Abstract
The use of surface readout BHP gages in ESP systems has become more prevalent with the development of cost effective installations. In spite of the improvement in their reliability, the BHP pressure sensors are subject to zero drift during their operational life. It thus becomes necessary to periodically check the indicated pressure and adjust its value to eliminate the zero shift. Since it is not economically justifiable to pull the tubing and the sensor, a method has been developed and field tested that permits to make the calibration check from surface measurements.
In an operating well, the method requires the periodic measurement of annular surface pressure using a pressure transducer with an accuracy of 0.1% and acoustic fluid level depth digitally acquired with a resolution of one millisecond while the casing valve is closed and produced gas is accumulating in the annulus. As fluid enters the wellbore the liquid is produced through the pump while the gas accumulates in the casing annulus. A software program determines the density of the gas in the annulus and computes the pressure at the gas/liquid interface accounting for the effect of the gas column hydrostatic. The process is continued until the gas/liquid interface is displaced near the pump intake, at which point the computed bottom hole wellbore pressure should be very close to the pressure indicated by the surface readout. Any discrepancy between the two measurements is used to estimate the zero shift and to correct the surface readout.
