Foam lift is a cost effective methodology for unloading gas wells. When injected continuously or intermittently to de-liquefy vertical gas wells, foam reduces the gravitational gradient and increases the frictional gradient, thus, the critical velocity at which liquid loading occurs is shifted to lower gas velocities. Currently, no methodology exists to predict the onset of liquid loading and the pressure drop in vertical gas wells under foam flow conditions.
To address this, we measured several foam flow characteristics in both bench top and large scale facilities using Anionic, two Amphoteric, Sulphonate and Cationic surfactants. We used the bench top facility to measure foam carryover capacity as a function of time and surfactant concentration. We used the large scale facility to measure liquid holdup, pressure drop, fraction of gas trapped in foam and foam holdup in 40 feet 2-inch and 4-inch tubing.
We developed closure relationships for liquid hold up, foam holdup, fraction of gas trapped in the foam and interfacial friction factor under foam flow by combining the bench top data with the data collected in the large scale experiments. A new transition criterion was developed to predict the onset of liquid loading under foam flow. Using a force balance over the gas core in annular flow, we developed a new procedure to calculate the pressure drop under foam flow conditions. In a different paper, our own experimental data was compared with our model and the results are satisfactory. In this study, these models were used to predict the onset of liquid loading and the pressure drop in seventeen vertical gas wells under surfactant application without plunger lift application. The wells considered have varying surfactant injection rates, daily gas rate, daily tubing and casing pressures. While there is no field data to compare the predicted critical velocity under foam flow, the predicted critical velocity under foam flow performed favorably when compared with experimentally observed critical velocity. Our pressure drop model was able to predict the field pressure drop within reasonable limits by application of appropriate scaling factor which is explained in this study.
The procedure developed is the only one currently available to calculate the transition boundary and pressure drop under foam flow conditions using the bench top facility data. It is superior to conventional annular flow pressure drop prediction models.