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Probabilistic Planning for More Efficient Well Abandonment

Probabilistic Planning for More Efficient Well Abandonment

Applying a risk-weighted probabilistic cost and schedule estimation software tool in well abandonment projects could represent a significant progression for more informed decisions.
For many decision-makers, the traditional deterministic cost estimate has produced satisfactory results. However, in an increasingly competitive business environment, project decisions need to be made in a more informed manner for organisations to become "Best in Class".
In the latest edition of the Platform Magazine, AGR Software BD Manager suggests that applying a risk-weighted probabilistic cost and schedule estimation software tool that can provide a comprehensive understanding of the time-cost risk spread in well abandonment projects could represent a significant progression.

The tool can help provide detailed answers to the three basic questions in managing project costs:
  • What is the likelihood of overrunning the project budget?
  • What is the level of exposure associated with any overrun that may occur?
  • Where do the individual risks lie that need to be mitigated in order to avoid overrun? 
If the risks can be mitigated, then the level of exposure can be reduced and projects can be delivered with greater budgetary certainty.
Probabilistic vs deterministic planning

There is some confusion as to what the difference is between probabilistic and deterministic planning. The normal deterministic approach allows for only one course of events, uncertainty about the outcome is considered by applying a safety factor either to each operation, or more generally to the complete project. This safety factor has to take into account all the uncertainties such as waiting on weather, bad cement jobs, inexperience in running special equipment etc. This approach makes it very hard to address all of the possibilities that may arise during an operation especially in complex operations such as a well abandonment.

Using probabilistic planning based on a simple spreadsheet offers benefits of ranges for time and cost, it is however difficult to enter information on the different paths that may present themselves during the planning operation. Using probabilistic software that is designed for drilling operations allows the identification of these paths and assignment of probability of their occurring. An example of this is shown in Figure 1:

p1 well abandonment

AGR were contracted to support the abandonment of a series of subsea wells. They needed to look at the most effective method of carrying out the abandonment and ensure that all of the local regulator requirements were met.
STEP 1: evaluating the type of vessels
The initial screening covered what type of intervention vessel could be used for the operations: 
1. A light intervention vessel could do some of the work but it would need a Mobile Offshore Drilling Unit (MODU) to carry out other areas.
2. A jack-up could carry out all of the work or the remainder of the work that the intervention vessel could not complete. However a jack-up would have a higher risk of waiting on weather as it moved between the wells.
3. A semi-submersible could carry out all of the work or the remainder of the work that the intervention vessel could not complete. The semisubmersible could kedge between wellheads so would not be affected by the weather, however there would be an additional cost that may not be offset by the possible time saved.
The P1™ software that was used for estimation gave detailed analysis of the time and cost for the many permutations of intervention vessel, jack-up and semi-submersible.  The probabilistic analysis took into account weather conditions, exchange rates, and rig rates among other elements to deliver a simple comparison that allowed an informed choice on the best vessel for the operations.

The abandonment now moved to the second stage, looking at the detailed steps to abandon each well.

STEP 2: probabilistic planning of each well with control over risk and cost

It was important to have a multidiscipline team when preparing the plan, the team consisted of personnel with experience in drilling, completions, sub-sea equipment and a facilitator. All of the team had operational and abandonment experience and could contribute to the process.

The process initially focused on the challenges below the mudline to ensure that the well could be abandoned to an acceptable level, in this case compliance with the UKOA Abandonment Guidelines.

By capturing all these possibilities and using the Monte Carlo simulation the team were able to present an AFE that they had confidence in, but were also able to show the level of risk that must be prepared for if everything took the worst case route as show in Figure 2:

p1 well abandonment apac
The software generates flow charts of the well abandonment steps, this ensures that all personnel involved in the operation can easily understand what will be happening at each step and reduces the chances of confusion. In addition it also gives a clear demonstration of what issues there may be at each stage of the operation. This method of displaying the risk makes it simple to communicate the risk to new team members or to other stakeholders such as senior management, JV partners or government bodies.

Generating these options allows the team to prepare solutions and ensure that the equipment, personnel, chemicals etc. are available either on the rig or poised for mobilisation as each phase is approached. The team found that printing a flow chart for each phase acted as check ensuring that they knew what the challenges were and that they had contacted all the correct suppliers and personnel before the operation started.

As can be seen from the photograph below planning was essential given the very limited deck space that was available during the operation.

aft deck 1

The project was completed successfully within 1.6% of the submitted AFE, this included unforeseen circumstances that, if deterministic planning had been used would have put the project well over the submitted AFE.

For more information about AGR's P1 software, please visit our webpage.

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