Well type
Gas
Max. temperature
304°F (151°C)
Max. pressure
3,812 psi
Max. deviation
54.4° @ 9,567 ft (2,916m)
Min. restriction
4.437” 28 ft (8.5m)
An operator was having problems with a wireline retrievable safety valve assembly impeding production in a vertical, cased hole oil well in Nigeria. The valve failed to seat properly in the tubing and flowed up hole with production to get stuck somewhere near the tubing hanger; precise position unknown.
The operator had made several slickline attempts to jar the fish free. The first of these involved more than 200 strokes in each direction using 2 1/2” heavy sinker bars, accelerator and a spring loaded jar with a pulling tool latched into the lock mandrel fish neck. When this was unsuccessful, another slickline attempt was made using a blind box on top of the inner packing mandrel sleeve to attempt to land the blows below the keys. In total, more than 1,000 strokes were made.
During the last phase, indicators showed that the lock mandrel fish neck was partially open, suggesting that its keys were jammed in a slightly inclined position, below the lower ID (4.795”) of the tubing hanger profile. The operator then turned to the contingency of using CT to perform a cut using sand erosion. But with little progress and lack of precision control they decided to engage Welltec® to mill out the obstruction and save the completion string.
Based on the unique problem description, Welltec engineered a custom solution consisting of a Well Tractor®, Well Stroker®, Well Miller® and a variety of bits.
The plan, as designed, was to mill through the safety valve in stages to remove the inner mandrel sleeves. This in turn would release the springs that were holding the dogs and subsequently dislodge the valve.
A System Integration Test (SIT) was conducted beforehand to confirm the solution for this project’s objective. Using the Well Tractor and Well Miller plus custom nipple milling bit in combination with the Well Stroker the team succeeded in milling out the safety valve as planned during the SIT.
The safety valve fell to the bottom of the well as desired, restoring access and production.
This successful milling operation enabled the operator to avoid having to pull the entire completion string and saved more than USD 2 million compared to the alternatives.
The operator was very pleased to be able to regain the well in a shorter time frame and estimated that the milling solution was twice as efficient as the other options.
