A valve lubricant, often referred to ‘valve grease’, is distinctly different to the standard, multi-purpose greases which are familiar to most people. The conditions a gate valve faces in the field are extremely severe and a grease must be able to withstand this environment and continue to provide lubrication and protection to the valve. Wide temperature variations, extreme pressures, flowing hydrocarbons, high levels of CO2 and Hydrogen Sulphide (H2S) all contribute to very difficult conditions for a grease, which is why it must be specifically formulated with these conditions in mind. Below are 6 essential attributes that are required to help enable it to do so:
Hydrocarbon Resistant – Be resistant to the produced fluids from the reservoir, so they are not broken down and dissolved by the fluids.
Cavity Retention – Be retained in the valve cavity so that it successfully provides lubrication and prevents contamination, blocking off the cavity ‘void’ space.
Wear Protection – Be an effective lubricant that can withstand the differential pressure encountered during valve cycling. This is especially important when the valves have begun to wear.
Effective Sealing Performance – An effective valve lubricant should also have optimal sealing performance to help maintain integral well barriers
Corrosion Protection – Provide resistance to corrosive liquids and gases.
Resistant to Drying Out – Resist drying out and hardening, even under extreme temperatures and pressures.
Hydrocarbon resistance
The main products produced from the reservoir are hydrocarbon gases, crude oil and water. It is therefore essential that the lubricant is insoluble and unaffected by these three materials. Although most lubricant greases are fully water resistant, they will readily dissolve in crude oil and the related but more aggressive gas well condensate. This generally leads to the disintegrating and washing out of the valve cavity. Produced hydrocarbon gases, both wet and dry, will also degrade the grease and remove it from the cavity. This often causes deposits made up from a combination of the grease’s thickener along with any solid lubricants the grease contains to remain in the cavity. These deposits will interfere with the valve’s operation, typically causing problems with valve cycling.
The absence of grease will also cause other problems. For example, when grease has been washed out of the valve cavity the protective layer of lubricating film is removed which makes valve damage more likely. Unlubricated valve internals are then likely to rub against each other and cause scoring on the surfaces which create leak pathways which affects the ability of the valve to seal when in the closed position. Furthermore, this then leaves a void, allowing contaminants to enter the valve cavity such as sand or other formation debris, leading to abrasion and erosion of the metal valve components.
Cavity retention
Valve greases often feel stickier than multi-purpose greases and this is a design feature required for valve lubricants to achieve optimal cavity retention. In addition to the tackiness required, the valve grease also needs to provide adhesive strength to surfaces and high cohesive strength to resist being washed away by pressurised liquids and gases flowing through the valve.
For a valve lubricant to excel though, more is required. As stated above, the base oil, thickener and additives must be matched to ensure that water resistance and hydrocarbon resistance are maintained. If any one component is incompatible the grease could easily fail and be washed away.
The temperatures of the reservoir fluids can be very high therefore, a valve grease must also be thermally stable so that it does not become too fluid and easily washed out of the valve cavity.
Wear protection
Another key requirement for valve lubricants is to provide the internal valve components with protection against extreme pressure and wear. In most valve greases some anti wear additives are used to ensure the grease can withstand the forces experienced when valves are opened against pressure and they are also important in reducing wear rates, particularly when valve internals are already partially damaged. Failure to adequately protect and lubricate the contact surfaces of the gates and seats in a valve will result in the valve leaking and not being able to provide the necessary integral well barrier to perform intervention work.
Effective sealing performance
It is important for a valve lubricant to have good sealing performance to help maintain well barriers when the valve is in the closed position. Very often when a valve has been in operation in the field for some time, minor imperfections are created on the surface of the internal gates and seats of the valve. If a valve lubricant has poor sealing capability, these minor imperfections can result in pressure not being able to be held and the valve passing when in the closed position.
This is a problem as the valve must reliably seal to be able to carry out a planned work over or intervention work on a well, and delays in getting a zero leak rate across the valve can result in high expense with maintenance crew and equipment is ready and waiting. A lubricant with excellent sealing performance can help reduce downtime and maintenance costs by ensuring your valves form a reliable seal and integrity is maintained.
Corrosion protection
When a valve cavity is full, the grease acts as a physical barrier between corrosive liquids and gases, which is another reason that good cavity retention is necessary in a valve grease. Most premium valve lubricants will also have a chemical corrosion inhibitor system to act as a secondary line of defence. This is particularly important where either brines or H2S are being produced.
Resistence towards drying out
Some valve greases are prone to drying out if they are exposed to prolonged periods of high temperatures and this susceptibility is dependent on the grease’s thermal resistant properties. Certain types of grease can polymerise which leads to the grease thickening, then becoming rubbery and finally drying out and eventually becoming hard. In particular mineral oil greases are particularly vulnerable to this degradation, especially after prolonged periods at high temperature as often seen on wellhead gate valves.
Hydrocarbon resistance, cavity retention, wear and corrosion protection, good sealing performance and dry-out prevention are all important features, and they should drive your grease selection process, ensuring your equipment and the integrity of your operation is fully maintained.
