This paper describes the naturally occurring phenomenon of hydrostatic reduction and oscillation about an underbalance pressure when hydrostatic pressure is decompressed by the opening of a ball valve to the underbalance pressure when tubing conveyed perforating is performed. Downhole memory pressure gauges have recorded this event on wells on three separate zones. Laboratory testing was conducted and was successful in recreating and recording this pressure transient. All the data indicates that the magnitude of the hydrostatic reduction to be a function of the differential pressure being decompressed. The continued decrease of pressure below the underbalance pressure occurs at a rate of approximately 20 psi (.14 MPa) per 100 psi (.69 MPa)of differential pressure. The duration of the oscillating pressure wave (i.e., the time from the opening of the ball valve until the pressure wave is damped out) is a function of the volume of fluid being decompressed. Theduration time increases at a rate of one second per400 U.S.gals (1.514m3).

The data has been plotted on pressure vs. time graphs. The curves are characteristic of damped harmonic motion with a single degree of freedom and therefore, can be solved by using a linear second order differential equation.

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