Cone packing with PTFE inner seal Preventing oil spills at the wellhead has been a challenge for producers, production supervisors and pumpers (maintenance personnel) since the beginning of oil production. This article analyzes the different approaches and techniques used to solve this illusive problem.
When oil and gas wells were produced throughout the early 1900s, oil spills were not seen as an issue. The government had little or no regulation or control over oil leaks and spills. As the number of oil wells increased and environmental concerns rose in oilfields, the need to keep oil from leaking into groundwater, streams and waterways became a pressing concern. Pump jacks with downhole pumps have been used in the production of oil since the first oil wells appeared in the U.S. Downhole pumps have always been an efficient and cost effective way to produce oil, but with them comes the challenge of preventing the stuffing box from leaking when the packing has worn. This wear is caused by:
- Low oil cut
- High water volumes
- Pitted polished rods
- Misalignment of the pumping unit with the wellhead
The stuffing box packing prevents oil from spilling onto the ground and diverts the oil away from the wellhead and down the flow line.
Early Spill Prevention
As the oil and gas industry advanced, so did spill prevention. Stuffing boxes using crown ring packing and cone packing were developed to allow increased wellhead pressure and less maintenance. These newer stuffing boxes were designed to use rubber packing that could be tightened every day by the operator then replaced when the packing could no longer be tightened. Depending on the well conditions, this packing could last from days to months without needing replacement. The packing still needed to be tightened and monitored daily to prevent an oil spill, but it was much more reliable than the old leather-stuffed stuffing boxes. One of the drawbacks to rubber packing is the moving polished rod contacting the rubber, which creates friction and heat that burns the packing. This burning process can cause an oil leak. One attempt to remedy this problem involved the development of new rubber compounds for each type of well application. For example, sweet crude wells and high temperature steam floods used two different rubber compounds. Flakes of material were added to the rubber, polytetrafluoroethylene (PTFE) or brass and graphite to help lubricate and minimize other issues (such as pitted polished rods or scale buildup). Fluid seal style stuffing box rubbers use a thin rubber lip that surrounds the polished rod to reduce friction. The main drawback to this design was the risk that an operator may tighten the packing too much, which would damage the lip. If this occurred, the packing would need to be replaced immediately. Operators tried inverting the cone stuffing box rubbers to reduce friction. This would cause the oil pressure coming up the tubing to tighten the packing. Then the pressure was relieved when pumped off. All these improvements had one underlying problem: the rubber packing was in constant contact with the polished rod. Therefore failure was eminent. Some other solutions for the friction problem included:
- Operators installed grease fittings into the stuffing box and greased the packing daily. This required an operator to be on location to service the well daily.
- Some operators installed concrete cellars under the wellhead to capture the spilled oil. The cellars were covered with expanded metal. Then they were pumped out as needed by a vacuum truck.
- Polished rod lubricator boxes were installed above the stuffing box to lubricate the polished rod when the well pumped off.
- Some wells used self-tightening stuffing boxes. These were made by adding springs to the bolts on the stuffing box in an attempt to reduce packing failure when the well was unattended.
- Other companies used automatic stuffing box lubricators. In this system, an auto lubricator pumps grease through the grease fitting of the stuffing box at a set interval to reduce friction caused by the polished rod.
- Shutdown, overspill boxes were installed under the stuffing box. These boxes caught the oil that spills from the stuffing box and sends a shut down signal to the pumping unit to prevent a larger spill from happening.
While each of these solutions was beneficial in preventing oil spills, none were long-lasting. Stuffing box rubbers continued to wear, fail and create oil spills regularly. Every time the packing fails, it takes a pumper one to two hours to clean up the mess and replace the packing. This usually involves cleaning the wellhead, disassembling the stuffing box, removing old packing, reinstalling new packing, removing the spilled oil and covering the affected area with dirt by a shovel and drag. Most packing fails because of friction heat buildup from contact with the the metal polished rod during operation. Other failure occurs as a result of pump unit misalignment, pitted polished rods, pumping off the oil well or a well that produces salt water and brine. All these factors can wear the packing and create oil leaks and spills at the wellhead.
21st Century Technology
The most well-known technology to solve wellhead oil spills was developed early in the 21st century. As the environmental movement increased its focus on oil and gas operations, the industry felt greater pressure to solve the age-old problem of spills and leaks at the wellhead. New stuffing box rubbers were developed based on the use of rope packing as a flexible PTFE inner seal inside the rubber packing. Merging PTFE rope packing with the rubber was accomplished by molding a groove inside the rubber cone or crown ring then inserting the PTFE rope packing. The rubber became an outer housing that created the energy to drive the rope packing inner seal toward the polished rod. The PTFE inner seal reduced the friction by self lubricating the polished rod and keeping the rubber from contacting the polished rod. This change made the stuffing box rubber last 10 times longer than packing without an inner seal. Rope packing has been used in transfer pumps for years with great success. PTFE rope packing is flexible and made from nylon fibers. If a unit is misaligned, the inner seal can adjust. The nylon fibers in the rope packing also helped to re-polish pitted polished rods and remove scale buildup. The newly manufactured stuffing box rubbers can also be used in almost all types of wellhead stuffing boxes. Cone style, crown rings, type X and many other factory stuffing boxes could easily be changed over by simply changing the packing. These new stuffing box rubbers were developed in 2002. This inner seal design was tested by the Rocky Mountain Oilfield Testing Center (RMOTC), a federal oil and gas test site that conducts independent testing on new products for the oilfield. RMOTC’s report showed that the packing with the PTFE inner seal lasted 10 times longer than packing without an inner seal.
Solving a complex problem that has been challenging the oilfield for decades was as simple as changing the stuffing box packing to a new style of rubber with a PTFE inner seal. This simple yet effective change eliminates the need for daily service and oil spill cleanup. It is now possible to prevent oil spills while saving time and money.