Oliver C Mullins | Schlumberger-Doll Research (original) (raw)

Papers by Oliver C Mullins

Asphaltenes, Heavy Oils, and Petroleomics, 2007

A very large monotonic variation of asphaltenes and viscosity has been measured by downhole fluid... more 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 (8×) viscosity gradient and is a major production concern. In addition, these shallow reservoirs are undergoing active biodegradation at temperatures of 55−61 °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 FHZ-EoS. 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.

ABSTRACT: Asphaltenes are an important class of compounds in crude oil whose surface activity is ... more ABSTRACT: Asphaltenes are an important class of compounds
in crude oil whose surface activity is important for establishing
reservoir rock wettability which impacts reservoir drainage.While
many phenomenological interfacial studies with crude oils and
asphaltenes have been reported, there is very little known about
themolecular level interactions between asphaltenes and mineral
surfaces. In this study, we analyze LangmuirBlodgett films of
asphaltenes and related model compounds with sum frequency
generation (SFG) vibrational spectroscopy. In SFG, the polarization
of the input (vis, IR) and output (SFG) beams can be varied,
which allows the orientation of different functional groups at the
interface to be determined. SFG clearly indicates that asphaltene polycyclic aromatic hydrocarbons (PAHs) are highly oriented in the
plane of the interface and that the peripheral alkanes are transverse to the interface. In contrast, model compounds with oxygen
functionality have PAHs oriented transverse to the interface. Computational quantumchemistry is used to support corresponding band
assignments, enabling robust determination of functional group orientations.

Two important features of the molecular structure of asphaltenes remain unresolved; the size dist... more Two important features of the molecular structure of asphaltenes remain unresolved; the size distribution of the 8 asphaltene polycyclic aromatic hydrocarbons (PAHs) and the number of PAHs per asphaltene molecule. The relatively small 9 molecular weight of asphaltenes restricts the PAH size; if there are several PAHs per asphaltene molecule, they must be rather 10 small. Optical spectroscopy especially when coupled with molecular orbital (MO) calculations is an excellent probe of asphaltene 11 PAH populations and thus asphaltene molecular architecture. Previously, singlet−singlet transitions for asphaltenes were 12 analyzed using both experiment and MO theory. Here, we describe MO calculations performed to treat triplet−triplet transitions 13 from the ground triplet state for 103 PAHs. Qualitative comparisons with corresponding triplet−triplet transition measurements 14 for asphaltenes are discussed. In addition, spin-forbidden transitions between the singlet and the triplet states, corresponding to 15 phosphorescence, are calculated and discussed in terms of the probability of intersystem crossing of PAHs in asphaltenes and 16 crude oils. Conclusions obtained here are consistent with the corresponding study of singlet−singlet transitions and support the 17 model of a single, relatively large PAH per asphaltene molecule as the predominant asphaltene molecular architecture: the island 18 model. This is consistent with a most probable asphaltene PAH of seven-fused aromatic rings (7FAR) with a width of four to ten 19 fused aromatic rings (4FAR-10FAR). This molecular architecture is a central feature of the Yen−Mullins model of asphaltene 20 nanoscience.

Hydrocarbons in subsurface reservoirs are generally found to be compositionally graded, with flui... more Hydrocarbons in subsurface reservoirs are generally found to be compositionally graded, with fluids deeper in connected and equilibrated reservoirs being relatively enriched in asphaltenes. These gradients result from effects such as gravity, entropy, and solubility. However, it is unclear if those same effects lead to gradients in the detailed molecular composition of asphaltenes. Here, we investigate the sulfur chemistry of asphaltenes from a reservoir with a large gradient in asphaltene content. Measurements of the sulfur content from combustion as well as measurements of sulfur speciation from K-edge X-ray absorption near edge structure (XANES) spectroscopy find no significant difference in the composition of the asphaltenes. Thus, different locations within this reservoir contain oils with different asphaltene concentrations, but the asphaltenes from throughout the reservoir all have the same sulfur chemistry. This result suggests that gradients in asphaltene content can be successfully modeled with the simplifying assumption that the asphaltene molecular composition is not graded in connected and equilibrated reservoirs.

Spatial gradients in the chemical composition of crude oil are now routinely observed; oils near ... more Spatial gradients in the chemical composition of crude oil are now routinely observed; oils near the top of a reservoir can be enriched in lighter ends, whereas oils near the bottom of a reservoir are typically enriched in asphaltenes. Equations of state capable of modeling these gradients have numerous practical applications, such as predicting variations in fluid properties, the presence of tar mats, and flow connectivity. Because of the great chemical complexity of crude oil, successful modeling of these gradients requires both an understanding of the physical drivers of the gradients and a set of reasonable simplifying assumptions for describing the composition of the crude oil. Gravity is often a dominant force driving fluid gradients, resulting in separation of low-density gas above medium-density oil above high-density water when those isolated phases are present and frequently driving gradients within a phase as well. The impact of gravity in driving gradients depends in part upon the mass and volume of molecules or aggregates in the crude oil. Here, we explore the impact of gravity in segregating asphaltenes of different masses. Asphaltenes were extracted from crude oils from a connected reservoir with a large gradient in asphaltene content and studied with two mass spectrometric techniques: laser desorption laser ionization mass spectrometry (L 2 MS), which measures the mass of asphaltene molecules, and surface-assisted laser desorption/ionization (SALDI) mass spectrometry, which measures the mass of asphaltene nanoaggregates. No significant gradients in the molecule or nanoaggregate mass were observed, suggesting that gravity causes almost no segregation of different molecular or nanoaggregate masses within the asphaltene class. That is, the large concentration gradient of asphaltenes in the reservoir is not accompanied by a large molecular weight gradient within the asphaltene class. These results indicate that asphaltene gradients can be modeled with the simplifying assumption that gravity drives gradients in the concentration of asphaltenes but not the chemical composition of asphaltenes in crude oil.

Important properties of a crude oil, such as viscosity, are determined by the crude oil's chemica... more Important properties of a crude oil, such as viscosity, are determined by the crude oil's chemical constituents. In 17 particular, viscosity is highly dependent on the asphaltene content. Various processes that act on reservoir crude oils can alter 18 chemical composition, such as aspahltene content, and therefore impact important crude oil properties. In many basins, such as 19 the Llanos basin in Colombia, processes such as biodegradation, water washing, and multiple charging can contribute to 20 asphaltene and viscosity variations. The considerable complexity of the problem requires a multidisciplinary workflow to 21 understand the main factors that influence the quality of reservoir crude oils and their gradients. Here, we perform 22 comprehensive two-dimensional gas chromatography with flame ionization detection and with mass spectrometry on samples of 23 known provenance, combined with petroleum system modeling, to develop an understanding of the primary factors controlling 24 asphaltene content and viscosity in a reservoir. The crude oils in our study show the impact of biodegradation, water washing, 25 and multiple charging. Some variation of composition is observed laterally in the subsurface formation. These observations help 26 constrain the petroleum system model; the timing of paleo-pasteurization appears to be key in establishing the quality of the oils.

Production of crude oil from subsurface reservoirs is greatly impacted by many complexities such ... more Production of crude oil from subsurface reservoirs is greatly impacted by many complexities such as reservoir fluid 7 flow, connectivity, viscosity gradients, and tar mat formation. In situ fluid analysis in oil wells has enabled facile measurement of 8 fluid gradients of dissolved gases and dissolved solids in reservoir crude oils; these gradients have proven very useful for analysis 9 of reservoir complexities. The analysis of solution gas generally uses the cubic equation of state. However, until recently, there 10 had been no predictive equation of state to model asphaltene gradients. Recently, the different nanostructures of asphaltenes in 11 crude oil have largely been resolved and codified in the Yen−Mullins model. In turn, this has enabled equation of state 12 development for asphaltene gradients in crude oils. For example, the Flory−Huggins−Zuo EoS is now ubiquitously utilized in 13 modeling asphaltene gradients. Here, the magnitude and dependencies of the three terms of this EoS, gravity, solubility, and 14 entropy, are considered in detail. Simple expressions for ratios of these terms are obtained as a function of the gas/oil ratio of the 15 crude oils. In particular, the transition from gravity dominance to solubility dominance is examined. A variety of heuristics are 16 developed to guide interpretation of asphaltene gradients that are so routinely measured. Expressions are given that could be used 17 for real-time interpretation upon measurement of these gradients. The utility of EoS modeling of asphaltene gradients is 18 significantly enhanced when incorporating these heuristics.

Asphaltenes are known to be interfacially active in many circumstances such as at toluene-water i... more Asphaltenes are known to be interfacially active in many circumstances such as at toluene-water interfaces. Furthermore, the term micelle has been used to describe the primary aggregation of asphaltenes in good solvents such as toluene. Nevertheless, there has been significant uncertainty regarding the critical micelle concentration (CMC) of asphaltenes and even whether the micelle concept is appropriate for asphaltenes. To avoid semantic debates we introduce the terminology critical nanoaggregate concentration (CNAC) for asphaltenes. In this report, we investigate asphaltenes and standard surfactants using high-Q, ultrasonic spectroscopy in both aqueous and organic solvents. As expected, standard surfactants are shown to exhibit a sharp break in sonic velocity versus concentration at known CMCs. To prove our methods, we measured known surfactants with CMCs in the range from 0.010 g/L to 2.3 g/L in agreement with the literature. Using density determinations, we obtain micelle compressibilities consistent with previous literature reports. Asphaltenes are also shown to exhibit behavior similar to that of ultrasonic velocity versus concentration as standard surfactants; asphaltene CNACs in toluene occur at roughly 0.1 g/L, although the exact concentration depends on the specific (crude oil) asphaltene. Furthermore, using asphaltene solution densities, we show that asphaltene nanoaggregate compressibilities are similar to micellar compressibilities obtained with standard nonionic surfactants in toluene. These results strongly support the contention that asphaltenes in toluene can be treated roughly within the micelle framework, although asphaltenes may exhibit small levels of aggregation (dimers, etc.) below their CNAC. Furthermore, our extensive results on known surfactants agree with the literature while the asphaltene CNACs reported here are one to two orders of magnitude lower than most previously published results. (Previous work utilized the terminology " micelle " and " CMC " for asphaltenes.) We believe that the previously reported high concentrations for asphaltene CMCs do not correspond to primary aggregation; perhaps they refer to higher levels of aggregation or perhaps to a particular surface structure.

Downhole fl uid analysis (DFA) is used to characterize compositional fl uid gradients, and equati... more Downhole fl uid analysis (DFA) is used to characterize
compositional fl uid gradients, and equations of state
(EoS) models are used for analysis to delineate reservoir
fl uid variations, connectivity and other complexities. A
series of reservoirs is examined to assess the state of the
contained fl uids in terms of thermodynamic equilibrium
in the reservoir. Substantial, systematic fl uid variations
are found using DFA. The cubic EoS is used for gasliquid
analysis, and the Flory-Huggins-Zuo EoS and the
Yen-Mullins model of asphaltenes are used for analysis of
dissolved solid-solution equilibria of reservoir crude oils.
‘Young’ reservoirs exhibit large, nonmonotonic variations
of fl uids (and solids), moderately aged reservoirs exhibit
monotonic, yet disequilibrium properties and ‘aged’
reservoirs are fully equilibrated even when in massive
scale. Nevertheless, these old reservoirs retain signifi cant
fl uid and organic solid variations as a result of sequential
fl uid-related processes in geologic time.
The dynamic behaviors of fl uids within reservoirs
that account for these variations are obtained by linking
a fundamental understanding of petroleum with basic
concepts from fl uid mechanics. In particular, the location
of tar deposition within reservoirs is clarifi ed when formed
due to asphaltene instability upon a secondary reservoir fl uid
charge. Tar deposition can be formed upstructure for rapid
gas charge, as is regularly seen in young reservoirs, or can
be formed at the oil-water contact for a slower gas charge,
as seen in many older reservoirs. The state of the reservoir
fl uids within the context of geologic time is shown to be
tightly coupled to key reservoir concerns for production.
Thus, understanding the context of the reservoir within
the overall geology and petroleum system can be used to
optimize reservoir evaluation. The expanding capabilities
of DFA, plus major advances in asphaltene science, have
revealed dramatic systematic variations of reservoir
fl uids and are becoming indispensable for optimization of
production.

In the present work, we investigate, by means of theoretical simulation, the preferred orientatio... more In the present work, we investigate, by means of theoretical simulation, the preferred orientation of a model asphaltene molecule at the oil−water interface (monomer). The coarse-grained model molecules at the mesoscale level, using dissipative particle dynamics (DPD), are adopted. The central polycyclic aromatic hydrocarbon (PAH) core and the peripheral alkanes in the asphaltene are considered. The asphaltene model construction by coarse grain mapping is proposed and analyzed, as well as the effect of using different solubility parameters in the construction of the potential interaction of the beads in the coarse grained asphaltene model. Also, the effect of surface coverage for a structure where steric effects dominate is presented as well as the effect of asphaltene coarse-grain nanoaggregates at the oil−water region. Finally, the orientation at the oil−water interface of an asphaltene with peripheral oxygen moieties is studied. Toluene is used as a model of oil. Three different orientations of the asphaltene model are used as starting configurations: horizontal to the oil−water interface, perpendicular to the oil−water interface, and tilted 45° with respect to the oil−water interface. In all cases, it is found that the asphaltene molecule stays at the oil−water interface with the preferred orientation where the aromatic region lays in the plane of the oil−water interface while the aliphatic chains are perpendicular to the oil−water interface and in the oil region. This molecular orientation remains in the case of higher asphaltene surface coverage. Due to steric hindrance, some of the asphaltenes migrate to the bulk and some remain at the oil−water interface. Asphaltene molecules in the bulk oil are found to interact by π−π stacking interaction of the aromatic cores. For the case of the nanoaggregate, where the aromatic core is surrounded by alkyl chains, it is observed that the aggregate migrates to the bulk of the oil region; thus these nanoaggregates do not load onto the oil−water interface. For the case of the coarse grain asphaltene with peripheral oxygen moieties, it is found that the oxygen moieties orient in plane at the oil−water interface while the PAH orients out of plane and into the toluene region. These many findings are consistent with extensive experimental results as discussed. The combination of coarse grain and DPD dynamics, while maintaining the major structural characteristics in the coarse grain mapping of asphaltene species, represents a powerful tool that will help to answer questions about oil−water emulsions.

Electric field gradients (EFG) at the metal center were studied in seven chloroindium porphyrin c... more Electric field gradients (EFG) at the metal center were studied in seven chloroindium porphyrin complexes and in chloroindium phthalocyanine by the technique of perturbed angular correlation of gamma rays. These complexes were synthesized with the probe nucleus ["In for use in polycrystalline and solution sources. The magnitudes of the EFGs of the metalloporphyrins span a wide range and are shown to correlate with the electron donating properties of the peripheral substituents. For the phthalocyanine complex the derived EFG is quite large in spite of the electron withdrawing properties of the phthalocyanine ring. The porphyrin complexes all exhibited appreciable distributions in the magnitudes of the EFGs, which may be correlated with the deformable nature of the porphyrin macrocycle in contrast to the relatively planar, inflexible structure of the phthalocyanine ring which showed no such spread. The PAC spectra of metalloporphyrins dissolved in organic solvents exhibited two attenuation components, whose parameters were consistent with porphyrin properties and provide evidence for metalloporphyrin aggregation.

Asphaltene precipitation from live crude oils that occurs due to pressure reduction can foul and ... more Asphaltene precipitation from live crude oils that occurs due to pressure reduction can foul and clog oil production equipment, at the well surface, in the borehole, and even in the subsurface formation, thus is of considerable interest to oil operating companies. We employ near-infrared (NIR) spectroscopy to characterize this asphaltene precipitation process; in particular, the independent measurements on asphaltene flocculation of wavelength dependence of optical scattering and of sedimentation rates are performed. Here, it is established that different asphaltene flocs form during depressurization of crude oil. Furthermore, the initial precipitate is probably not problematic in the production of crude oil, relaxing constraints imposed by asphaltene considerations. Additionally, the asphaltene precipitation process is shown to be largely reversible in the minutes time frame, but subtle irreversibilities are suggested. Compressibility is measured using NIR techniques to validate our methods. Optical spectroscopy on optically thin samples is found to be a powerful and indispensable tool to characterize asphaltene precipitation.

Endpoints of the OBM filtrate and virgin fluid a b s t r a c t Accurate quantification of oil-bas... more Endpoints of the OBM filtrate and virgin fluid a b s t r a c t Accurate quantification of oil-based drilling mud (OBM) filtrate contamination of hydrocarbon samples is still one of the biggest challenges in formation fluid sampling with formation testers. There exist contamination quantification techniques, but they can be technique sensitive, lack a confident level of quality control and apply only to a limited combination of probe types and formation fluid types. In particular, current techniques rely on an assumed absence of mud filtrate coloration at relevant optical channels and on sufficient optical density contrast between mud filtrate and virgin fluids. Such assumed fluid properties may not materialize when, for example, drilling muds are reused in multiple wells or when virgin fluids exhibit little color due to the absence of asphaltenes. In this paper, new mixing rules have been developed for mass density and shrinkage factor and for the newly defined " f-function " and " q-function ". The f-function, also referred to as the modified gas/oil ratio, is essentially a combination of gas/oil ratio with shrinkage factor. Similarly, the q-function is referred to as modified composition and is a combination of composition with mass density. OBM filtrate contamination in volume fraction, optical density, f-function, mass density, shrinkage factor, and q-function at a specified downhole sampling station have been found to be mutually linearly related as predicted by the mixing rules. The mutually linear relations are further confirmed by laboratory data for different mixtures of mud filtrate and formation fluids. Application of these mixing rules enables accurate OBM filtrate contamination in hydrocarbon samples if the filtrate properties and virgin fluid properties can be determined. A new methodology has been developed to determine the properties of the virgin formation fluid and mud filtrate, which are referred to as endpoints. The mud filtrate properties are obtained by extrapolating the mutually linear relations established from the cleanup data to zero gas/oil ratio or other known filtrate values such as zero methane composition, and/or zero optical density at specified wavelengths. The virgin fluid properties are determined by the power-law fitting of cleanup data coupled with the flow regime identification which is confirmed by large number of downhole fluid analysis datasets from wireline formation testers and numerical simulation. This novel methodology enables accurate quantification of the OBM filtrate and pure virgin formation fluid. Furthermore, the self-consistency of using multiple independent sensors provides confidence and greatly improves the robustness and quality control of OBM filtrate contamination monitoring downhole. Finally, contamination results can be expressed in volume or weight percent and as live fluid or stock-tank liquid fraction for easy comparison to laboratory results. A latest generation downhole fluid analysis (DFA) tool was employed to measure fluid properties downhole in real time on more than 30 DFA stations acquired with either conventional probes or 3D radial probes. The new methodology was applied to each of the acquired datasets. All the results from the new method are in good agreement with the results of the laboratory analysis.

Asphaltene molecular size and weight have been of concern since asphaltenes were first isolated f... more Asphaltene molecular size and weight have been of concern since asphaltenes were first isolated from crude oils. Despite previous divergent results on this topic, in recent years, there has been a growing consensus among all mass spectral ionization techniques and all diffusion measurements that asphaltenes are fairly small molecules. In this paper, fluorescence correlation spectroscopy (FCS) is used to determine translational diffusion coefficients of asphaltene and model compounds under a variety of conditions. These FCS studies provide several stringent tests on asphaltene molecular size and architecture. A broad range of concentrations including ultralow concentrations is investigated to ensure the lack of potential aggregation difficulties. Large temperature variations are used to test the application of the simple diffusion equation. FCS results here clearly show the dependence of the diffusion constant on the molecular weight. Finally, FCS results on asphaltenes are in quantitative agreement with those of time-resolved fluorescence depolarization on asphaltenes. A comparison of the results herein with previous FCS and time-resolved fluorescence depolarization (TRFD) results on the same asphaltenes confirms the correlation between molecular size and asphaltene chromophore size; this supports a molecular architecture with one or two polycyclic aromatic hydrocarbons (PAHs) per molecule and counters proposed structures with many PAHs per asphaltene molecule.

nheptane, asphaltenes are a highly aromatic, polydisperse mixture consisting of the heaviest and ... more nheptane,
asphaltenes are a highly aromatic, polydisperse mixture
consisting of the heaviest and most polar fraction of crude oil.1
The chemistry of asphaltenes is critically important to many aspects
of the exploration and production of conventional crude oil, and
asphaltene composition and behavior have been correspondingly
scrutinized.2 Traditionally, asphaltenes are thought to control the
phase behavior of oil. More recently, asphaltenes have emerged as
an important class of geochemical markers that reflect the geology
of oil reservoirs.1 In the future, the relevance of asphaltenes is poised
to increase dramatically, as asphaltenes are present in high
concentration (∼15%) in unconventional resources, such as heavy
oils and tar sands, which are expected to feature prominently in
the world’s energy mix in the decades to come.3 The efficient
production of these resources requires a thorough understanding
of asphaltene chemistry.
Despite the significant and growing importance of asphaltenes
in the world’s energy supply, even basic aspects of their chemistry
are currently poorly understood. For example, since Boduszynski’s
initial report in 1987,4 numerous experimental techniques have been
applied to measure the molecular weight distribution of asphaltenes.
These experiments have generated a controversy. Reported mean
molecular weights range over 2 orders of magnitude for similar
samples; recent overviews summarize these different points of
view.1,5,6 Briefly, mass spectrometric measurements with electrospray,
chemical, and field desorption ionization, along with
fluorescence-based diffusion measurements and electron microscopy
experiments that relate molecular size to molecular weight, suggest
an average molecular weight in the range of 500-1000 Da. On
the other hand, fast atom bombardment and plasma desorption mass
spectrometry show a pronounced tail extending at times beyond
10 000 Da, and size exclusion chromatography can give a bimodal
distribution containing an intense peak at >106 Da.

Downhole fluid analysis data from several deepwater oil wells in the Gulf of Mexico are examined.... more Downhole fluid analysis data from several deepwater oil wells in the Gulf of Mexico are examined. The
primary question addressed is whether there is lateral fluid-flow connectivity of the ‘‘A’’ Sand that spans
two wells. The predominant fluid gradient observed in the A Sand is the variable dissolved asphaltene
content. To perform thermodynamic modeling of the asphaltene gradients, the Flory–Huggins–Zuo Equation
of State is used. This formalism relies on using proposed asphaltene colloidal sizes from the Yen–
Mullins Model of asphaltenes. In particular, in the reservoir crude oil in Sand A, asphaltenes were presumed
to be dispersed as 2 nm nanoaggregates, which is typical for corresponding black oils. In this
case, the Sand A crude oil was shown to be equilibrated and thereby indicating reservoir connectivity,
which was subsequently proven in oil production. This new analytic methodology is shown to complement
a variety of more traditional analyses, and is shown to be superior in analyzing the most important
reservoir properties. The combination of new petroleum science and new measurement capabilities is
yielding many important advances in reservoir evaluation including understanding of fluid-flow connectivity,
viscosity gradients, tar mat formation and large gradients associated with fluid disequilibrium

Hydraulic connectivity of petroleum reservoirs represents one of the biggest uncertainties for bo... more Hydraulic connectivity of petroleum reservoirs represents one of the biggest uncertainties for both oil production and petroleum system studies. Here, a geochemical analysis involving bulk and detailed measures of crude oil composition is shown to constrain connectivity more tightly than possible with conventional methods. Three crude oils from different depths in a single well exhibit large gradients in viscosity, density and asphaltene content. The oils were collected with a wireline sampling tool to provide samples from well-defined locations and relatively free of contamination from drilling fluid; the known prove-nance of the samples minimizes uncertainties in analysis. The detailed chemical composition of almost the entire crude oil was determined by use of comprehensive two-dimensional gas chromatography (GCÂGC) to interrogate the non-polar fraction and negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to interrogate the polar fraction. The simultaneous presence of 25-norhopanes and mildly altered normal and isoprenoid alkanes was detected, suggesting that the reservoir experienced multiple charges and contains a mixture of oils bio-degraded to different extents. The gradient in asphaltene concentration is explained by an equilibrium model considering only gravitational segregation of asphaltene nanoaggregates; this grading can be responsible for the observed variation in viscosity. Combining the analyses affords a consistent picture of a connected reservoir in which the observed viscosity variation originates from gravitational segregation of asphaltene nanoaggregates in a crude oil with high asphaltene concentration resulting from multiple charges, including one charge that suffered severe biodegradation. Observation of these gradients having appropriate magnitudes suggests good reservoir connectivity with greater confidence than possible with traditional techniques alone.

Asphaltenes, Heavy Oils, and Petroleomics, 2007