Advanced Reservoir Evaluation Using Downhole Fluid Analysis and Asphaltene Flory-Huggins-Zuo EOS
Authors: Zuo Julian Y. , Mullins Oliver C. , Elshahawi Hani , Ramaswami Shyamalan , Dong Chengli , Dumont Hadrien , Zhang Dan , Ruiz-Morales Yosadara 
Published: April 2013

Understanding reservoir complexities such as compartmentalization and compositional gradients early on is crucial for optimal field development, especially in deepwater environments. Downhole fluid analysis (DFA) measures composition, gas/oil ratio (GOR), density, optical density (linearly associated with asphaltene content), and fluorescence intensity. Based on the Yen- Mullins model of asphaltene science and DFA measurements, the industry's first predictive asphaltene equation of state (EOS), the Flory-Huggins-Zuo (FHZ) EOS has been developed. It has been successfully used to estimate asphaltene concentration (optical density, OD) gradients and help predict reservoir connectivity – subsequently proven by production data. This provides an advanced reservoir evaluation tool, which reduces uncertainty in reservoir characterization.

In this paper, DFA and the FHZ EOS were used to analyze a couple of case studies: The first deals with a black oil column with a steep asphaltene gradient; the second and third deal with a light (near critical) oil with a large compositional gradient. For the black oil column, detailed analysis of recently available pressure data suggests that this oil column is disconnected from the aquifer and from the regional pressure regime. For the light oil columns, the delumping technique (Zuo et al., 2008) was used to obtain compositions from the DFA data which compared well with gas chromatography data. The cubic EOS was applied to describe the large variations observed in composition, GOR and density. The obtained results were also in good agreement with the measurements. Because of very low optical absorption in this light oil column, the FHZ EOS was employed to analyze the fluorescence intensity gradient, which is correlated with a fraction of heavy resins. The FHZ EOS has been successfully extended to light oil with very low optical absorption but a large fluorescence intensity gradient for the first time. The results show that the heavy resin is molecularly dispersed in this light oil column, and the GOR gradient creates the fluorescence intensity (heavy resin) gradient via the solubility term of the FHZ EOS. The equilibrium heavy resin distribution suggests this oil column is connected, which is also proven by other log and production data with the latter indicating a variable mix of the end members.

In addition, tar mat formation is reviewed showing that the physical chemistry approaching embodied by the FHZ EOS and the Yen-Mullins model can treat asphaltic fluids as well. In particular two fundamental methods of tar mat are identified; one mechanism is from gas addition, where solution gas increases causing tar mat formation with a discontinuous increase of asphaltene content at the oil-tar contact. The other mechanism is by asphaltene addition, where the concentration of asphaltene is increased beyond its solubility limit. This type of tar yields a continuous increase in asphaltene concentration at the oil-tar contact. Note that in contrast to other putative explanations, water plays no role in either of these mechanisms for tar mat formation.