Plugging depleted reservoirs can be a challenge. Many operators spend lots of money and effort to solve the associated P&As.
Therefore, it is essential to identify the potential difficulties that can lead to failure early so they can be dealt with in the P&A planning phase.
Low pressure – Too little pressure can result in a cement plug disappearing. The weight of the cement is heavy in relation to the reservoir pressure, and it could just disappear. Depleted reservoirs typically have decreased pore pressure, thus an increased possibility of losses during plugging operations.
In depleted reservoirs, water contamination of the cement slurry is usually a challenge. Cement mixed with water (or other wellbore fluids) will not properly cure. Therefore, you need proper placement procedures and preparation of the mixture to minimize the associated risk.
An old well can have issues with old
Damaged and sometimes collapsed
Other challenges include high permeability channels around the wellbore which can cause the loss of the plug to the formation.
In most cases, the reservoir conditions are not as expected due to mechanical failure and formation changes that has occurred during the production phase. Hence, you will have insufficient or wrong data to plan effective plugging of the well.
Well diagnostics and analysis are essential because of the unpredictability of depleted reservoirs. You can expect most depleted reservoirs to be on loss in the dynamic condition. However, you need to identify the flow paths that may have a direct impact on the plugging operation.
Several factors are required to ensure a successful plugging operation, these include material choice and placement technique.
Material choice is important in the plugging operation for depleted wells. Because the hydrostatic column will be too heavy for the plug to remain in the desired location, high density cement can rarely be used. Therefore, you must use a light weight plugging material to ensure that the pill will stay in place. One important goal is to have the well in equilibrium.
It is vital to ensure that the pressure does not increase above the reservoir pressure. There is usually a discrepancy between the modelled pore pressure and the reservoir pore pressure after depletion.
Pumping material with solids across a collapsed tubular is not recommended. The risk of bridging off, resulting in incomplete placement, is great.
In cases where a work string can be used, the operation tends to be easier. With a work string, you can use almost any type of plugging material since accurate placement is possible. Besides, an LCM material can be used to reduce losses during the operation.
In the cases where a work string is not available, the best option will be a lightweight material to reduce problems caused by the hydrostatic column. Use the hydrostatic balance method to guarantee that the pill stays in the location it was planned. The hydrostatic balance method is limiting the amount of displacement fluid and thereby reducing the hydrostatic head to avoid the pill not being pushed out of place and further into the formation.
Plugging depleted reservoirs successfully is all about proper planning, fit for purpose material and efficient techniques.
This blog post was originally published 7. August 2017 and updated for new readers and followers of The Well Integrity Blog.
Image: Flickr user Glenn Beltz