Foamer evaluation by the sparging test method for application to gas well deliquification

Abstract

Natural gas wells produce small amount of liquids along with the gas. During the initial part of the reservoir life, the gas velocity in the well tubing is large enough to drag the liquid upwards. As the well matures, the reservoir pressure starts to decline leading to a decline in the gas velocity in the tubing; here the gas momentum is not enough to carry the liquid to the top and the liquid starts to accumulate at the bottom of the well. One of the ways to remove the liquid is the use of foam assisted lift technique where surfactant is injected downhole and turbulent mixing between the gas and the liquid generates foam which can easily be transported by the gas to the surface. However, the liquid produced in a gas well may differ from well to well. Depending on the fluid composition (gas, condensate, water) and the physical conditions of the well, a surfactant may or may not be able to remove the liquid from the bottom of the well. Hence it is imperative to evaluate the surfactant before its use. There are various methods available to evaluate a surfactant; one of the prominent methods is a small scale sparger test, where gas is injected into a surfactant solution to generate foam. We attempt to understand the foam behaviour in the small scale setup and we will use the insights to possibly standardize surfactant evaluation. The present work focuses on understanding the effect of three parameters on the foam generated in a small scale sparger setup, namely: gas velocity, surfactant concentration and pressure. We conduct three tests for each variation of the above three parameters, namely: build-up test, collapse test and carryover test. Two commercial surfactants: Foamatron and Trifoam Block 820 have been studied in the present work. In addition to these measurements, the characteristics of the dynamic surface tension of the surfactants are measured by using the maximum bubble pressure method. Increasing the pressure (in the range of our measurement) is found to have a stabilizing effect on the foam but it does not seem to directly affect the ability of the surfactant to unload liquid. The amount of foam formed in the small scale test strongly correlates with the gas velocity but is found to have a weak correlation with the surfactant concentration. Even though the hydrodynamics of the flow differ between the test scale and the flow loop scale, the small scale tests can still give important information which can help in evaluating surfactants. To be more specific, the carryover test is found to have similar trends as with data from intermediate scale tests (i.e. flow loop test) in context of ability of surfactant in unloading liquid. However the results from the flow loop scale suggest that the small scale sparger setup is not a good predictor in terms of pressure drop. We find that for an increasing velocity in the small scale setup, we operate within two flow regimes: bubble flow and slug flow. We recommend conducting the test in the slug flow regime which is closer to the actual hydrodynamics in the gas wells during liquid loading (churn flow). However, there are certain limitations to conducting the test in the slug flow: since the gas velocities are high the limit of carryover could affect the interpretation of results.