Since computer modelling of petroleum reservoirs was developed in the late 1980's, the technique has become almost ubiquitous. But, most of the experienced geoscientists that really understand the capabilities of these models are sceptical about the predictive capability of reservoir modelling and acknowledge that even their best models are either highly subjective or are quantified guesses. In recent years a trend has been emerging in which reservoir models are increasingly demanded by regulatory authorities prior to development approval which raises the question: "What is a good reservoir model?"

This paper considers the concept of a ‘good model’ since, without clear guidelines on good models it is difficult to determine who is qualified to build a model or who is qualified to audit a model. The process of verification and validation of reservoir models is then considered and petroleum industry practices are compared with computer modelling in other industries. There is increasing public concern over the development of unconventional resources, and industry concern over possible litigation, so this paper suggests a set of acceptance criteria for sub-surface models with the aim of identifying procedures that are auditable, defendable and robust for adoption by regulators.

During the development of these criteria, a number of key areas in modelling workflows were identified as being of critical concern to the construction of a ‘good model’; model scope, prior elicitation, scale issues, and multiple scenarios. These four areas are discussed in detail and their impact on model quality and predictive capability are assessed. It is hoped that the topics highlighted in this paper encourage enlightened discussion, and ultimately consensus, on the concept of good quality sub-surface modelling of unconventional resources between the regulators and the industry.

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