Determining Producing Bottom-Hole Pressures in Wells Having Gaseous Columns
Authors: McCoy J.N. 
Published: December 2007

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Introduction
An accurate method to determine the producing bottomhole pressures in oil wells, using acoustic liquid-level pressures in oil wells, using acoustic liquid-level instruments, is desirable when measuring producing well efficiency. When gas is produced from the casing annulus, the oil column is lightened. If gas bubbles are present in an oil column, a correction for the oil gradient present in an oil column, a correction for the oil gradient is necessary to get an accurate producing bottom-hole pressure. This paper presents a simple technique to find pressure. This paper presents a simple technique to find the gradient of a gaseous oil column and the producing bottom-hole pressure, using acoustic liquid-level instruments, in oil wells that produce gas through the casing annulus. The data needed to obtain the producing bottom-hole pressure area liquid-level test,the casing pressure at the time of the test,the time required for an increase in casing pressure (when the casing valves are closed and the well continues to pump), andthe amount of casing pressure increase.
The test can be performed in less than 30 minutes. performed in less than 30 minutes. Gaseous Liquid Columns
When gas bubbles through a liquid column, the gradient of the column is reduced. A correction factor is necessary to compensate for the amount of gas present in the annular space: Fig. 1 shows the gradient correction factor in the annulus of a well for different flow rates and casing annulus areas at various pressures. The gradient correction factor indicates the amount of liquid present in a gaseous liquid column compared with a completely liquid column. This curve was prepared by W. E. Gilbert and is presented in Ref. 1.

Application of Gilbert's Curve to Producing Oil Wells
When an acoustic liquid-level test is run on a well, the volume of casing annulus above the liquid level can be determined. If the casing valve is closed and the time required for a certain buildup in casing pressure is obtained, the gas flow rate through the gaseous liquid column in the annular space can be found. Thus, a correction factor is obtained from the Gilbert curve and is applied to the gaseous liquid column.
Fig. 2 is the adaption of the Gilbert curve to data obtained easily during a liquid-level test. The flow rate, instead of being expressed in thousands of cubic feet per day, is expressed by the increasing casing pressure in a specific time and the length and area of the gas column. Because the Gilbert relationship divides the flow rate by the casing annulus area, the annular area is cancelled in the new relationship. This allows the relationship to be expressed by the change in casing pressure, the time for the pressure buildup, the length of the gas column, and the pressure to the 0.4 power at the point of interest. By indicating the different pressures in Fig. 2, the calculation of pressure to the 0.4 power is not necessary. Thus, the liquid level, pressure buildup, and time of the buildup give the operator a relationship that measures the gradient correction factor in a producing oil well at any pressure.
JPT
P. 117