A knowledge of the producing bottomhole pressure is desired in most artificial lift wells to determine if the well is being produced efficiently. An acoustic liquid level test and casing pressure measurement permits calculation of the PBHP. If the well contains liquid above the formation and the well produces gas from the casing annulus, the liquid column is aerated with gas bubbles. These bubbles are continuously moving upward through the gaseous liquid column. The gas is vented at the surface. The gradient of this gaseous liquid column is not known with a high degree of precision.
One technique for determining the gradient of the gaseous liquid column is to depress the liquid level by closing-in the casing valve. Stopping the flow of gas from the casing annulus at the surface of the well causes the casing pressure to increase. The casing pressure increase depresses the height of the gaseous liquid column. The test can be continued to determine the gas/liquid interface pressure as the top of the gaseous liquid column is depressed. This data can be used to calculate the gradient of the gaseous liquid column and the producing bottomhole pressure.
Several wells were tested which contained bottomhole pressure sensors. The increase in producing bottomhole pressure was measured as the liquid from the casing annulus was displaced into the pump which necessarily reduces liquid flow from the formation. The casing pressure and top of the gaseous liquid column was also determined. Several examples are presented to show the effect of closing-in the casing annulus gas vent valve on the producing bottomhole pressure, casing pressure and height of the gaseous liquid column.
The determination of the bottomhole pressure in a producing well is important The producing bottomhole pressure should be low when compared to the reservoir pressure when the maximum production is desired. The producing bottomhole pressure (PBHP) and the static bottomhole pressure (SBHP) are used to determine the producing rate efficiency of the well. The PBHP and the SBHP are used in conjunction with the well test to determine the maximum production capability of the well.1 The PBHP can be determined by surface acoustic surveys without the need of lowering pressure gauges into the well in most cases. The PBHP is the summation of the casing pressure, gas column pressure and the pressure exerted by the liquid column if a liquid column exists above the pressure datum.
If the well does not produce gas from the casing annulus, the PBHP can be calculated accurately by summing the casing pressure, gas column pressure and the gas free liquid column pressure. The casing pressure should be measured accurately. The specific gravity of the gas in the casing annulus can be calculated from the measurement of acoustic velocity when an acoustic liquid level depth test is performed. These permits calculation of an accurate gas column pressure and a gas/liquid interface pressure. The gradient of the gas free liquid column can be determined accurately using various correlations or the referenced sources.2,3
