Productive reservoirs appraisal is relevant at all stages of hydrocarbon field development as it provides information for planning and continuous optimization of development. Justification of the value of information obtained during subsurface appraisal is a methodologically complex task, the relevance of which increases under conditions of technical and logistical constraints in offshore operations, as well as when considering costly and non-standard methods to reduce geological uncertainties. In the world practice quantitative justification of appraisal is performed by calculating the value of information (VOI) with its subsequent comparison with the related cost of information gathering. The article formulates the problem of VOI assessment, introduces the key necessary terminology and provides theoretical approaches for VOI estimation both for the simplest idealized cases and more general scenario that takes into account the incompleteness of information. In addition, the paper considers a practical example of the described methods application for the offshore field development: VOI estimation for appraisal of non-developed reservoir of Piltun-Astokhskoye field located on the shelf of Sakhalin Island is discussed. Based on the results of the evaluation, a decision was made to drill a pilot hole to the deeper gas layer with reservoir of interest coring and formation fluid sampling. The appraisal program implementation allowed significantly reducing the key uncertainties in further development planning.
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