Sealing a well’s fate

Steve Jagels, Precision Polymer Engineering (PPE), a unit of IDEX Sealing Solutions, USA, addresses the challenges of modern upstream sealing.

Since the dawn of the modern petroleum industry in the nineteenth century, the sector has grown exponentially, with oil becoming one of the most valuable and politically significant commodities on the global marketplace. Today, existing global oil reserves are depleting quickly, and as such the search for new hydrocarbon sources has intensified in recent decades. Now companies are rapidly progressing into hard to reach, previously untapped areas that are commonly found in harsh, arduous terrains such as deep water, or in complex rock formations. This kind of activity is leading operating companies to drill even deeper and push the boundaries of equipment capability further, as they explore increasingly challenging environments.

The implications of sealing failure

This new era of exploration has presented a wave of new technical challenges that equipment manufacturers need to consider carefully if they are to ensure operational stability in the upstream sector. In the face of the increasingly extreme conditions found in an ever-changing operating environment, the industry must ensure that sealing solutions are implemented effectively, and can deliver high quality performance against the most demanding conditions. Fundamentally, selecting the appropriate seal for an application, such as a valve, subsea equipment or tree, will guarantee and achieve safe, reliable operations while making sure that all possible outcomes, such as failure due to extreme temperature and pressure or chemical exposure, are considered and planned for.It is perhaps obvious to state that the implications of a sealing failure in upstream exploration can be considerable, especially when drilling offshore, where crude oil can be leaked into the environment and cause extensive damage to local ecosystems. In addition, the financial repercussions for the operating company can be substantial, losing revenue through downtime as well as additional costs incurred from the repair and retrofitting of seals in difficult to reach assets such as valves, pumps and pipelines. Industry experts must also be aware of the high risk to personnel safety, as a seal failure can potentially result in fire and explosions.

As a result of these potential pitfalls, engineers must ensure innovative and technologically advanced sealing solutions are inbuilt into equipment from the outset, and considered a unique and high priority for each specific application in order to minimise risk and maximise reliability of all operational assets.

The key challenges: Low temperature and high pressure

While oil-based operations on land must cope with demanding terrains, offshore drilling presents an even tougher proposition, especially when considering sealing integrity. Due to remoteness, temperature and pressure of underwater wellhead locations, this type of activity requires an extremely high performance sealing technology to cope with the demands of the environment, and maintain performance against the conditions over the expected lifetime of the asset. Specifically, the combination of low temperature sea waters (typically ranging between 0°C and 3°C) and the high pressures common to deep wells can have a twofold destructive impact on the integrity of sealing elastomers in subsea equipment, increasing the likelihood of failure.

The impact is that high pressures are known to increase an elastomer’s ‘glass transition’ temperature, in turn decreasing the low temperature sealing performance. To understand this better, it is perhaps important to consider the key benchmarks and tests of the sealing elastomer material for specific low temperature/high pressure performance:

The above are measured via ASTM tests (D1329, D2137 and D7426 respectively), which are conducted at ambient atmospheric pressure. Both temperature of retraction and Tg are better indicators of low temperature sealing as they indicate a level of elasticity of the material, which is required to maintain a robust seal in application.

To overcome the challenge of working in high pressure, low temperature sealing applications, it is essential to utilise seals with very low glass transition capabilities to deliver performance. This is because the pressure exerted on the elastomer by the water causes the glass transition temperature to rise, in turn decreasing the free volume of the material, which will reduce its molecular mobility and thus its ability to form an effective seal.

Table 1 shows how high pressure may change an elastomer’s ability to seal at various temperatures. As a generalisation, for every 5 MPa (725 psi) increase in pressure the Tg increases by 1°C.

Therefore engineers must seek and specify an elastomer which is proven and tested beyond the temperature and pressure requirements, ensuring equipment is able to withstand the severities found at seabed and deep well environments.

Selecting the appropriate seal for an application, such as a valve, subsea equipment or tree, will guarantee and achieve safe and reliable operations while making sure that all possible outcomes are considered and planned for.

Steam

Another sealing challenge that is becoming increasingly common in the upstream oil and gas industry is the presence of steam, as increasingly high pressure steam injection is being turned to as a method of thermal stimulation of reserves. One method in particular utilises steam as the key means by which extraction is achieved: known as Steam Assisted Gravity Drainage (SAG-D).

SAG-D is a technology that enhances total oil recovery during the extraction of heavy crude oil and bitumen from oilsands. The extraction process involves drilling horizontal wells in the oilsands, into which high temperature, high pressure steam is injected to liquefy the bitumen, which is in turn recovered. The process has proved popular since its launch in the 1970s, as the surge in oil prices over recent decades has enabled enhanced oil recovery technologies such as SAG-D to become economically viable.

Consequently, in the SAG-D process it is important to consider three crucial factors for sealing integrity; the temperature, pressure and chemical environment that the seal will be exposed to. If these factors are not acutely considered, the risk of seal failure will be intensified ultimately leading to downtime.

A recent development in elastomer technology has improved seal performance in high temperature, high pressure steam environments. An improved type of ethylene propylene diene monomer (EPDM) technology from Precision Polymer Engineering (PPE) can now outperform even the highest grade perfluoroelastomer (FFKM), some of which are shown to be incompatible with high temperature steam as the conditions can cause the material to become brittle and crack. This is also an issue with materials such as nitrile butadiene rubber (NBR) and hydrogenated nitrile butadiene rubber (HNBR) elastomers as they can also crack and break down when exposed to high temperature steam. After testing, results have shown that the EPDM material retains its properties and forms an effective leak-tight seal after 168 hours of steam ageing at 600°F (315°C). For the SAG-D process, the new EPDM seals offer greater performance and high temperature steam resistance. Using these seals alongside SAG-D equipment for upstream sealing could considerably improve reliability and maintain the integrity of the seal whilst also reducing system downtime that is required for maintenance.

In addition, the new EPDM material demonstrates exceptional resistance to rapid gas decompression (RGD). When elastomer seals are exposed to high pressure gas at elevated temperatures for a prolonged period of time the gas absorbs into the polymer compound. When the external pressure is reduced, the gas dissolved within the material comes out of solution to form micro bubbles. As the gas expands, it will permeate out of the material. However, if the rate of decompression and expansion is high, the trapped gas within the seal expands beyond the materials ability to contain the gas bubbles, causing fissuring potentially resulting in seal failure.

When tested to NACE TM0297 RGD standard the EPDM material was found to show minimal signs of damage.

EnDura E90SR (EPDM) steam resistant O-rings used in SAGD operations.

Chemical resistance: sour gas

The harsh chemical conditions faced by operating companies are additionally making it harder for elastomer seals to perform as required. Similar to temperature and pressure considerations, engineers must ensure that they prepare for the chemical conditions of the operating environment, and use a sealing material that is specifically designed and manufactured to withstand exposure to corrosive and damaging chemicals.

Sour gas is the name given to any natural gas containing more than 2% hydrogen sulphide (H2S) by volume. This highly toxic substance commonly found within oil extraction environments has the potential to cause extensive damage to seals in pipelines and exploration equipment, particularly when combined with water. Increasing occurrences of sour gas in oil production now present specific challenges for engineers, who must ensure effective, high performance elastomer seals are employed that can withstand the harsh chemical exposure.

Conclusion

The intricate operating conditions such as those outlined in this paper require proven, tested and reliable sealing materials to ensure the demands of the modern exploration landscape can be met head on. As operating companies reach further and further into unknown drilling territory, it is probable that sealing materials will be required to perform against even greater exposure to extreme chemicals, temperatures and pressures, and as such companies must ensure high performance sealing technology is implemented as a top priority, to deliver safety and reliability at all times.

Sealing providers such as PPE, a unit of IDEX Sealing Solutions, work to prevent the costly repercussion of elastomer failure, and eradicate common faults such as cracks and embrittlement of seals by developing solutions tested above and beyond their requirements. However, it must be noted that no single type of elastomer is appropriate across all applications, but rather specific solutions should be chosen per application.

Due to industry trends over recent years, elastomer seals must now, more than ever, perform beyond traditional requirements. New sealing materials are now available which are proven to exceed the demands of increasingly extreme operating environments, reassuring engineers of their ability to function faultlessly, time and time again.

Read the article online at: https://www.oilfieldtechnology.com/special-reports/02092015/sealing-a-wells-fate/