Put a plug in it

Plugging operations on oil and gas producing wells are conducted for a number of different reasons, such as extending the life of a producing field or shutting it down permanently.

Plugging a well in order to conduct a slot recovery operation can extend the lifetime and value of existing platforms and processing equipment. In addition, plugging is used in well repair operations through the restoration of zonal isolation in wells, thereby increasing production and oil and gas recovery ratios. At the other end of the spectrum there is the plugging and permanent abandonment (P&A) of wells, which cuts off both the hydrocarbon and revenue stream for operators and license holders.

Section milling

Since the 1970s, section milling has been the favoured method of oil companies when plugging an oil and gas well where the annulus integrity is lacking, either for slot recovery, restoration of zonal isolation or plug and abandonment operations. However, the section milling method can be both time consuming and difficult to execute safely and effectively.

Briefly explained, section milling incorporates the removal of a section of casing by milling operations which allows for the installation of a rock to rock plug for hydraulic isolation purposes. After the section of casing is milled, the well needs to be properly cleaned out by removing the swarf cuttings (the metal shavings left behind by a cutting tool) and other debris. After the new formation is exposed, a balanced cement plug is placed in the section.

A traditional section milling job is time consuming. Typically, it takes 10 - 14 days to perform a 50 m section milling operation using an expensive drilling rig.

Proper hole cleaning is a challenge and the fluids designed for section milling must have sufficient viscosity to suspend and transport swarf and debris to the surface. In the worst case, poorly transported swarf and debris can build up downhole and result in stuck pipe. Section milling also presents health, safety and environmental challenges, as the swarf cuttings from the well, which are brought to surface, requires specialised material handling and disposal. The cuttings are razor sharp, demanding protective equipment when handling. On average, milling a 50 m section of an offshore well generates approximately 4 t of swarf cuttings.

Offering an alternative

The oil companies challenged the industry to find a more effective method of plugging wells, given the challenges of the existing techniques. As a result, the Norwegian technology company HydraWell was founded in 2010.

HydraWell worked to create a technology that could improve the operational efficiency of plugging through multi-activity tool combinations. From this HydraWell developed its single-trip PWC, Perf-Wash-Cement^® technology.

Briefly explained, HydraWell’s PWC HydraHemera^TM system consists of three parts. Placed at the bottom of the assembly are the perforation guns (supplied by third party), which for P&A operations are dropped into the well upon firing. Above the perforation guns there is a jetting tool, with a cementing tool just above.

Figure 1. An illustration of the HydraHemera perf, wash, cement tool. 

The jetting tool is used to wash and clean out debris in the annuli behind perforated casings. It features jet nozzles, which are configured at irregular angles and engineered for optimum fluid velocity and annuli cleaning efficiency. The jets penetrate and clean thoroughly behind single or multiple perforated casings. In this process, debris, old mud and cement traces are replaced by clean fluid. The jetting tool ensures clean conditions in the casing annuli prior to placing the plugging material in the cross section.

Using a ball drop mechanism after jetting, the cementing tool is activated. This enables the placement of a barrier material in the entire cross section of multiple annuli, and hence, establishing a proper rock-to-rock barrier in the well. The system is available for all casing sizes.

Just as in section milling, the mud weight must be sufficient to maintain the stability of the exposed formation. However, high viscosity fluids are not required to lift metal debris from the wellbore. In addition, the PWC system creates an abandonment plug that can be verified. After placing the barrier material, it is possible to drill out the plug and perform a cement bond log to provide verification of integrity.

Time and cost saving

Since introducing the technology, 16 operators – including supermajors, national and independent oil companies – have accepted and used the technology, installing close to 250 plugs worldwide. The PWC system has repeatedly demonstrated that it is capable of plugging offshore wells in 2 - 3 days instead of the 10 - 14 days it takes with traditional plugging methods such as section milling.

ConocoPhillips Norway was the first company to use the technology on its Ekofisk field in the North Sea in 2010. At a presentation held at the Norwegian Plug & Abandonment Forum in 2016, ConocoPhillips stated that at the Ekofisk Alpha field had achieved a 70% improvement in P&A performance based on the number of days saved per well, thereby concluding the P&A campaign almost one year ahead of schedule. This delivered significant cost savings to the operator. The technology played a major part in these savings.

In total, by employing the PWC method, more than 1500 rig days have been saved for oil companies, with the additional benefit of a significantly smaller environmental footprint as no swarf cuttings have been brought to surface for further disposal. The latter accumulates to about 950 t, equivalent to 12 000 m of casing that has not been milled.

Rigless P&A

The next development in HydraWell’s technology portfolio will deliver a step change in P&A efficiency: plugging offshore wells without removing the production tubing by using the HydraArtemis^® technology.

Typically, thousands of metres of production tubing is brought to surface in the current P&A activity. Today, oil companies often have to upgrade the drilling unit on their platforms, or hire a drilling rig, in order to execute P&A operations to retrieve this production tubing to surface.

The new proposed solution that is being developed, enables the plugging of wells from the production platform, without a conventional drilling rig, but by the use of well intervention equipment such as coiled tubing, wireline or e-line. Performing this type of P&A work, without a drilling rig, can significantly reduce costs and environmental footprint for both oil companies and society as a whole.

An additional benefit, to avoiding the tubing retrieval, is that the method reduces costs and risks associated with heavy lift vessels, transport of material onshore plus demolition and disposal of the tubulars recovered from the well.

HydraWell is in dialogue with oil companies to further developing the HydraArtemis technology and start testing it onshore and offshore.

Figure 2. A cross section of a well showing how the HydraHemera jetting tool washes and cleans out debris in the annuli behind perforated casings.

Importance of cost efficient plugging operations

A wave of oil and gas wells drilled in the last 50 years are reaching the end of their useful life. These wells now need to be plugged and abandoned in a safe manner, so as to avoid costly liabilities for oil companies. Plugging and abandoning wells represents an unwanted but necessary cost to the industry. Any improvements in efficiency and reduction of cost in this activity would deliver significant financial benefits to operators and license holders.

Furthermore, as plugging and abandonment activities in many cases are subsidised through government fiscal regimes, cutting the cost of such operations will ultimately benefit the tax payers too. In a world where the oil and gas industry is coming under increased scrutiny, contributing towards lowering society’s overall cost and environmental footprint makes perfect sense.

Read the article online at: https://www.oilfieldtechnology.com/offshore-and-subsea/05092018/put-a-plug-in-it/