two stage cementing

Thief zone

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Thief zones are areas in the well where circulating fluids can be lost, typically caused by high permeability formations. These zones have a higher permeability than the surrounding reservoir, leading to early water breakthrough and decreased recovery efficiency. Identifying and understanding the impact of loss zones on production is crucial for developing effective mitigation strategies. Common causes of thief zones include fractured or highly permeable formations. This kind of formation can disrupt circulation during cementing operations and threaten the integrity of the wellbore. Lost circulation in these zones can lead to significant financial losses for the industry.

Possible ways to handle thief zones and lost circulation

To address the effects of lost circulation, operators may use a variety of techniques, such as adding lost circulation materials (LCMs), increasing mud weights, drilling deeper, increasing velocity, using bridging agents, and employing mechanical solutions. Lost circulation materials are materials added to the drilling fluid to help plug or seal the formation. Increasing mud weight, on the other hand, will help reduce fluid loss by increasing the hydrostatic pressure.

For more severe cases, it may also be necessary to drill deeper to avoid thief zones. You can also increase the velocity of the drilling fluid. This can indeed reduce the time it takes to reach the bottom of the hole. Lastly, using bridging agents and mechanical solutions, such as plugs or diverters, can help plug a portion of the hole or divert the fluid away from the loss zone.

Using multi-stage cementing operations

Multistage cementing is a method of cementing a casing string in a borehole. This approach involves the use of open hole annulus packers and stage collars to isolate and prevent the cement slurry from entering the thief zone (definition here). This method allows for multiple cementing operations to perform at different depths, enabling a continuous and homogeneous cementing process.

thief zone

The packers serve to isolate the sections of the casing string, while the stage collars provide a means of sealing off the annular space between the casing and the borehole wall. This approach eliminates the need for a single large volume of cement to pump through the thief zone, and thus reduces the risk of the cement plug failing due to excessive pressure. Operators may also use cost-effective alternatives, such as Xpandable steel external casing packers (ECP), to ensure annulus integrity. These annular barriers can serve to form a tight seal around the outside of the casing.

Once you have permanently deformed the packer, you can then cement it back to the surface. The combination of the cement and the packer in the annulus creates a permanent barrier between the wellbore and its surrounding environment. This barrier helps to prevent the flow of fluids between the wellbore and the surrounding ground, ensuring the safety and integrity of the well and protecting the environment from contamination. As a result, the stage cementing kit allows bypassing the thief zone.

Saltel Xpandable steel ECPs can serve in openhole, deviated, and under-gauged holes and can withstand difficult RIH conditions. The ability to rotate and apply weight to the packer during RIH is also advantageous.