The PHI system is a retro-fit surface termination system designed for all semi-permanent deliquification installations.

Liquid loading in gas wells is a well known phenomenon in mature reservoirs. At depletion stage, the energy of the reservoir is not high enough to transport liquid droplets to the surface. They begin accumulating at the bottom of the well bore and cause backpressure on the reservoir, reducing gas production and eventually killing the well.

It is estimated there are around 775,000 active gas wells globally — excluding Russia and China — with 43,000 new wells drilled annually. With approximately 90% of wells suffering from liquid loading, there is an increasing demand for reliable, cost-effective deliquification solutions.

Caledyne has produced the first complete deliquification system suitable for offshore applications. The first installations are due in several locations over the coming months, including the southern North Sea, mainland Europe, and offshore Indonesia.

Challenges in different environments

Various artificial lift technologies are used to mitigate the consequences of liquid loading and to extend the production life of mature gas reservoirs. Most systems have been developed and used successfully in onshore environments. Approaches range from lowering wellhead pressures by using compressors or eductors to downhole artificial lift using plunger lift, surfactant technology, or pumping technologies. Depending on the individual well/field characteristics, these technologies have proven highly successful in mitigating losses caused by liquid loading.

Challenges range from offshore environmental considerations and field characteristics, such as liquid loading with hydrocarbon condensate, to cost pressures. In the main, the industry tries to address these challenges through the evolution of deliquification technologies from onshore to offshore.

Offshore environments technical challenges

The main challenges in offshore environments occur when legislation requires sub-surface-safety valves (SSSV). SSSVs need to be able to shut in wells in case of an emergency, which is done with a flapper-type arrangement valve. Hydraulic pressure is taken off of the control line, and the flapper of the SSSV closes and shuts in the well. The dilemma with the traditional SSSVs is that by definition nothing can be run through the flapper-valve when it is closed in emergency. Most artificial lift technologies (surfactant injection lines, rods, power cables) need to be run through the SSSV when being retrofitted to wells. Several operating and service companies have addressed this by developing enhanced SSSVs, although a reduction in the effective flow area is often the result.

Another challenge is wellhead penetration. Most forms of artificial lift technology such as surfactant injection lines, coiled tubing for gas lift applications, and power cables need to penetrate the wellhead. This can be a technically challenging exercise if no spools can be changed out or a very costly exercise if — as a consequence of wellhead modifications — flowline heights need to be changed.

Reservoir, cost challenges

Some of the fields experiencing liquid loading have been characterized as retrograde gas condensate fields. This causes two factors that can potentially impact production in such fields:

1. The drop-out of hydrocarbon condensate from gas in the reservoir causes a reduction of relative permeability to gas, leading to less inflow of gas into the wellbore, known as condensate banking.

2. Liquid loading with hydrocarbon condensate can lower the success of surfactant injection, one of the most widely and successfully used deliquification technologies, as hydrocarbon is technically very different from foam than it is to water. Several condensate foamers have been developed, but their operating range is narrow.

Potential economic deliquification benefits are significant. And the number of wells in need of these technologies is high — in part because wells subject to liquid loading are mostly mature and were not originally completed with the purpose of deliquification in mind. Because the remaining lifespan of these wells is relatively short, there is an added reluctance to invest significant amounts of money as there is increased risk of not recouping the original investment.

Many mature offshore wells produce to unmanned installations or platforms with limited crane capacities and deck space, which makes any type of workover activity a costly exercise.

Deliquification technologies have to be cost-effective in the long run, i.e., run with the minimum requirement for intervention, monitoring, and optimization.

Complete solution

Caledyne has produced the first complete deliquification system including its balance pump, Torus Valve, and pin hole injection (PHI) system.

The patented Caledyne Torus Safety Valve was developed in response to industry demand for an “enabling solution” to retrofit artificial lift technologies into existing wells, previously restricted due to the flapper-type SSSV.

Development of the 41?2-in. size was facilitated through a joint industry collaborative project by the Industry Technology Facilitator (ITF), with contributions from major operators BP, Chevron, ConocoPhillips, and Total. The Torus SSSV allows any artificial lift solution to be installed in offshore operations. It is a fail-safe closed insert safety valve, offering a permanent conduit through the center of the valve. It is installed in the safety valve nipple profile and operated by the existing hydraulic control line, using a sliding sleeve mechanism to shut in production. The sleeve is operated via a power spring and piston arrangement akin to industry standard flapper-type safety valves. Using the sliding sleeve mechanism results in a far greater flow area than that made available by flapper valves using a bypass mechanism for umbilicals. During API 14A qualification, the valve has proven to be a zero leakage valve even at low pressures.

The Caledyne balance pump, which can be installed through the Torus Safety Valve, provides a cost-effective, reliable deliquification method for operators of both onshore and offshore wells suffering from liquid loading. It is a retrofit solution that allows operators to avoid costly workovers.

The hydraulically actuated reciprocating low-volume pump is operated by two hydraulic lines, one filled with oil, the other with water. The differential hydrostatic pressure between the water and oil acts to move the internal piston downward. Applying pressure to the oil line drives the piston upward, lifting the water trapped above the traveling check valve and drawing water into the lower chamber. Removing the pressure from the hydraulic line allows the water line to return the piston, after which pressure is reapplied to the hydraulic line.

The pump is designed for maximum reliability and increased run-life. All metal work is constructed from AISI 420 80ksi stainless steel for increased corrosion resistance to H2S and CO2.

The PHI is a retrofit surface termination system that minimizes disruption at the surface when umbilicals are fed through the wellhead and avoids the insertion of spool pieces or flow line modifications. It has been designed for all semi-permanent deliquification installations, both onshore and offshore.

The PHI system provides communication via a hollow tubing hanger tie bolt through a sub landed in the tubing hanger profile. Should a tie-down bolt not be available, an alternative entry point can be created above the tubing hanger. For offshore applications, the system is generally used in conjunction with the Torus Insert Safety Valve configured for capillary installations.

This allows communication into the well with the following advantages — no disturbance to topside infrastructure, no compromise on safety or well integrity, no significant downtime, and no need to kill the well.

Acknowledgements
The author thanks BP (Matt Dunning, Andrew Patterson, Werner Schinagl) for support in writing the article.