: Advantages and Limitations

Advantages:	The advantages of this test procedure include:.
Nonintrusive. Rapid Inexpensive Can show chronological history. Can compliment other testing measurements. Appropriate for calibrating fracture models.
Current Limitations:	Be careful with the following:
Are frictional effects in the tubulars adequately handled? Coupled with two-dimensional fracturing model? May be susceptible to changes in fracture compliance due to cake. Some doubt about selection of the average fracture width? Some concern about the effects of near wellbore tortuosity and height? Some anecdotal inferences as to insensitivity to fracture height? Some possibility for interpretative confusion due to out-of-gauge but unfractured wellbores? Bias towards uppermost fractures or washouts? Anecdotal references to excessive attenuation if the fracture is too long?

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References

Paige, R.W., Roberts, J.D.M., Murray, L.R. and Mellor, D.W.: "Fracture Measurement Using Hydraulic Impedance Testing," SPE 24824, 1992.
Paige, R.W., Murray, L.R. and Roberts, J.D.M.: "Field Application of Hydraulic Impedance Testing for Fracture Measurement," SPE 26525, 1993
Holzhausen, G.R. and Egan, H.N.: "Fracture Diagnostics in East Texas and Western Colorado Using the Hydraulic Impedance Method," paper SPE 15215, presented at SPE 1986 Unconventional Gas Technology Symp., Louisville, KY (May 18-21).
Wylie, E.B. and Streeter, V.L: Fluid Transients, FEB Press, Ann Arbor (1982) 384 pages.
Holzhausen, G.R. and Gooch, R.P.: "Impedance of Hydraulic Fractures: Its Measurement and Use for Estimating Fracture Closure Pressure and Dimensions," SPE/DOE 13892, Denver, CO (May 1985).
Simpson, A.J. and Paige, R.W.: "Advances in Forties Field Water Injection," SPE 23140, paper presented at the Offshore Europe Conference, Aberdeen, Scotland (September 3-6, 1991).
Sneddon, I.N., "The Distribution of Stresses in the Region of a Crack in an Elastic Solid," Proceedings of Roy. Soc. A, (1946) 187-229.
Clifford, P.J., Mellor, D.W. and Jones, T.J.: "Water Quality Requirements for Fractured Injection Wells," SPE 21439, paper presented at the SPE Middle East Oil Show, Bahrain, (November 16-19, 1991).
Paige, R.W. and Murray, L.R.: "Produced Water Re-Injection -Field Experience and Current Understanding," venue unknown.

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