Historically, deep HPHT exploration wells in South Oman have been designed in a deterministic manner. In the design phase certain assumptions are made somewhat risk-averse, resulting in well designs catering for "worst-case" conditions. With hindsight this method, although proven to be sound, has resulted in over-designed wells.

The cost conscious Well Engineer, by better accounting for uncertainty, needs to minimise the over-design without risking the loss of pressure control and its possible effects on Health, Safety and Environment. It is evident that a careful balance needs to be found between risk and the level of over-design.

A novel approach to reducing the over-design is through applying a "Probabilistic Well Design" method. This method differs from the conventional design methods by defining a model in which the geological variables are defined in a probabilistic rather than a deterministic manner.

Once the model is described outcomes are simulated with statistical "Monte Carlo" software to calculate the distribution in the range of maximum design loads. Based on this distribution curve the Well Engineer can define the maximum safe operating limits of the well. The method results in a "fit for purpose" well design, with the clear understanding that if the worst case conditions (which have a low probability of occurring) are approached, some of the well objectives may not be achieved. If this should turn out to be the case the downside is that a subsequent well that does cater for these conditions is then required.

The technique has been successfully applied to two deep onshore HPHT wells during 1997 and is planned for two further wells in 1998. In the first application the technique resulted in a well, that if designed in the deterministic manner would have been drilled as a 15000psi well. After applying the probabilistic analysis it was determined that there was a 93% chance that the objectives could be met without the maximum surface pressure exceeding 10,000 psi. The well was drilled and all the objectives were met within the 10,000 psi design rating.

The probabilistic design technique is now applied to all the HPHT exploration wells and it is anticipated that it will result in considerable reductions in drilling costs in the future.

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