LIQUID LOADING IN GAS WELLS. The presence of a liquid phase in gas reservoir can affect the flowing characteristics of the gas well. Liquids can come from condensation of hydrocarbon gas (condensate) or from interstitial water in the reservoir matrix. The liquid phase must be in the mist flow regime where gas is the continuous phase and liquid the discontinuous phase. Liquid loading or accumulation in gas wells occurs when the gas phase does not provide adequate energy for the continuous removal of liquids from the wellbore. The accumulation of liquid will impose an additional back pressure on the formation, which can restrict well productivity. In low pressure wells the liquid may completely kill the well; and in high-pressure wells slugging will occur, whcih can affect well test results.
METHODS FOR UNLOADING LIQUID
Many gas wells do not provide adequate amounts of transport energy for continuous liquid removal from the wellbore. Liquid accumulation or loading will therefore occur in these gas wells. Fluctuations of daily gas rates and casing pressures are characteristics of liquid accumulation in gas wells. There will usually be a buildup of casing pressure which may be sufficient to blow the liquid accumulation in the casing/tubing annulus into the tubing and then to the surface. If enough casing pressure cannot be achieved, the well weill die and swabbing becomes neccesary to revive it.
Another symptom of liquid loading is abnormally high casing pressure. A casing pressure of more than 200 psi higher than the flowing tubing pressure generally is an indication of excessive liquid accumulation. Flowing bottom-hole pressure surveys and tubing pressure traverse curves can also be used in determining liquid-loading tendencies.
Various methods have been employed to remove liquids from gas wells, those are as follows:
Beam Pumping Units:
When beam pumps are used to unload liquid from gas wells, the liquids are pumped up the tubing and the gas is produced out of the annulus. It is desirable for the tubing to be set as close to the bottom preforation as possible, and preferably below it. This prevents gas interference problems in the pump by having a liquid cushion. If a diverter is used, no liquid cushion is required.
Beam pumps do not depend on gas velocity for lift. They are quite cheap for shallow wells but become very expensive for deeper and morehighly pressured wells. They are best for low gas rates and liquid producing rates of greater than 10 bbls liquid/day.
A successful method of liquid removal in gas wells is by plunger lift. A steel plunger with a valve is located in the tubing string. At the bottom of the tubing is an opening through which gas and liquid can pass into the tubing. When the plunger is at the bottom of the tubing, the tubing is closed and all production goes into the annulus. The casing pressure builds up energy is stored in the annulus for
moving the plunger and liquid above the plunerg to the surface. A motor valve , operated by a time clock on the flowline, may ve used to control the cycle rate of the plunger. Following plunger arrival at the surface a bumper opens the valve in the plungerc and the well is allowed to flow for a preset period of time. Plunger lift is one of the most successful methods of unloading gas wells, especially when used with a surface controller.
Small Tubing String:
The objective of using small tubing strings is to reduce the flow area and therefore increase the gas velocity so that liquid will be carried to the surface. This is a very succesful method of preventing liquid loading in gas wells.
These are essentially gas-lift valves with time control. They operate on the principles of intermittent gas lift. These flow controllers may not provide consistent productiob rats because of gas slippage and liquid failback.
Surface flow controllers in conjunction with plunger lifts have been found to be superion to time
controllers. A surface flow controller operates on the principle of permitting the well to flow until a less than critical gas rate occured; it would then shut in the well. Wells having suffient deliverability to maintain rates above the critical rate for short periods are good candidates for rate or critical velocity control.
Injection of forming agents on noncondensate producing wells has become a major method of unloading gas wells. Schematics of soap injection systems are shown. Foaming agents are injected into the casing/tubing annulus with a chemical pump and time clock and water is loaded continuously in a foamed slug rate. Foam injection has not been as successful in condensate producing wells.