Multivariate Characterization, Modeling of Paleo Zone Improves History Matching
Authors: Carpenter Chris 
Published: May 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 22034, “Rigorous Multivariate Characterization and Modeling of Paleo Zone for Simulation Model History Matching Under Reservoir Uncertainty,” by Nader Y. Bu-Khamseen, SPE, Marko Maucec, SPE, and Anwar R. Awan, SPE, Saudi Aramco, et al. The paper has not been peer reviewed. Copyright 2022 International Petroleum Technology Conference. Reproduced by permission.

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The presence of a paleo zone, which frequently exists below free-water-level (FWL) surfaces, can affect dynamic reconciliation of reservoir simulation models. In the complete paper, the authors present an approach that integrates characterization of paleo zones, parameterization of paleo-zone conductivity, and application of flow profiles as a guide in an accelerated history-matching study of a large-scale dual-porosity/dual-permeability model. The approach to probabilistic parameterization of paleo-zone conductivity has contributed to a model with high quality and rendered a reservoir simulation model with reliable predictive capability in accelerated time.

Methods and Models
Geological Modeling of Paleo Zone. Structural growth evidences the filling history and displacement occurred by regional tilting, stamping a record of relic (or paleo) oil in the pore system below the current FWL. Applicable saturation-height models are generated based on dedicated core petrophysical rock typing in conjunction with a robust FWL and paleo-oil FWL inversion, a process that allows reproduction of the saturation profile as seen from logs (original logs not affected by production) and the oil-saturation tail as seen below the FWL.
The primary methodology behind the approach presented in the paper consists of determining the volume of paleo oil by modeling FWL and paleo-oil FWL surfaces, defining the corresponding paleo zone within the reservoir, and dynamic evaluation of communication between the aquifer, paleo oil, and oil zones; the two surfaces come from an inversion process of the saturation height function.
Current developments for the incorporation of the most-important variables affecting dynamic conditions (assisted history matching, uncertainty quantification, multivariate parameterization, and static and dynamic calibration) in the reservoir have allowed incorporation of, and sensitivity studies of, some overlooked parameters, including paleo-oil characterization, modeling, and simulation.
Adjusting Petrophysical Properties Inside the Paleo Regions. Petrophysical properties (porosity and permeability) inside the paleo-oil region are modeled based on the bulk volume of water (BVW) from log interpretation. The higher the BVW, the higher the chance of effective communication between the oil leg and the aquifer. These modeled multipliers represent the probability of the sealing character of the paleo-oil region because this accumulated hydrocarbon may not be entirely homogeneous nor uniformly distributed, as suggested by dynamic data and corroborated by core samples.
The methodology implemented to represent the presence of the paleo zone in the interpreted and modeled region between the current FWL and the paleo-oil FWL incorporates the use of multipliers in porosity and permeability, allowing the reproduction of the blocking or baffling effect caused by the presence of residual-oil saturation.