| dc.description.abstract | RLWI is an important part of Statoil’s goal of increasing the recovery factor from subsea wells from 42% to 55%. Subsea well intervention has traditionally been performed by expensive workover rigs which have made intervention for subsea wells less desirable than for platform wells. The introduction of RLWI has made subsea well intervention more desirable. To make it even more desirable, limitation and problems needs to be identified and improved.
After investigating 68 RLWI operation performed for Statoil by the Island Offshore Alliance, the three problems with the highest frequency were identify to be; scale related, difficulties pulling the THCP and fish left in the hole.
A HXT vs. VXT time analysis showed that RLWI operations takes about two days longer to perform on wells with HXT. The main reason for this difference is because a THCP needs to be pulled, which runs into difficulties 40% of the time. A RLWI will always take longer time for HXT because the THCP needs to be pulled, but the time difference could be reduced by applying a proper THCP pulling procedure. An important part of improving the THCP pulling procedure is to improve the jarring operation.
Drag force acting on the wireline by the sea current have the potential to reduce the sensitiveness of the jarring operation. The drag force can reached 1 380 N on the 7/32” braided line during worst case scenario, which creates a deflection of 23m at 300m water depth and a TS weight of 2 000 N. The drag force can be reduced by developing a wireline with higher specific weight, smoother surface and smaller diameter. Or the drag force can be eliminated by placing the wireline winch on the sea bottom.
Oil field scale deposit is a big problem for many fields world wide as many fields are getting older. Scale deposit in the well is the reason for many intervention complications such as; high friction in the well, unable to install/RIH downhole equipment and stuck downhole valves and chokes.
The introduction of the rotating PTA tractor has provided a cost efficient scale removal technique from electrical wireline. The technique has however two major disadvantages compared to CT and snubbing; the power available downhole is very limited and fluid circulation is impossible. Because of these limitations the technique can only remove relative short soft scale bridges due to the limitation in available downhole power, and the scale bridge cannot completely block the fluid because that makes it impossible to remove the debris. A new wireline called CC can however dramatically increase the available downhole power, but fluid circulation will be impossible to achieve with wireline.
Whenever a fish is left in the well it must be investigated what went wrong to avoid the similar incidents in the future. Three cases have been looked into in this report where a fish was left in the hole:
o Fish left in hole due too to small ID in the TFL completion solution, the TFL is not a very intervention friendly completion
o Fish left in hole because of perforating in too high overbalance. If high overbalance is suspected the TS could be anchored in place to avoid it from being flushed up the well, or the well could be shut in prior to perforating.
o Fish likely left in hole because of wrongly installed muleshoe in the tailpipe. The self aligned muleshoe has a spring loaded inner part which has a cushioning effect on the jarring operations, which makes it difficult to jar the TS free. Whenever a muleshoe is installed a caliper run should be run to see if it is properly installed.
RLWI has limitations and disadvantages compared to CT and snubbing, but with the development of new techniques and equipment the RLWI clearly has a huge potential to increase the recovery factor for subsea wells. New equipment and techniques will also be needed to perform intervention in constantly deeper and more complex wells. | nb_NO |
