Field-Development Process Revealing Uncertainty-Assessment Pitfalls

Summary The amount of information available for field-development planning is limited, forcing the production strategy (PS) to be designed with a great amount of uncertainty. During its implementation, new information allows the adaptation of the strategy for economic gain. This work reproduces the field-development process under geological uncertainty in case study UNISIM-I-D (benchmark case that is based on Namorado Field in Brazil). The main objectives are to evaluate the process and to observe the evolution of risk curves, all in a controlled environment with real-field features. The methodology generates new geostatistical images on the basis of new well logs, assimilates production data with an ensemble-based method, and reoptimizes the PS with a hybrid algorithm. The field development is carried out by repeatedly applying this framework with human supervision. Each step is customized with algorithms to simplify the implementation and to reduce computational effort, making this methodology more appealing for practical use. New data are collected from a high-resolution reference model that does not belong to the ensemble of models. The process starts with a PS, previously optimized under the uncertainties of the case study, which yields the real economic outcome within the original uncertainty range. Results show high-quality history matching (HM) that excessively reduced the risk range and the variability of the updated model sets. Optimizations on the PS, on the basis of the updated ensembles, consistently increased the expected monetary value (EMV) of the project without guaranteeing an increment in the real net present value (NPV). Applying the methodology repeatedly throughout the field development increased the EMV by 29% (from 1.532 to 1.975 billion USD), whereas the real NPV decreased 2% (from 1.346 to 1.319 billion USD), falling out of the expected range and revealing that the model sets did not fully represent the real field. The lack of good representation is aggravated by heterogeneities inherent to the unknown reservoir, which are difficult to identify with only well logs and production data. The results from the application of a closed-loop reservoir-development process in a controlled environment warn against similar hidden mechanisms happening on real-field developments under similar circumstances. They reveal intrinsic pitfalls in reservoir modeling that may contribute to production-forecast problems and call for a reflection on how reservoir uncertainty assessment is performed. We prove that large sets of models do not guarantee coverage of geologic uncertainties because they do not fully represent the real reservoir. The field-development process naturally changes the risk curves, contributing to revealing the lack of representation.

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