Gas locking in oil wells is a common challenge in the oil industry. In this article, we will delve into the intricacies of gas locking, its causes, consequences, and the solutions employed by the industry to mitigate its impact.
What is ‘Gas Locking’ in Oil Wells that also Produce Entrained Gas?
Gas that remains in the solution when the liquid enters the pump increases the volume of total fluid through the pump compared to the liquid measured at the surface by the formation volume factor at pump-intake conditions. The gas also decreases the density of the fluid and, thus, the head or pressure to be pumped against in the tubing.
Free gas that enters the pump must be compressed to a pressure equivalent to the head required to lift the fluid. This free gas will reduce the volume of both the produced liquid that enters the pump and the liquid measured at the surface. Any time the pump does not compress the free gas to a pressure greater than that exerted on the pump by the fluid column in the producing string, production ceases and the pump is said to be “gas locked.” This condition can exist in both plunger and centrifugal pumps.
Gas locking is particularly challenging in sucker rod pump (SRP) artificial lift systems. According to research from the Society of Petroleum Engineers, “Gas locking has been a problem accompanying the ball and seat sucker rod pump ever since its inception in the oil industry. A gas lock occurs when, contrary to the normal functioning of an SRP, due to a gas influx from the standing valve, the pressure exerted by the fluid below the traveling valve in the closed chamber does not overcome the weight of liquid column lying above the traveling valve in the pump barrel. This fails to lift the ball off the seat.”
Why is Gas Locking of a Sucker Rod Pump So Detrimental to Production?
When a pump gas locks, the area in the pump barrel below the plunger contains mainly gas – and gas is compressible, where fluid is not. In this scenario, the hydrostatic pressure of the column of fluid and gas above the traveling valve of the plunger is greater than the compressed pressure of the gas below the plunger in the barrel of the pump. Therefore, on the downstroke, all that happens within the pump is gas compression and expansion.
Effectively, on the upstroke you have gas expansion, and on the downstroke you have gas compression. In both instances, the traveling valve ball remains on its seat with no new entry of oil into the pump on the downstroke, and subsequently, no new fluid produced into the tubing and displaced to the surface as production. Essentially the pump jack on the surface is just going up and down without any fluid produced to the tanks.
How Does the Slimline® Tubing Anchor Catcher Aid in the Reduction of Sucker Rod Pump Gas Locking?
When produced fluid and gas enters the casing from the reservoir via the perforations below the pump, it exhibits a pressure drop that causes gas to break out of the solution and rise towards the surface in the casing. With the increased flow area of the Slimline® design between the anchor OD and the casing ID (+245% over the standard anchor), the gas is able to be produced up the casing / tubing annulus rather than through the sucker rod pump where it creates the problems mentioned above. The standard tubing anchor catcher is a “choke-point,” causing restrictive turbulent flow. In contrast, the Slimline® from TechTAC®, with its tapered design and smaller diameter, allows for a laminar flowpath.
Validated by CFD Analysis
A recent computational fluid dynamics (CFD) study from the independent consulting firm Imaginationeering finds that the net pressure drop around a standard Baker B2-style tubing anchor, as fluid/gas passes through the annular cavity around the anchor, is more than double the pressure drop around TechTAC’s Slimline® TAC. A significant pressure drop, like that around the standard anchor, is the primary issue causing gas locking in rod pumps and a major contributor in the formation of scale, iron sulfide, and paraffin.
According to Bruce Friesen, former global artificial lift advisor for BP, “[The CFD study highlights] multiple benefits of running the TechTAC Slimline anchor. Number one, reduce pressure and temperature drop, reduce turbidity and turbulence, which all four of those things contribute and cause the formation of scale, iron sulfide, paraffin and solids. [Eliminate] all those parameters … then you don’t have plugging. You don’t have stuck anchors. You don’t have workovers. You don’t have fishing jobs. And you also don’t have what is one of the biggest issues and problems and headaches in the oil patch is gas locking of your rod pump.”
Conclusion
Gas locking is a common and challenging issue in the oil and gas industry that can have significant consequences for production rates and operating costs. Understanding the causes and consequences of gas locking is essential for oil producers to implement effective mitigation strategies. Among several possible solutions – from gas injection to costly workovers – one of the most cost-effective ways to mitigate gas locking in SRP systems is by deploying a Slimline® TAC from TechTAC®. The Slimline’s unique design dramatically increases flow-by capacity, which reduces the occurrence of gas locking by directing gas around the anchor instead of through the pump. By minimizing the frequency and impact of gas locking, production companies can significantly reduce cost, increase run time and increase the production of oil well operations.
