The Modified Yen Model of Asphaltenes coupled with Downhole Fluid Analysis to Address Flow Assurance
Authors: Mullins Oliver C. , Freed Denise , Zuo Julian Youxiang , Elshahawi Hani 
Published: July 2010

For condensates the most important compositional variation is GOR, and the Equation of State (EoS) treatment of GOR variations is well developed and successful. Consequently, key reservoir fluid attributes of condensates that impact Flow Assurance can readily be addressed, in particular by downhole fluid analysis (DFA). In contrast, black oils are defined as having low GOR; while black oils can exhibit GOR variations, the most important compositional variation of black oil is the heavy ends, essentially the asphaltenes. Of course the asphaltenes have a major impact on Flow Assurance. However, interpretation of asphaltene variations in reservoirs had been severely limited because there had been no corresponding understanding of the governing principles of asphaltene science. Indeed, the field of asphaltene science had been in disarray. Consequently, there had been significantly reduced motivation to perform DFA profiling of black oil columns.

In recent years, several major advances have taken place in asphaltene science; these advances are in use by research groups around the world to address many issues such as heavy end upgrading and interfacial properties of asphaltene. For Flow Assurance objectives, these scientific advances greatly improved the capability to use heavy ends to characterize reservoirs containing black oils as well as lighter crude oils including condensates. The confluence of advanced asphaltene science with the third generation of DFA tools enables a powerful new approach to address important and critical reservoir fluid issues such as Flow Assurance. Case studies prove the merit of this approach for a variety of different crude oils.

Introduction

Hydrocarbon reservoir fluids can undergo a large variety of phase transitions that can be deleterious for the production of oil and gas. In particular, asphaltenes represent a particular enigmatic problem.[1] In contrast to light ends and gas-liquid phase equilibria, characterization of asphaltenes from a theoretical standpoint has been challenging. In particular, there has been no predictive theoretical treatment describing asphaltene distributions in the reservoir. Reservoir fluids can exist largely in equilibrium or in great disequilibrium.[2] Proper asphaltene Flow Assurance treatment depends critically on this distinction, in particular, regarding the ability to extrapolate specific laboratory results throughout the reservoir. Fortunately, basic advances in asphaltene science coupled with new theoretical treatments enable new efficient work flows to address asphaltene Flow Assurance.

Asphaltene phase transitions can be induced by a variety of mechanisms and represent a traditional Flow Assurance problem. Figure 1 shows various asphaltene solids that can form upon change of pressure, temperature and/or composition.[3]

Figure 1.

In early years, asphaltene science first advanced by characterization of many bulk properties of asphaltenes even though much more difficult questions of molecular and colloidal characterization remained unresolved. This bulk chemical knowledge enabled an effective model to be proposed about asphaltenes. For forty years, the Yen model proposed by Professor Teh Fu Yen and coworkers [4], had dominated understanding of asphaltene solids.