Recent experimental studies on intermittent gas lift have shown that the lift efficiency of this method decreases drastically as the viscosity of the fluid to be lifted increases. As the viscosity increases more gas is needed to keep the fallback losses at a minimum. One way to get around this problem and eliminate fallback losses is to implement the use of Gas Chamber Pumps (GCP's).
GCP's are highly appropriate for shallow wells producing heavy oil in places where high-pressure injection gas is available. That is the case of some wells in Lake Maracaibo and in some places in the eastern oil fields in Venezuela that are currently producing oil of 14 to 23°API from reservoirs located at depths between 2000 and 3500 ft.
A variety of different GCP configurations can be found in the literature, from highly complex and compact units to simple types of completion that can be implemented with minor changes of current gas lift completions. The advent of simple and highly reliable programmable surface controllers is making it possible to simplify subsurface completion. The simplicity of these new completions implies a new and economical way of implementing GCP's where they are appropriate.
A description of how different GCP's work and the most popular configurations are given in this paper. It is also explained in detail a new and simple engineering procedure to estimate the liquid production and gas consumption of a well producing with a GCP. This procedure takes into account the inflow capability of the well and couples this capability with the pressure losses across the different parts of the completion and the flow and pressure capacity of the gas lift system.
Even though in many operational situations GCP's are more efficient than sucker rod pumps, currently they are not widely in use. One reason for this to happen might be the lack of injection gas in areas where sucker rod pumps are being used.
The early methods, such as the one depicted in Fig. 1, consisted in alternately injecting gas into a down hole accumulation chamber and bleeding it off to allow it to refill. In Fig. 1, surface valve # 1 is open and surface valve # 2 is closed while gas is being injected. For this type of completion, during the gas injection stage the liquids are forced into the well annulus through a down hole valve installed in a standard side pocket mandrel. This valve is equipped with an internal check valve that does not allow the liquids to return to the tubing chamber. Once the liquid level has reached a minimum, which is at the down hole valve depth or above it, surface valve # 1 would close and surface valve # 2 would open allowing the injection gas to be vented to the surface flow line. The pressure in the tubing chamber then drops to a pressure close to the separator pressure so that the liquids can flow from the formation to fill the tubing chamber again. There are many different types of gas chamber pumps that are explained below.
The advantages of using GCP's over other types of artificial lift methods are as follows:
They are able to manage sand production with fewer problems.
They can lift gassy or viscous fluids better and high fluid temperatures will not affect the subsurface completion.
Some types of completions can be very simple. More complicated pumps can be wireline retrievable which is ideal for offshore installations.
Their use reduces the changes of creating emulsions.
Compared to gas lift, GCP's can significantly increase the drawdown on the formation. They can also be combined with gas lift to get maximum formation drawdown at maximum production rate.
GCP's can provide full pump stroke at any depth or cycle rate.
They can be installed in deviated wells and can be run and pulled with wireline equipment.