One of the key most important factor when repairing a leak in oil & gas wells is the correct placement of sealant at the desired point. The designed sealant pill is typically spotted with help of a work string, coiled tubing (CT), or a dump bailer. This would be normal and straight forward in wells you are drilling or work over.
But what if the well is completed?
Using these methods to repair leaks, might require pulling the completion to have access to the leak point. That is a big and very expensive operation.
In this article, we will discuss how we remediate some leaks in wells by using the gravity feed method which dramatically reduces operation costs and enables placement of sealant in difficult-to-reach places.
How do we seal off leaks by gravity feeding ThermaSet® sealant?
Wellcem’s ThermaSet® sealant has been utilized to solve a variety of well integrity challenges in hundreds of oil and gas wells around the world. ThermaSet® is pumped downhole in a liquid state and with time and temperature will transition into a hard set solid and a permanent seal.
Additional reading: Different materials for oil and gas wells
In several instances, Wellcem has applied a gravity feed technique to spot and squeeze the designed ThermaSet® sealant into the leak pathway and re-establish the integrity of leaking wells. This is sometimes the only possibility of reaching the leak path when you have a large volume and distance to the leak path, but next to no injectivity.
The unique set of ThermaSet® properties which allows it to be gravity fed thousands of meters downhole are:
- Immiscibility in water-based fluids: when mixing with water-based fluids such as water-based mud and completion brines, ThermaSet® resin quickly separates with no contamination.
- Maintaining mechanical properties: tests showed that there is no reduction in mechanical properties of cured ThermaSet® after liquid phase was mixed with water-based fluids and separated.
- ThermaSet is a permanent seal: The long-term integrity of ThermaSet has been tested and proven in accelerated downhole tests simulating hundreds and probably thousands of years in a real well.
- Very low gel strength and low viscosity: this allows ThermaSet® to always sink or rise in fluids until they find density equilibrium or a mechanical base. Meaning that ThermaSet® freely fall through lighter weight fluids or float above a fluid with higher density.
- Adjustable density: The ThermaSet® density can be adjusted so that it is denser than the well fluid and sinks through it.
- Adjustable setting time according to operational requirements. ThermaSet® is a thermosetting resin meaning that it starts setting only when exposed to the design temperature. Wellcem can control the gel time of ThermaSet® very accurately. The ThermaSet® recipe is designed to provide enough gel time to enable placement of the pill at desired depth prior to it reaching its gel stage.
Prior to the operation, a specific recipe is engineered for each well in the Wellcem laboratory, where the ThermaSet® pill is designed to be heavier than fluid(s) present in the well.
The ThermaSet® is fed into the desired well entry (annulus or tubing) from the surface and allowed to fall by gravity to the lowest point. Since the ThermaSet® pill is cohesive and not affected by surrounding fluids in the well (as discussed above), it sinks through the fluids, and settles down on the foundation (e.g., top of cement or packer).
Once the pill covers the leak point, pressure is applied from the surface to inject some resin into the leak paths or the formation.
More to read: Effective ways of reducing well integrity problems
This placement method has been successfully used by Wellcem in many applications;
Here are some examples:
- Eliminating sustained casing pressure (SCP) in the B-annulus:
The designed ThermaSet® pill was gravity fed into the B-annulus from the surface and allowed to fall through annular fluid (brine) to the TOC at around 1200-m. The pressure was applied to squeeze some resin into the leak pathway in the cement.
- Sealing four leaks in the tubing string:
The leaks were located in the tubing at 2015-2035m MD, just above the production packer (PP) where the angle of deviation is around 60 degrees. The designed ThermaSet® pill was gravity fed into the A-annulus and allowed to fall through the brine present in the annulus to the top of pp and cover area from PP to above the top tubing leak.
- Repairing a deep plug in the tubing:
A gas storage well had serious integrity issues due to a stuck wireline conveyed mechanical tubing plug inside the x-mas tree. AA plug which was placed in sub hydrostatic reservoir and inside tubing was leaking. A heavy ThermaSet® pill was gravity fed into the tubing and allowed to fall through the sea water present in the tubing to the top of leaking plug at around 2780 m.
Benefits of gravity feed as placement method are:
- Enables sealing leaks in places where it is hard to access by conventional methods.
- Simpler footprint and lower personnel requirements: the designed pill is batch mixed most often in small dedicated mixers and pumped from the surface using very simple equipment that is easily mobilized to a well site.
- The job can be executed riglessly.
- Significant reduction in cost and time.