Abstract

The evaluation of downhole fluid analysis (DFA) measurements of asphaltene gradients provides the ability to determine the extent of asphaltene equilibrium and the operative reservoir fluid geodynamics (RFG) processes. Typically, equilibrium of reservoir fluids indicates reservoir connectivity, a primary concern in field development planning. Currently, the modeling of asphaltene gradients is done through the manual evaluation of the DFA optical density gradients. The optical density measurements are fit to an equation of state (EOS), such as the Flory-Huggins-Zuo EOS, and evidence for asphaltene equilibrium is concluded if the inferred asphaltene diameter corresponds to that of the Yen-Mullins model for asphaltene composition.

In this work, we present an automated Bayesian algorithm that proposes multiple hypotheses for the state of asphaltene equilibrium. The proposed hypotheses honor DFA measurements; physical models for asphaltenes in equilibrium, such as the Yen-Mullins model; and prior domain knowledge of the reservoir, such as geological layers, faults, and flow units. The leading hypotheses are reported, and evidence for or against asphaltene equilibrium is concluded from inferred quantities. Our proposed method provides a faster way for domain experts to explore different reservoir realizations that honor the theory of asphaltenes gradients and previous knowledge about the reservoir.

We verify our novel method on three case studies that are undergoing different RFG processes through comparison of the interpretation done by domain experts. While there are many reservoir complexities associated with each case study, we focus on whether the underlying RFG process corresponds to the asphaltenes in equilibrium. The first case study is a light oil reservoir in the Norwegian North Sea that is mostly in fluid equilibrium with exceptions at the flanks. The second case study is a black oil reservoir that has undergone a fault block migration after the reservoir fluids had a chance to achieve equilibrium. The last case study is a black oil reservoir in quasi-equilibrium due to biodegradation in the lower portion of the well.

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