Asphaltene gradients and tar mat formation in reservoirs under active gas charging

Zuo, Julian Y.; Elshahawi, Hani; Mullins, Oliver C.; Dong, Chengli; Zhang, Dan; Jia, Na; Zhao, Hongying

doi:10.1016/j.fluid.2011.11.024

Cited by 26 publications

(24 citation statements)

References 17 publications

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“…[3,4] The FHZ EoS with the gravity term only can be used to estimate the asphaltene gradient due to low-GOR oil. [14][15][16][17][18][19][44][45] That is, the solubility term of the FHZ EoS is largely invariant for a relatively homogenous low-GOR crude oils at low pressure as predicted by the cubic EoS. The predictions by the FHZ EoS with the gravity term only are illustrated in Figure 11.…”

Section: Crude Oil Properties Of the Reservoir Fluidmentioning

confidence: 99%

Diffusion Model Coupled with the Flory–Huggins–Zuo Equation of State and Yen–Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

Zuo

Jackson²,

Agarwal³

et al. 2015

Energy Fuels

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A very large monotonic variation of asphaltenes and viscosity has been measured by downhole fluid analysis (DFA) in crude oils in five-stacked sandstone reservoirs in the northern part of the Barmer Basin, northwest India, undergoing active biodegradation; each of the five-layered sand bodies shows overlaying fluid gradients with depth providing replicate validation of the measurements. Fluid data from four wells across the field shows that the gradients are uniform across the formation. The crude oil in the upper half of the oil column exhibits an equilibrium distribution of asphaltenes matching predictions of the Flory-Huggins-Zuo Equation of state (FHZ EoS) with the gravity term only using asphaltene nanoaggregates of the Yen-Mullins model. However, the bottom half of the reservoir reveals a large asphaltene gradient approximately three times larger than the equilibrium predictions from the FHZ EoS. This increase in asphaltenes creates a very large (8x) viscosity gradient and is a major production concern. In addition, these shallow reservoirs are undergoing active biodegradation at temperatures of 55 °C to 61 °C.A simple diffusive model coupled with the FHZ EoS is shown to account for the entire observed asphaltene distribution in each of the five sand layers. Alkanes (and some aromatics) are rapidly consumed at the oil-water contact at the base of the oil column. The rate-limiting step is the diffusion of these compounds to the oil-water contact. The loss of these oil components decreases the oil volume yielding an increase in asphaltene concentration and a significant increase in viscosity. The limited geologic time of the oil in the reservoir limits the vertical extent of the diffusive process accounting for the observation of asphaltene equilibrium at the top of the column. Specifically, petroleum system modeling of this basin indicates that the oil commenced undergoing biodegradation approximately 50 million years ago, and this duration matches the analysis using the diffusion model plus the FHZEoS. Gas chromatography applied to the oils from the top to the bottom of the oil column provides detailed compositional confirmation of the diffusive mechanism proposed. In particular, all measured compositional properties of the oil column are shown to be consistent with this simple diffusive model. The ability to account for asphaltene and viscosity variations in the five stacked sand layers with a simple diffusive model coupled with the FHZ EoS and the Yen-Mullins model provides a robust model for improving efficiency of reservoir engineering and oil production.

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Section: Crude Oil Properties Of the Reservoir Fluidmentioning

confidence: 99%

Diffusion Model Coupled with the Flory–Huggins–Zuo Equation of State and Yen–Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

Zuo

Jackson²,

Agarwal³

et al. 2015

Energy Fuels

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“…Recently, the Flory-Huggins-Zuo (FHZ) EoS was developed, which models gradients in asphaltene concentration based on the sizes of asphaltene molecules and aggregates described in the Yen-Mullins model, which synthesizes many of the results presented here. − Figure shows how the FHZ EoS can match gradients in a variety of fluid types, if the appropriate asphaltene particle size is used. The broad applicability of the equation has enabled its use in a variety of reservoirs, including those that are out of thermodynamic equilibrium, undergoing active gas entry, or undergoing biodegradation. − …”

Section: Application: Formation Evaluation In Conventional Reservoirsmentioning

confidence: 99%

Toward Molecule-Specific Geochemistry of Heavy Ends: Application to the Upstream Oil Industry

Pomerantz

2016

Ind. Eng. Chem. Res.

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Detailed molecular analysis of the composition of petroleum is typically performed on only the light (volatile) fraction, using analyses involving gas chromatography (GC). Heavier (nonvolatile) components such as kerogen, bitumen, and asphaltenes, which are relevant to conventional reservoirs and particularly to unconventional gas shale and tight oil formations, are not amenable to analysis by GC and instead are typically studied by more phenomenological techniques. This report describes a suite of analytical techniques that can provide sufficient average molecular information on the heavy ends (defined as the nonvolatile fraction) to be useful for a variety of industrial applications, expanding the discipline of molecule-specific geochemistry into the non-GC amenable fractions of petroleum materials. Heavy ends are composed of the elements C, H, N, S, and O, and the bonding environments of each can be measured by various solid state spectroscopies: nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies for C, H, and O; X-ray absorption near-edge structure (XANES) spectroscopy for S and N. Additionally, advances in mass spectrometry, including techniques such as laser desorption laser ionization mass spectrometry (L 2 MS), can be used to measure the molecular weight and dominant molecular architecture of asphaltenes. Moreover, higher-order structures can be measured: samples of kerogen with representative pore geometry can be prepared, and those pore systems are analyzed with techniques such as X-ray diffraction and neutron scattering. Colloidal structures of asphaltenes can be measured with a variety of techniques, which reveal a hierarchical aggregation structure. These analyses provide sufficient structural data to enable structure−property and structure− function relationships to be determined, which can be applied to the characterization of conventional and unconventional resources. In gas shale and tight oil formations, which contain abundant inorganic minerals as well as kerogen, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is used for simultaneous analysis of mineralogy, kerogen content, and maturity, typically performed at the wellsite. In conventional reservoirs, spatial variations in the asphaltene concentration measured by downhole fluid analysis logging tools are being interpreted with a recently developed equation of state to address upstream issues such as reservoir connectivity, tar mat formation, and fault block migration. As more heavy end analyses are developed, further scientific knowledge and industrial applications of heavy end geochemistry are anticipated.

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“…The solvent is also a mixture whose properties are calculated by a cubic EOS (for details, see Zuo et al 2009aZuo et al , 2009bZuo et al , 2010aZuo et al , 2010bZuo et al , 2010cZuo et al , 2012aZuo et al , 2012b. The Flory-Huggins-Zuo EOS for asphaltene grading is given by (Freed et al 2010;Zuo et al 2010aZuo et al , 2010bZuo et al , 2010c: …”

Section: The Flory-huggins-zuo Eosmentioning

confidence: 99%

“…Another major success of the FHZ EOS is its capability to account for the gigantic asphaltene gradient in a single near-critical oil column deepwater, Gulf of Mexico (Zuo et al 2012b). Carbon isotope analyses were conducted on the flashed gases from the live fluid samples.…”

Section: Oil Columns With Late Gas Chargingmentioning

confidence: 99%

Integration of Downhole Fluid Analysis and the Flory-Huggins-Zuo EOS for Asphaltene Gradients and Advanced Formation Evaluation

Zuo

Dumont

Mullins

et al. 2013

SPE Annual Technical Conference and Exhibition

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The Yen-Mullins model of asphaltenes has enabled the development of the industry's first asphaltene equation of state (EOS) for predicting asphaltene concentration gradients in oil reservoirs, the Flory-Huggins-Zuo (FHZ) EOS. The FHZ EOS is built on the existing the Flory-Huggins regular solution model, which has been widely used in modeling the phase behavior of asphaltene precipitation in the oil and gas industry. For crude oil in reservoirs with a low gas/oil ratio (GOR), the FHZ EOS reduces predominantly to a simple form-the gravity term only-and for mobile heavy oil, the gravity term is simply based on asphaltene clusters. The FHZ EOS has been applied to different crude oil columns from volatile oil to black oil to mobile heavy oil all over the world to address key reservoir issues such as reservoir connectivity/compartmentalization, tar mat formation, nonequilibrium with a late gas charge, and asphaltene destabilization by integrating downhole fluid analysis (DFA) measurements and the Yen-Mullins model of asphaltenes. Asphaltene or heavy-end concentration gradients in crude oils are treated using the FHZ EOS explicitly incorporating the size of resin molecules, asphaltene molecules, asphaltene nanoaggregates, or/and asphaltene clusters. Field case studies proved the value and simplicity of this asphaltene or heavy-end treatment. Heuristics can be developed from results corresponding to the estimation of asphaltene gradients. Perylene-like resins with the size of ~1 nm are dispersed as molecules in high-GOR light oils (condensates) with high fluorescence intensity and without asphaltenes (0 wt% asphaltene). Heavy asphaltene-like resins with the size of ~1.5 nm are molecularly dissolved in volatile oil at very low asphaltene content. Asphaltene nanoaggregates with the size of ~2 nm are dispersed in stable crude oil at a bit higher asphaltene content. Asphaltene clusters are found in mobile heavy oil with the size of ~5 nm at even higher asphaltene content (typically >8 wt% based on stocktank oil). All these studies are in accord with the observations in the Yen-Mullins model within the FHZ EOS analysis.Furthermore, the cubic EOS and FHZ EOS have been extended to a near critical fluid column with GOR changing from 2600 to 5600 scf/STB and API gravity changes from 34 to 41 °API. Data from the real-time third-generation of DFA were used to establish the early time EOS for advanced formation evaluation. The early-time EOS was updated after the laboratory PVT data were available. The results from the early-time EOS based on the new-generation DFA data were in accord with those from the updated one based on the pressure/volume/temperature (PVT) data. The large GOR gradient is well modeled by the cubic EOS assuming a small late gas charge from the crest to the base. The FHZ EOS with 1-nm diameter was employed to predict the fluorescence intensity gradient. This agrees that perylene-like resins with the size of ~1 nm are dispersed as molecules in high-GOR light oil (rich gas condensate) with high fluorescence intensi...

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Asphaltene gradients and tar mat formation in reservoirs under active gas charging

Cited by 26 publications

References 17 publications

Diffusion Model Coupled with the Flory–Huggins–Zuo Equation of State and Yen–Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

Diffusion Model Coupled with the Flory–Huggins–Zuo Equation of State and Yen–Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

Toward Molecule-Specific Geochemistry of Heavy Ends: Application to the Upstream Oil Industry

Integration of Downhole Fluid Analysis and the Flory-Huggins-Zuo EOS for Asphaltene Gradients and Advanced Formation Evaluation

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