ABSTRACT Reservoir fluids frequently reveal complex phase behaviors in hydrocarbon columns owing to the effects of gravity, thermal diffusion, biodegradation, active charging, water washing, seals leaking, and so on. In addition, the formation compartmentalization often causing discontinuous distributions of fluid compositions and properties makes the proper fluid characterization and reservoir architecture even more challenging yet compelled. The recognition of compositional grading and flow barriers becomes a key to accurate formation evaluation in a cost effective manner. Downhole fluid analysis (DFA) of asphaltene gradients provides an excellent method to delineate the complexity of black oil columns. In this paper, a methodology was developed to estimate downhole asphaltene variations with depths using an equation-of-state (EOS) approach coupled with DFA measurements. DFA tools were used to determine fluid compositions of CO2, C1, C2, C3-C5, C6+, gas-oil ratio (GOR), density and the coloration (optical density) associated with asphaltene contents at downhole conditions. The delumping and characterization procedures proposed by Zuo et al. (2008) were employed to obtain the detailed compositions excluding asphaltenes. In addition, a molar mass distribution of asphaltenes was described by a three-parameter Gamma probability function. The Gaussian quadrature method was used to generate asphaltene pseudocomponents. Five pseudocomponents were employed to represent the normal asphaltene nanoaggregates. Asphaltene distributions in oil columns were computed by tuning the molar mass of asphaltene nanoaggregates against the DFA coloration logs at a reference depth. The methodology was successfully applied to investigate black oil reservoir connectivity (or flow barriers) for offshore field cases. The analysis results were consistent with the subsequent production data and analytical chemistry. Furthermore, for simplicity, it is reasonable to assume that asphaltenes have average properties such as molar mass in entire oil columns. 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 the DFA logging during acquisition.
Cite this paper
nullJ. Zuo, O. Mullins, C. Dong and D. Zhang, "Modeling of Asphaltene Grading in Oil Reservoirs," Natural Resources, Vol. 1 No. 1, 2010, pp. 19-27. doi: 10.4236/nr.2010.11003.
 C. Mullins, S. S. Betancourt, M. E. Cribbs, J. L. Creek, F. X. Dubost, A. Ballard and L. Venkataramanan, “Asphaltene Gravitational Gradient in a Deepwater Reservoir as Determined by Downhole Fluid Analysis,” Paper SPE 106375 presented at the SPE International Symposium on Oilfield Chemistry, Houston, 28 February–2 March 2007.
S. S. Betancourt, F. X. Dubost, O. C. Mullins, M. E. Cribbs, J. L. Creek. and S. G. Mathews, “Predicting Downhole Fluid Analysis Logs to Investigate Reservoir Connectivity,” Paper SPE IPTC 11488 presented at IPTC, Dubai, UAE, 4-6 December 2007.
J. Y. Zuo, O. C. Mullins, C. Dong, D. Zhang, M. O’Keefe, F. X. Dubost, S. S. Betancourt, and J. Gao, “Integration of Fluid Log Predictions and Downhole Fluid Analysis,” Paper SPE 122562 presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Jakarta, Indonesia, 4-6 August 2009.
L. Hoier and C. Whitson, “Compositional Grading-Theory and Practice,” SPE Reservoir Evaluation & Engineering, Vol. 4, 2001, pp. 525-535.
O. C. Mullins, G. Fujisawa, M. N. Hashem and H. Elshahawi, “Coarse and Ultra-Fine Scale Compartmentalization by Downhole Fluid Analysis Coupled,” Paper SPE IPTC 10034 presented at ITPC, Doha, Qatar, 21-23 November 2005.
K. Indo, J. Ratulowski, B. Dindoruk, J. Gao, J. Zuo and O. C. Mullins, “Asphaltene Nanoaggregates Measured in a Live Crude Oil by Centrifugation,” Energy & Fuels, Vol. 23, 2009, pp. 4460-4469.
S. S. Betancourt, G. T. Ventura, A. E. Pomerantz, O. Viloria, F. X. Dubost, J. Zuo, G. Monson, D. Bustamante, J. M. Purcell, R. K. Nelson, R. P. Rodgers, C. M. Reddy, A. G. Marshall and O. C. Mullins, “Nanoaggregates of Asphaltenes in a Reservoir Crude Oil and Reservoir Connectivity,” Energy & Fuels, Vol. 23, 2009, pp. 1178- 1188.
A. Hirschberg, “Role of Asphaltenes in Compositional Grading of a Reservoir’s Fluid Column,” Journal of Petroleum Technology, Vol. 40, No. 1, 1988, pp. 89-94.
J. Y. Zuo, O. C. Mullins, C. Dong, S. S. Betancourt, F. X. Dubost, M. O’Keefe and D. Zhang, “Investigation of Formation Connectivity Using Asphaltene Gradient Log Predictions Coupled with Downhole Fluid Analysis,” Paper SPE 124264 presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 4-7 October 2009.
L. X. Nghiem, M. S. Hassam and R. Nutakki, “Efficient Modeling of Asphaltene Precipitation,” Paper SPE 26642 presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3-6 October 1993.
L. X. Nghiem and D. A. Coombe, “Modeling Asphaltene Precipitation during Primary Depletion,” Paper SPE 36106 presented at the SPE IV Latin American/Caribbean Petroleum Engineering Conference, Trinidad and Tobago, 23-26 April 1996.
X. Qin, P. Wang, K. Sepehrnoori and G. Pope, “Modeling Asphaltene Precipitation in Reservoir Simulation,” Industrial Engineering Chemistry Research, Vol. 39, 2000, pp. 2644-2654.
K. S. Pedersen and P. L. Chritensen, “Phase Behavior of Petroleum Reservoir Fluids,” CRC Press, Taylor & Francis Group, Boca Rston, Florida, 2007.
A. Gisolf, F. X. Dubost, J. Zuo, S. Williams, J. Kristoffersen, V. Achourov, A. Bisarah and O. C. Mullins, “Real Time Integration of Reservoir Modeling and Formation Testing,” Paper SPE 121275 presented at the EUROPEC/EAGE Annual Conference and Exhibition, Amsterdam, 8-11 June 2009.
A. E. Pomerantz, M. R. Hammond, A. L. Morrow, O. C. Mullins and R. N. Zare, “Asphaltene Molecular-Mass Distribution Determined by Two-Step Laser Mass Spectrometry,” Energy & Fuels, Vol. 23, No. 3, 2009, pp. 1162-1168.
O. C. Mullins, B. Martinez-Haya and A. G. Marshall, “Contrasting Perspective on Asphaltene Molecular Weight. This Comment vs. the Overview of A. A. Herod, K. D. Bartle, and R. Kandiyoti,” Energy & Fuels, Vol. 22, No. 3, 2008, pp. 1765-1773.
O. C. Mullins, “The Physics of Reservoir Fluids: Discovery through Downhole Fluid Analysis,” Schlumberger, Sugar Land, Texas, 2008.
C. H. Whitson, T. F. Anderson and I. Sorede, “Application of the Gamma Distribution Model to Molecular Weight and Boiling Point Data for Petroleum Fractions,” Chemical Engineering Communications, Vol. 96, 1990, pp. 259-278.
C. H. Whitson, T. F. Anderson and I. Soreide, “C7+ Fraction Characterization,” in: L. G. Chorn and G. A. Mansoori, Ed., Taylor & Francis New York Inc., New York, 1989, p. 35.
C. H. Whitson, “Effect of C7+ Properties on Equation of State Predictions,” Soc. Pet. Eng. J., December 1984, pp. 685-696.
A. K. Tharanivasan, W. Y. Svrcek, H. W. Yarranton, S. D. Taylor, D. Merino-Garcia and P. Rahimi, “Measurement and Modeling of Asphaltene Precipitation from Crude Oil Blends,” Energy and Fuels, Vol. 23, 2009, pp. 3971- 3980.
F. Montel, J. Bickert, A. Lagisquet and G. Galliero, “Initial State of Petroleum Reservoirs: A Comprehensive Approach,” Journal of Petroleum Science and Engineering, Vol. 58, 2007, pp. 391-402.
K. S. Pedersen and N. Lindeloff, “Simulations of Compositional Gradients in Hydrocarbon Reservoirs under the Influence of a Temperature Gradient,” Paper SPE 84364 presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, 5-8 October 2003.
J. Y. Zuo and D. Zhang, “Plus Fraction Characterization and PVT Data Regression for Reservoir Fluids near Critical Conditions,” Paper SPE 64520 presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Brisbane, Australia, 16-18 October 2000.
J. Y. Zuo, D. Zhang, F. Dubost, C. Dong, O. C. Mullins, M. O’Keefe and S. S. Betancourt, “EOS-Based Downhole Fluid Characterization,” Paper SPE 114702 presented at the SPE Asia Pacific Oil & Gas Conference and Exhibition, Perth, Australia, 20-22 October 2008.
K. Akbarzadeh, A. Dhillon, W. Y. Svrcek and H. W. Yarranton, “Methodology for the Characterization and Modeling of Asphaltene Precipitation from Heavy Oils Diluted with n-Alkanes,” Energy & Fuels, Vol. 18, 2004, pp. 1434-1441.
H. Alboudwarej, K. Akbarzadeh, J. Beck, W. Y. Svrcek and H. W. Yarranton, “Regular Solution Model for Asphaltene Precipitation from Bitumens and Solvents,” AIChE Journal, Vol. 49, 2003, pp. 2948-2956.
J. X. Wang and J. S. Buckley, “A Two-Component Solubility Model of the Onset of Asphaltene Flocculation in Crude Oils,” Energy & Fuels, Vol. 15, 2001, pp. 1004- 1012.