Monitoring - Interference Testing

Interference Testing

Contacts

Ahmed Abou-Sayed ahmed@advantekinternational.com Advantek International

Summary

The pressure response to a stimulus at one well is measured at another well. The second well is relatively remote from the active well (the well where the stimulus is applied). Interference tests are carried out to test for reservoir continuity, to detect directional permeability and other major reservoir heterogeneities and to estimate reservoir volume.

In an interference test, a long duration rate modification in one well creates a pressure interference in an observation well that can be analyzed for reservoir properties. Interference and pulse testing require at least one active (producing or injecting) well and at least one pressure observation well. The observation well is shut-in while the testing is performed.

Key Issues

Basic Concepts Design References

Key Issue 1: Basic Concepts

Concept 1.1:

If asked, "What are some of the advantages?"

Only two wells need to be involved in the test at a time, and the other wells in the field can continue injection/production. One well is the active well that is pulsed and the other is the passive (measurement) well. More than one observation well is required to determine anisotropy.
Usually, both the mobility-thickness product, kh/m, and the porosity-compressibility-thickness product, fc_th, can be estimated. In reservoirs with fluid contacts in the zone of influence, results may be unreliable.

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Key Issue 2: Design

Design 2.1:

Analysis Methods?

Theoretical analytical solutions (with numerous assumptions) exist for a single injecting well and a passive monitoring well.
If there is a history of injection/production in both the active and the passive wells, the situation is more complex.
For infinite acting situations there is a set of available type curves (Mueller and Witherspoon, JPT, April 1965; Earlougher, 1977). Deviations from fits on the type curves can be indicative of reservoir heterogeneity. There is no real guarantee of a unique fit to the type curves. Unless the reservoir is homogeneous and isotropic, the interpretation is ambiguous.
For bounded systems, dimensionless data have been published.

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Design 2.2:

Precautions

Additional considerations are required if boundaries are close.
The rates at the active wells should be fairly constant.
Wellbore storage and skin effects at the pulser and the responder (active and passive wells) can lead to complexities and uncertainties in the results, especially if the dimensionless distance between the two wells is less than 1,000 or if the duration of the pulse is too short.
There are other complicating factors for pulse testing (lunar-solar tides, oilfield activities, natural or hydraulic fractures, multi-layering, the presence of gas ….
Unless, and even if, the reservoir is homogeneous, isotropic and infinite, interpretation can be ambiguous and conjectural.

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References

Daltaban, T.S., and Wall, C.G.: Fundamental and Applied Pressure Analysis, Imperial College Press, London (1998).
Earlougher, R.C. Jr.: Advances in Well Test Analysis, Henry L. Doherty Memorial Fund of AIME, SPE, Dallas (1977).
Matthews, C.S. and Russell, D.G.: Pressure Buildup and Flow Tests in Wells, Henry L. Doherty Memorial Fund of AIME, SPE, Dallas (1967).
Sabet, M.A.: Well Test Analysis, Gulf Publishing Company, Houston (1991).

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