The acoustic determination of the pump intake pressure in wells producing viscous low API gravity crude is presented in this paper. These wells have liquid above the pump and the pump is set above the formation and gas flowing upward through the gaseous liquid column. The method to determine the pump intake pressure involves the acoustic measurement of the liquid level and the casing-pressure buildup rate when the casing head valve is closed. When these data are used with this paper's new empirically derived correlation for viscous crudes, then an accurate gradient of the gaseous liquid column in the annulus is obtained. This new correlation provides a more accurate correction of the gaseous gradient of the fluid above the pump intake for wells producing viscous crudes.
Results are presented from field testing of numerous wells where the actual gradients of gaseous liquid columns were measured is wells having a variety of gas flow rates and an oil gravity in the range of 10 degrees API. A back pressure valve is used to increase and stabilize the casing head pressure and the valve is used to depress and to stabilize the liquid level at different depths while the well is produced at a constant rate. Over a period of many days the acoustic measurements to the stabilized liquid levels are acquired. The gradient of the gaseous liquid column is then calculated and the pressures are extrapolated to the pump intake depth. Development of a new correlation from the analysis of this field data improves the accuracy of pump intake pressure determination in wells producing viscous low API gravity crude.
This paper addresses the problem of determining the pump intake pressure in wells producing heavy crude (10–12 API) by means of pumping systems such as progressing cavity pumps, plunger pumps and other pumping means, using surface measurements of pressure and fluid levels. The technology is well established in industry and has been reported in several publications 1,2,3,4. It requires determination of the gradient of the gas-liquid mixture in the annulus above the pump intake based on knowledge of the oil gravity adjusted by an effective gradient factor that represents the effect of any gas flowing through the annular liquid and being produced from the casing at the surface.
Fig. 1 (left) illustrates the fluid segregation that occurs in a pumping well that is being operated at stabilized conditions. Stabilization implies that after pump start-up the following variables have settled at a relatively constant value:
The surface producing rate of oil, water and gas
The produced Water Oil Ratio
The casinghead pressure
The annular fluid level
This also implies that the pump has been operating at a constant speed for a sufficient time to reach stabilization.
At stabilized producing conditions, the oil in the casing annulus becomes saturated with the gas that is continuously flowing to the surface. Consequently, if gas is being vented from the casing annulus at the surface at a constant rate, free gas is being produced from the formation simultaneously with the oil. This condition results in a gaseous annular liquid column. Generally, most oil is produced through the pump while most free gas is produced up the casing annulus. Pump intake pressure and bottom hole pressure calculation is undertaken from a measurement of casinghead pressure, knowledge of oil and gas properties, and an estimate of the oil fraction in the annular liquid. The fraction estimate is required to obtain the gradient of the gas/liquid mixture. This problem has received considerable attention by numerous authors. These techniques involve the determination of the gas flow rate up the annulus and, in turn, the calculation of the amount of liquid present in the gaseous liquid column by use of such well conditions as casing/tubing sizes, liquid properties, and pressure.