The principle of mass balance of gas phase in the annulus may be used to estimate annular gas rate and flowing bottomhole pressure of a pumping oilwell. The method enables one to calculate the annular gas volume, including gas bubbles in the oil column. The knowledge of annular gas volume can then be used to find pseudo-length of both the oiland gas columns and hence the flowing bottomhole pressure to allow calculation of the well's productivity index. This technique is particularly applicable in wells having a foaming annulus, since under such circumstances acoustic well sounding method yields questionable results due to the absence of a distinct oil-gas interface.
This paper demonstrates that the improper application of the material balance equation may lead to serious errors in estimating the annular gas rate and the flowing bottomhole pressure. The inaccuracy stems from the lack of knowledge of the true annular gas rate when the material balance equation is applied to the total gas in the system (tubing andannulus). Since only a fraction of the total gas is produced through the annulus, the actual annular gas rate is not known a priori. unless measured at thesurface. The method suggested in this work develops the material balance equation for the gas phase inthe annulus only, thereby avoiding the problem.
It is proposed that a short buildup test be conducted to extrapolate the buildup data to zero time for obtaining the Flowing bottomhole pressure. The proposed method avoids the erroneous application of the material balance equation for saturated condition of the oil present downhole.
The knowledge of flowing bottomhole pressure (Pwf) of an oilwell operating either at pseudosteady state or steady state condition is an essential element in predicting its deliverability potential. The flow rate corresponding to the Pwf is the first differential (i.e., the productivity index) of the inflow performance curve (IPR). The stable solution obtained by superimposing the tubing performance curve on the IPR: curve yields the maximum possible flow rate l. This information is necessary for proper pump design. Since about 72 percent of all oilwells in the United States2 are currently being operated by sucker-rod pumps, the importance of accurate estimation of flowing bottomhole pressure cannot be overemphasized.
The in-situ measurement of bottomhole pressure in flowing oilwells can be accomplished by the use of sub-surface pressure recorders. However, the pumping oilwells present special problems. A conventional pressure recorder cannot be run down the tubing string in a sucker-rod pumping oilwell. The alternatives are nitrogen tube or other permanent downhole equipments. But the initial cost is high for such a permanent pressure recording devices and the same wells must be always be tested.
Acoustic well sounding technique (AWS) offers the potential for obtaining reliable flowing bottomhole pressure data in certain cases. Unfortunately, estimating the fluid density in the tubing-casing annulus is not easy when the oil is in a saturated condition, i.e., the flowing bottomhole pressure is below the bubble point pressure (Pwf < Pb).
