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Iterative ensemble smoothers are gaining recognition as robust data assimilation methods for modeling geological reservoirs in various applications, including hydrocarbon exploitation, water resources management, geothermal energy, and CO$_2$ storage. Despite numerous publications with both synthetic and field applications, there is a significant lack of discussion on the practical aspects of the implementation of these methods in operational settings. This work aims to bridge this gap by discussing practical considerations for effectively using iterative ensemble smoothers in reservoir data assimilation. The topics covered include:
(1) Setup of the data assimilation method: This includes a discussion about the number of iterations, ensemble size, truncation of singular values, and localization.
(2) Problem parametrization: This involves the selection and representation of uncertain model parameters, encompassing petrophysical properties like porosity and permeability, as well as more general scalar parameters such as relative permeability curves and faults. Special attention is dedicated to the representation of geological facies using a parametrization based on truncated plurigaussian simulation.
(3) Production data: This involves an examination of the type and frequency of selected production data and the associated data-error covariances.
(4) 4D seismic data: This includes a discussion about data preparation and selection, scale transfer, petroelastic modeling, and data-error covariance.
(5) Quality control of prior and posterior models: This involves metrics for measuring the representativeness of the prior model realizations, data-match quality, ensemble variance, and model changes.
The discussions in this work are backed by real-world examples from over a decade of hands-on experience in developing and applying ensemble methods to reservoir data assimilation for hydrocarbon recovery. While the paper’s primary focus is on the method ES-MDA (ensemble smoother with multiple data assimilation), most discussions and recommendations are applicable to other iterative ensemble smoothers. The goal is to provide guidelines for researchers and practitioners in the field.
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