New economical and efficient technologies are required to stimulate and produce low pressure gas reserves. Liquid lifting from low pressure gas wells is cited by the industry as one of the main problems that requires new technology. This paper presents the experimental equipment, procedure, and results of a testing program to study foam lift technology. Foam as a liquid carrier is evaluated for stability and flow regimes under different flow conditions.
A tubing/casing vertical model has been constructed to simulate the flow conditions in a typical gas well experiencing liquid loading problems. The model consists of a 40-ft long, 1-inch diameter tubing connected to a 6-inch OD casing. Pressure data along the tubing string are recorded with a data acquisition system. The model allows visual observation of fluid flow inside the tubing as well as tubing and casing flow interactions.
Surfactant solution is injected to induce foam. Foam lift efficiency is evaluated in terms of the minimum gas velocity required to prevent liquid fallback. In order to evaluate the incremental advantage foam has to lift liquid, two-phase flow studies has been examined without foam (base case). The two-phase flow studies included visual observation of liquid fallback phenomena under different gas and liquid velocities. Foam flow experiments were conducted at similar flow conditions and visual observations were made. The performance of the two phase flow with and without surfactant present in the liquid phase is then compared and analyzed. Foam lift is shown to reduce the minimum critical gas lifting velocity by order of magnitudes which will positively impact the production economics from marginal producers.
Liquid loading problems occur in gas wells when the gas velocity becomes insufficient to continuously carry liquids from the tubing. The onset of such a critical gas flow condition is typically associated with sluggish well production behavior and a dramatic decrease in well productivity. Correct diagnosis of well behavior can be a difficult problem for the production engineer. In many cases, a well experiencing a liquid loading problem is suspected of having formation damage problems. In these wells, acidizing or fracturing are unnecessarily aggravating the situation.
Current technologies are in some cases adequate and successful to prolong the life of a gas well with a liquid load problem by continuously or intermittently lifting the liquids to the surface or disposing them to a downhole disposal zone. However, economical and practical limitations remain obstacles. Some of the current liquid lifting methods in use are: Velocity string, plunger lift, gas lift, submersible pumps, sucker rods, swabbing, foaming, jetting, in addition to downhole disposal for environmental concerns. These methods can be applied separately or in combination with others intermittently or continuously to fit certain gas well production conditions.
The application of methods that utilize reservoir energy (such as velocity string, plungers, and foaming) are by far the most economical. P. 317^