Technical Paper: Investigation of Formation Connectivity Using Asphaltene Gradient Log Predictions Coupled with Downhole Fluid Analysis

Society: SPE
Paper Number: 124264
Presentation Date: 2009
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Reservoir fluids often demonstrate complicated phase behaviors in a single column as a result of the impacts of gravity, thermal gradients, biodegradation, active charging, water washing, leaky seals, and so on. Moreover, reservoir compartmentalization can cause discontinuous distributions of fluid compositions and properties, making the proper characterization of fluids and reservoirs even more challenging yet compelling. The recognition of flow barriers or compartmentalization is key to effective and efficient reservoir characterization, production, and management. Downhole fluid analysis (DFA) is an essential tool for determination of the compositional gradients in real time at reservoir conditions. However, analysis of flow connectivity in the reservoir by DFA can be complex, especially when the reservoir fluid compositional gradients are small with depth. In this case, the analyses of bulk fluid properties may not be sufficiently sensitive. However, DFA of asphaltene gradients provides an excellent method to delineate the complexity of black oil columns. Moreover, DFA measurements are very sensitive to and often linear in the asphaltene content.

A methodology has been developed to estimate downhole fluid coloration variations with depths using an equation-of-state (EOS) approach coupled with the DFA tools. The DFA tools are used to determine downhole fluid compositions of CO2, C1, C2, C3–C5, C6+, and the coloration associated with asphaltene contents. Recent additions to DFA measurements include fluid density and viscosity. The delumping and characterization procedures proposed by Zuo et al. (2008) are applied to obtain the detailed compositions including asphaltenes and the parameters of the EOS model. Fluid profile and coloration logs are computed by “tuning” the molar mass of asphaltene nanoaggregates against the DFA coloration logs. The methodology has been successfully applied to investigate reservoir connectivity for offshore and laboratory centrifugation cases. The adjusted molar mass of asphaltene nanoaggregates is determined to be in a range of 3,000–8,000 g/mol for the cases studied, yielding molecular aggregation numbers of roughly 7 in reservoir fluids, which is in agreement with laboratory studies. The results obtained in this work demonstrate that the proposed method provides a useful tool to reduce the uncertainties related to reservoir compartmentalization and to optimize DFA during the logging run. In addition, the results indicate that treating part of the Cn+ (e.g., C36+) fraction as an asphaltene component (monomer) in the traditional cubic EOS approach is contradicted by the recent observations that asphaltenes are nanoaggregates in crude oils.

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