An Analytical Description of Liquid Slug Flow In Small-Diameter Vertical Conduits

Abstract

A wide range of intermittent gas-lift tests was conducted in a 1,500-ft experimental well through 1 ~-and 1 ¥Z-in. nominal size tubing. The well was equipped w~th two gas-lift valves, four Maihak electronic pressure transmitters and surface facilities to measure casing and tubing pressures, temperature, liquid production and the rate and volume of gas injected. Fo'r each tubing size, tests were conducted varying the slug length, gas volume injected and liquid viscosity and surface tension. From these tests, an empirical "fallback" or liquid loss correlation has been developed. No genoeral fallback correlation could be obtained , and the correlation is presented in the form of one curve for each slug length of a given liquid and tubing size. A conceptual model has been developed which combines the fallback correlation with equations which govern liquid slug flow. A mathematical simulation of the model in the form of a digital computer program is used to calculate total recovery, gas volume used and clearance time for a given test. Tubing pressures at the gas-lift valve and at the gas-slug interface are calculated as a function of time. Tubing pressures at the surface during slug clearance are also calculated. The results compare favorably with the test data and verify the conceptual model. The extent to which the model can' be used for other tubing sizes, depths, liquids and gas-lift valves can only be determined with additional experimental data. Data are also necessary so that a fallback curve can be constructed for the particular test conditions studied.