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.
The objective of this paper is to present a field method by which it is possible to determine the accuracy of down-hole pressure sensor with surface readouts which are often installed in wells produced by means of electrical submersible pumps. Surface readings of producing and static bottom hole pressure are used in analysis of well performance and also for diagnostic evaluation of the performance of the pumping system. Since these sensor may be subject to zero drift it is important to be able to periodically check that the readings correspond to the actual pressure in the well without having to pull the sensor for calibration. The proposed method is based on accurate measurement of the casing head pressure and calculation of the down hole pressure distribution to obtain a best estimate of the pressure at the same depth as the sensor and thus conclude whether the surface readings are reliable or whether the sensor requires overhauling.
Knowledge and understanding of the fluid distribution and gradients that exist in the annulus of a pumping well is a requirement for analysis of well performance. These basic concepts have been described in detail in numerous publications but they will be reviewed briefly in the following section. The assumption is that in most pumping wells the annulus is open to the formation (no packer) and thus the fluids are free to move in the annulus under the action of buoyancy and convective gradients. In addition the following applies to steady state conditions which are defined as a constant flow rate and fluid composition being produced by the well. P. 531