Pressure transient analysis has undergone a significant development over the last two decades. Sophisticated methods of flow regime identification were developed along with the models of complex heterogeneous systems. The technology is strictly applicable to data obtained on flowing wells. Majority of wells, however, are on pumps; in such a case, the standard methods of data interpretation cannot be used because the presence of a pump hardware in a tubing string prevents the bottom-hole pressure measurements. Removing a pump and rods in order to run the pressure gauges may not only prove to be costly but also would not allow early time data gathering. Thus, a different approach is needed to test the sucker-rod pumping wells.
In principle, the bottom-hole pressure may be indirectly determined as the sum of the casing-head pressure and the hydrostatic pressures of the gas and liquid columns:
Equation (1) (Available In Full Paper)
The casing-head pressure is readily available since it is continuously monitored during a buildup test. The pressure exerted by the gas column may be obtained by using either the Cullender Smith method1 or the Average pressure and temperature method2 for calculating the static pressure.
The calculation of both Pgc and Pic requires the information on gas and liquid columns lengths variation in time. The method widely used in industry to track the gas-liquid interface is the acoustic well sounding technique (AWS).
The estimation of the hydrostatic head of the liquid column is complicated by the presence of free gas and requires information on gas void fraction variation not only in time but also along the wellbore.
The upward gas-liquid flow has been investigated by numerous researchers. Over the period of last two decades various correlations and mechanistic models have been proposed; the methods of Orkiszewski3, Aziz et. al.4 and Hassan et. al.5 are mentioned to name just a few. It is common to all these methods first to establish the correct flow regime and then to use the appropriate holdup and friction factor correlations.
Orkiszewski3 selected for the bubbly flow and the mist flow regimes the correlations by Grifith and Wallis6 and Duns and Ros7 respectively whereas for the conditions of the slug flow Orkiszewski proposed a new method.
Aziz et. al.4 employed the drift flux concept for the bubble and the slug flow regimes and the Duns and Ros7 methods for the mist flow.
Hassan et. al.5 used the results of the modeling efforts of Taitel et. al.8 for predicting the flow pattern transitions. Similarly to the method of Aziz et. al.4, they used the drift flux approach for the gas void estimation with a modification accounting for the fluid flow conditions in the tubing-casing annulus. They proposed pressure drop calculation for annular flow regime which is also based on mechanistic principles.
Estimation of the pressure at the gas-liquid interface presents no difficulty as it amounts to adding up the casing-head and hydrostatic head pressures at various shut-in times.