Quantification of uncertainty in reservoir performance is an important part of proper economic evaluation. The uncertainty in our understanding of a given reservoir's performance (e.g. reserves at the economic limit) arises from the uncertainty in the information we have about the variables that control reservoir performance (e.g. permeability, oil water contact, etc.). The problem is complex since the influence of the variables on the reservoir performance is often non-linear. Conventional techniques often ignore this.

This paper describes the application of the method of Experimental Design to this problem. It consists of the following five steps :- (i) Identify the ranges of the important variables that influence the recovery, (ii) Experimental Design to identify the values of the variables at which the recovery is forecast, (iii) "Conduct the experiments" - use reservoir simulation to identify the recovery for each of the experiments identified in step (ii), (iv) Analyse "the results of the experiments" and establish a generalised multivariate correlation of the recovery as a function of the variables, (v) Validate the correlation arrived at in step (iv), and (vi) Predict the distribution of the uncertainty in the recovery. Here the term ‘recovery’ is used in a generic sense; the technique can be employed to any parameter of interest e.g. gas flow rate after ten years of production. The paper discusses an example of the application of the Composite Design to a North Sea gas field.

This technique has potential value in providing input to evaluation of economic risks in a project in the appraisal and development stages.

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