The Suppression of Sonic Shocks in Geothermal Wells


John Reid and Peter Nielsen

Key Words:

sonic shock, wet steam, speed of sound


Stanford Geothermal Workshop




Emerging Technology



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The speed of sound in wet steam is only a small fraction of its value in either pure steam or water. As a result, sonic shocks form easily in wet steam.

Behaviour of wet steam under the action of gravity is of great importance in the operation of geothermal systems. Few systems create water with sufficient specific enthalpy to form dry steam and so the steam is of low quality. In a production well a balance point is reached at which the weight of wet steam provides precisely the correct pressure for boiling to commence. This point, “the depth of vaporization” is strongly dependent on the reservoir exit temperature. Thus small variations in reservoir exit temperature can cause large, rapid changes in the depth of vaporization. This can give rise to incompressible slugs of liquid water alternating with compressible slugs of wet steam in production well channels leading to complex, transient, resonating behaviour. This situation is analogous to a complex spring pendulum with a multiplicity of weights and springs. If a rapidly moving wet steam slug should locally exceed the speed of sound relative to the well-casing a sonic shock will form spontaneously even though there is no change in the diameter of the well. Extremely high pressures will then be manifested instantaneously in the fluid behind the shock, possibly causing damage to the well-casing. Such shocks would be recorded as earthquakes at the surface.

These phenomena can be suppressed as follows:

1. In a production well, by using a down-hole valve to maintain a high pressure in the production well in order to prevent the formation of wet steam.

2. Following stimulation, by releasing pressure very slowly, so as to avoid shock formation when wet steam forms.

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