Myths and misperceptions about RLWI

Milling bit packed with set asphaltenes Copyright Welltec A/S

Various myths and misperceptions surround riserless light well interventions (RLWI), preventing operators from performing these cost-efficient, production enhancing maintenance operations on their subsea wells.

In fact, since the start of the millennium more than 2000 RLWI operations have been performed in more than 400 wells. Welltec has milled hydrates, asphaltenes and malfunctioning hardware, performed scale clean-outs and pulled plugs—all as RLWI operations.

The previous five years have seen advances in vessel and deployment technologies and the water depth has more than tripled. RLWI water depths now exceed 3900ft (1200m). The only limitations to water depths are the ratings on subsea lubricator components and umbilical length, and this boundary is being extended every year.

Experience shows that frequent interventions enable increased recovery from subsea wells. On average a well will need some form of intervention every four years to keep optimum production. Production optimization through sand, scale and other debris removal, as well as replacement of worn systems and components have not traditionally been feasible operations, but RLWI technology has changed this, making it possible to close the 10-30% production rate gap that currently exists between dry tree and wet tree wells.

Since the introduction of mechanical intervention services in 2003, numerous RLWI operations have proved that it is possible to do much more than just logging or perforating. An estimated 80-90% of the intervention scope can now be performed using RLWI including complex mechanical or clean-out operations.

In addition to these production related benefits, reconnection to well after disconnect is much faster with RLWI than with a rig, and vessels can be farmed out to other locations, which facilitates greater scheduling flexibility and vastly improved operational efficiency compared to using a drilling rig. The e-line solutions offer robotic precision irrespective of the well’s inclination and the lightweight nature means equipment can be mobilized quickly by vessel or helicopter.

Contrary to common belief, RLWI operations are being performed year round, as seen by operators in parts of the North Sea. Some areas of the world will always hold extreme weather conditions, but given the fast transit speeds of these vessels, they are able to be redeployed to other areas very efficiently.

Bailer section packed with dense asphaltenes after the filter is removed. Asphaltenes set once exposed to lower temperature; when in the well under a certain temperature the asphaltenes are stickier. Copyright Welltec A/S

Current effects are handled through thorough planning and downline management. For example, the West of Shetland is infamous for its current conditions: 1.5-3.5 knots from 50-160m water depths and 1.2 knots from 360-460m depths.

Welltec has performed numerous successful RLWI interventions in the area. A proprietary simulation model based on relevant metocean data allows the Welltec crew to determine which torque to expect on the cable head and adjust as needed.

RLWI operations are considered safer than rig-based operations, as there is no pipe or riser connected to surface. No hydrocarbons at surface means the emergency disconnect scenario is easier to handle if a blowout situation should occur, as the e-line is much easier to shear and seal than pipe.

With built-in Welltec release devices configured at different points in the toolstring, should the tool become stuck at any point in the operation, it can be released from surface or with a timer at various points, to remove as much of the toolstring as possible and leave a standard fishing neck on the remaining part of the tool. Proper planning allows additional lubricator sections to be stacked to hold fishing-string and fish for contingency.

More and more operators are recognizing the benefits of RLWI interventions and as the number of subsea wells is steadily growing, RLWI is gaining ground.

Written by Bevan Morrison and Ole Eddie Karlsen, Welltec A/S