The challenges of Slot Recovery

Slot recoverySidetracking existing production wells is commonly a cost-effective way of accessing additional production from untapped or poorly drained elements of the reservoir. This is particularly attractive for offshore platform wells where there are limited well slots within the platform design.

Sidetracks assist with maintaining production levels by providing access to additional elements of the production reservoir not sufficiently serviced by the original wellbores. This increases hydrocarbon recovery factors, extends field life, and can significantly improve field economics.

Kick off point

Commonly referred to as slot recovery, this is usually undertaken by abandoning the wellbore below a pre-determined point where a new, secondary, well bore will be drilled. Referred to as the “kick-off point”, this location is chosen to permit access to the new production formation target. The kick-off is usually achieved by setting a device known as a whipstock in the wellbore. This forces the drill bit to drill through the casing (and cement if present) in order to create a new wellbore on the required trajectory to the target.

How to re-enter into the original wellbore?

However, this arrangement effectively prevents any future re-entry into the original, primary, wellbore. Thus, all abandonment work required in the redundant primary wellbore must be carried out prior to setting the whipstock.

Design of the plugging of the current access to the reservoir formation, and any other formations below the whipstock requiring isolation, should be considered with the same rigor as permanent plug and abandonment, as the results will form part of the subsequent final abandonment at the end of the well’s life.

This can pose a number of challenges, many caused by incomplete data and deterioration in the integrity and condition of the construction components.

An ongoing challenge for the industry

This is an ongoing challenge for the industry. Wells that were drilled over 20 years ago and even younger wells present engineers and managers with a number of technical and commercial challenges. Each well drilled has a unique story which must be understood to best plan for a successful abandonment.

The collection of data to understand well architecture, changes in formation pressure over time, metallurgical changes to tubulars in well, cement condition in annulus, and annular pressures at wellhead are few examples of key information that should be gathered and analysed in order to best propose a successful abandonment plan.

These issues are often manifested as a loss of integrity, or a lack of confidence in the primary cement behind the casings. In a significant number of cases a remediation of annular cement is required to restore isolation between formation pressures and surface, in order to satisfy Operator and regulatory requirements.

Your choices

There are several choices available to best suit the needs of the well. These range from Section milling, Cement Squeeze, Perf Wash Cement, as well as other technologies. Each comes with their own of risks and commercial consequences.

1. Cutting and Pulling

Cutting and Pulling the casing to expose the annulus may not be an option when the casing is needed to provide a foundation for a whipstock to kick-off the new wellbore. Where it is an option, it can only be applied when the desired barrier is known to be above the existing cement. Even then operations are often problematic, with corroded casing parting or debris, such as fallen-out barite causing the casing to get stuck in hole.

Both instances can be very time consuming to resolve with little predictability over solution effectiveness, additional durations and cost overruns.

2. Section Milling

The other alternative of section milling the casing also often poses serious technical challenges and demands a certain specification of surface equipment, further driving up costs. Swarf handling at surface poses an HSE risk and cleaning the hole and the BOPs of swarf can be time consuming and problematic.

Finally, the simpler approach of merely perforating a small section of the annulus and attempting to squeeze away a volume of cement may appear attractive but can often in reality result in failure to achieve the desired isolation, the requirement to repeat the operation a number of times and difficult to predict overall durations and costs.

3. Perforate, Wash and Cement

Perf, Wash and Cement (PWC) is a cost effective, predictable and efficient alternative method of restoring a competent cement barrier behind the casing as part of the installation of isolation barriers during permanent P&A. The method has been demonstrated to be significantly quicker than section milling and provides a higher confidence in achieving a long-term isolation barrier, compared with simple perf and squeeze techniques.


Value delivered by use of PWC single casing

Savings achieved

This graph illustrates an example of how savings are achieved at typical offshore spread costs and operational durations.

How quickly costs can escalate

In addition, the graph below illustrates how quickly costs can escalate when using the ‘perf and squeeze’ approach and how, even at low spread costs, PWC quickly becomes the cheaper option, without the additional uncertainties of the supposedly simpler alternative.

Rig spread rate at USD 100K

HydraWell offers a complete service of preparatory engineering and specialist tooling provision and operation, including managing the critical interfaces with cement, fluid, and perforating gun providers, in order to integrate our services into an optimised operational procedure. Our experience has taught us that it is critical to ensure that perforation sizes provide sufficient access to the annulus and the washing programme parameters, including that flowrates, pass speeds, and jetting velocities are optimised within any constraints, such as ECD and pore pressure limits posed by the wellbore configuration. In addition, fluid choice driven by existing annular contents is key to ensuring effective cleanliness of the annulus prior to commencing cementing.

Service costs including preparatory engineering are usually of the order from USD150K per plug, but are dependent on location, tooling size, scope of supply and work scope volume.

As the inventors of PWC, and specialists in this particular area, we pride ourselves on having the best and most extensive track record in this sector of the industry, with our latest tooling design bringing the cost benefits of significantly reduced operations durations, compared to alternative versions of the technique available.

HydraWell’s PWC methodologies are routinely accepted in many regulatory environments including in UK, Norway, Netherlands, Canada and Malaysia, and are being employed by an increasing number of operators including Shell, BP, ConocoPhillips, Chevron, Petronas and Equinor with a success rate of over 98%.

What factors do I need to consider in order to successfully install an annular abandonment barrier?

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What are the common reasons for failure?

- How can they be avoided?

Historically, three basic methods have been used to achieve an annular barrier. Each carries the risk of technical failure or of significant delays and increased cost beyond the forecast.

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