Schlumberger

Technical Paper: Downhole Fluid Analysis Coupled with Novel Asphaltene Science for Reservoir Evaluation

Society: SPWLA
Paper Number:
Presentation Date: 2010
 

Abstract

In elementary school science, one learns that matter is composed of solids, liquids and gases. True to form, crude oils obey this basic tenet with constituent solids (asphaltenes), liquids, and hydrocarbon (and other) gases. Modeling of gasliquid properties of reservoir crude oil such as gradients of GOR, saturation pressure, etc. can be accomplished using standard cubic equations of state (EoS). Moreover, the new technology downhole fluid analysis (DFA) enables measurement of these corresponding gradients of reservoir fluid properties. When these DFAmeasured fluid gradients are shown to be in equilibrium, that is, properly modeled by a cubic EoS, then reservoir connectivity is implied. Equilibration of reservoir fluids requires massive fluid flow throughout the reservoir and is inconsistent with intervening sealing barriers.

In contrast, modeling the solid-fluid equilibria of reservoir crude oils had been precluded until now because of the lack of understanding of asphaltene fundamentals. For example, there had been a ~6 orders of magnitude debate about asphaltene molecular weight. Without knowing the asphaltene size, gravity and other terms cannot be determined and proper asphaltene modeling in reservoir crude oils is precluded. Major advances in asphaltene science have been made, codified in the modified Yen model and are based on extensive laboratory and oilfield studies. This new understanding enables proper modeling of solid-fluid equilibria of asphaltenes correcting perhaps the greatest limitation of petroleum science. Using DFA-measured asphaltene (and GOR) gradients coupled with this new asphaltene nanoscience, it is now possible to model heavy-end distributions of reservoir fluids from heavy oils to condensates.

Moreover, for low GOR crude oils, the GOR is homogenous; thus, only the heavy end gradients are useful for reservoir connectivity analysis. The new technology of DFA coupled with new asphaltene science is shown in numerous case studies to greatly broaden the understanding of reservoir crude oils and represents a powerful new tool to address reservoir connectivity.

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