Replacing the AC system in your home is a little like a prostate exam. As the equipment gets older, it’s going to be necessary, but it’s never fun. Such was the case for our family two summers ago when our AC unit finally gave out. Interestingly, one of the main factors contributing to the failure was also a necessary part of the system. The air filter that was needed to keep the circulating air clean was also restricting the airflow too much for the old system to handle. The result was a frequently frozen AC condenser and soaring temperatures in the house.
A very similar issue – where an essential part of the system causes unintended consequences – can occur with standard tubing anchors in oil wells. Tubing anchors are often an important part of the downhole production string. In wells using rod pump systems, the anchors can stabilize the string. That stability prevents unnecessary cyclic movement of the tubing that can cause tubing failure and reduced rod pump efficiency and life. However, that same standard anchor can create serious problems with well production.
The Impact of Tubing Anchors Above the Pump Intake
That was the finding of a research study conducted by Echometer Company. The Texas-based software firm offers solutions for analyzing and optimizing the performance of oil, gas, and water wells. In this case, the firm studied the production rates of 11 oil wells. Each well had high fluid levels and the pump intake was located below the perforations. Yet despite these seemingly favorable conditions, nine of the wells showed a pump fillage rate of less than 90%.
According to the report: “In these wells that exhibited high fluid levels … the fluid distribution in the wellbore below the liquid level was not uniform. The wellbore in the vicinity of the pump intake was primarily filled with gas with a minimal volume of liquid. …The presence of a tubing anchor set high above the pump intake is considered to be the main cause of this uneven distribution of fluids in the wellbore.
“The particular tubing anchor used in these wells provides a flow area of about 2.9 square inches between the body of the anchor and the casing compared to a flow area of 14.4 square inches between the casing (4.892-inch ID) and the tubing (2.375-inch OD). The small flow area could increase the velocity of the upwards flowing gas to the point where it would be difficult for liquid present in the upper part of the annulus to flow downwards past the depth where the tubing anchor is set. The anchor would essentially act as a choke and also cause an increase of the annular back pressure.”
In other words, the tubing anchor itself was restricting the flow of gas up through the annulus of the well. The gas that did escape had an increased velocity. That velocity made it difficult for fluid above the anchor to fall past it and reach the pump. In turn, the fluid around the pump had a high gas content, making highly efficient production impossible.
Tubing Anchors Can Provide a ‘False Positive’
Another key finding in the report was related to the fluid levels in these wells. In many cases, the fluid levels above the anchors appeared higher than they actually were. The report states: “Fluid level depression tests were used to confirm that free gas can collect below a tubing anchor and prevent the liquid present in the gaseous column above the tubing anchor from falling to the bottom of the wellbore and to the pump intake.”
Said differently, the fluid above the anchor was unable to fall past the anchor at a significant enough rate to allow full pump fillage and decrease the overall fluid level. This phenomenon essentially gave operators a false reading, since they thought they had high fluid levels. However, the reality was that fluid just couldn’t get past the anchor as quickly as it was being pumped out.
Possible Solutions
The Echometer team notes that removing the anchor or moving it below the perforations should resolve the issue. Of course, neither option is necessarily ideal. Removing the anchor would increase the flow area within the casing. However, it would also cause the cyclic movement of the tubing string. And although setting the anchor below the perforations can be an effective model with the right equipment, many engineers and field operators are wary of this approach.
Consider the Slimline® Tubing Anchor Catcher from TechTAC®
Thankfully there is another possible solution. The design of the patented Slimline® Tubing Anchor Catcher improves the flow-by capacity of fluid in the annulus by as much as 245% over a standard anchor. That increased flow-by area allows gas to more easily flow up the well, rather than being trapped around the pump. The Slimline® also features tapered flow deflectors on the top and bottom. That tapered design, along with the anchor’s reduced OD, virtually eliminates the risk of sediment or paraffin building up on top of the anchor. As such, it can be safely set above, in, or below the perforations, depending on the well design.
One U.S.-based oil and gas company saw these benefits firsthand. “We were having trouble with high fluid levels because the fluid could not sufficiently drop past a standard anchor,” said a senior completion foreman with the company. “Those elevated levels were causing some pump failure due to gas locking. We tried the Slimline TAC, and in every well we tested, fluid levels were at or just above the anchor.”
Conclusion
A tubing anchor is an important element of many downhole production strings. However, the study by Echometer found that those same standard anchors can restrict production rates and pump fillage. The Slimline® Tubing Anchor Catcher from TechTAC® provides an alternative to standard anchors. The Slimline’s increased flow-by capacity can reduce or eliminate many of the issues identified in Echometer’s research.
To access the complete Echometer study, titled Tubing Anchors Can Reduce Production Rates and Pump Fillage, visit the Society of Petroleum Engineers’ research website www.onepetro.org. For more information about TechTAC’s downhole tubing anchor solutions, including the Slimline® TAC, visit the company’s anchor product pages.
