Crude Oil Components Research Articles

Decomposition of Oxidized Sulfur-Containing Components of Crude Oil and Petrochemicals Under Conditions of Nanoheterogeneous Catalysis

The catalytic decomposition of dibenzothiophene sulfone is studied in the presence of nanosized iron(III) oxide with an average particle diameter of 10–12 nm; it is synthesized by the thermolysis of iron(III) acetylacetonate in the medium of diphenyl ether. The optimum conditions of the process are ascertained by varying the temperature and time of the reaction and the concentration of the nanosized catalyst. It is shown that the oxidative desulfurization of crude oil and petrochemicals may occur with the oxidized sulfur-containing components being decomposed under mild conditions in the presence of nanosized iron(III) oxide at the second stage. In some cases, the degree of desulfurization is above 80%.

Aerobic Biodegradation of Crude Oil Components by Acidophilic Mycobacteria

Biodegradation of crude oil components by strain AGS10, an acidophilic member of the genus Mycobacterium, was studied under extremely acidic conditions (pH 2.5). The degree of degradation of the same hydrocarbons in different kinds of oil was found to be different. The direction of biodegradation was, however, the same: the share of n-alkanes in oxidized oil decreased, while the share of branched alkanes increased. At the same time, the degree of redistribution of methane hydrocarbons in degraded oil varied significantly for different oils, although no strict dependence on the type of oil was found. After 28 days of incubation at 30°C and pH 2.5, the degradation of n- and iso-alkanes was 99 and 44%, respectively for the light, low-viscosity oil of the Nizhnevartovsk deposit, 58 and 32%, respectively for the medium-density oil of the Moscow oil-procesing plant, and 80 and 16% and 99 and 69%, respectively for the heavy, viscous oils of the Cheremukhovskoe and Usinkoye oil fields. Moreover, after extended cultivation time strain AGS10 completely utilized alkanes, as well as a significant part of the naphthene component of the aliphatic fraction. The studied strain was characterized by ability to oxidize a broad spectrum of methane hydrocarbons, including high-molecular C17–C30n-alkanes, in oils of different properties and composition. Apart from its scientific interest, farther investigation of biodegradation of high-paraffin oils and viscous oils with elevated paraffin content by strain AGS10 may be useful in view of the technical issues associated with paraffin accumulation in the course of recovery and transportation of these oils.

Medium Chain and Long Chain Alkanes Hydroxylase Producing Whole Cell Biocatalyst From Marine Bacteria

Alkanes are major component of crude oil that could be hydrolyzed by the enzyme of alkane hydroxylase. The are three types of alkane hydroxylase based on the chain length of alkane such as short-chain length/SCL (C2-C4), medium-chain length/MCL (C5-C17), and long-chain length/LCL (C>18). The aims of this study were to characterize and identify alkanes-degrading bacteria from these bacteria. The 30 strains from marine were grown on MCL (Pentane-C5H12, Decane-C10H22, and Pentadecane-C15H32) and LCL (n-Paraffin-C12H19C17 and branch of Pristane-C19H40). The study showed twenty-nine isolates have the ability to degrade alkanes compounds, whereas 14 isolates have grown ability on MCL and LCL medium, 11 isolates have the ability to grow on MCL and n-LCL, 3 isolates have the ability only to grow on MCL medium and 1 isolate has the ability only grow on n-LCL medium. The growth test result indicated that 29 isolates have medium-chain alkane monooxygenase and long-chain alkane hydroxylase. Based on 16S rDNA gene analysis, we obtained twenty nine of oil- degrading bacteria, namely a-proteobacteria (57 %), g-proteobacteria (30 %), Flavobacteria (7 %), Bacilli (3%) and Propionibacteriales (3 %). g-Proteobacteria and a-proteobacteria which seems to play an important role in the alkane biodegradation.

Toxicokinetics of Crude Oil Components in Arctic Copepods.

The risk of accidental oil spills in the Arctic is on the rise due to increased shipping and oil exploration activities, making it essential to calibrate parameters for risk assessment of oil spills to Arctic conditions. The toxicokinetics of crude oil components were assessed by exposing one lipid-poor (CIII) and one lipid-rich (CV) stage of the Arctic copepod Calanus hyperboreus to crude oil WSF (water-soluble fraction). Water concentrations and total body residues (BR), as well as lipid volume fractions, were measured at regular intervals during exposure and recovery. Bioconcentration factors (BCFs) and elimination rates ( ke) for 26 petrogenic oil components were estimated from one-compartment models fitted to the BR data. Our parameters were compared to estimations made by the OMEGA bioaccumulation model, which uses the octanol-water partitioning coefficient ( KOW) in QSAR (quantitative structure-activity relationship) predictions. Our parameters for the lipid-poor CIIIs generally agreed with the OMEGA predictions, while neither the BCFs nor the kes for the lipid-rich CVs fitted within the realistic range of the OMEGA parameters. Both the uptake and elimination rates for the CVs were in general half an order of magnitude lower than the OMEGA predictions, showing an overestimation of these parameters by the OMEGA model.

Coarse-Grained Molecular Simulation and Nonlinear Manifold Learning of Archipelago Asphaltene Aggregation and Folding.

Asphaltenes constitute the heaviest aromatic component of crude oil. The myriad of asphaltene molecules falls largely into two conceptual classes: continental-possessing a single polyaromatic core-and archipelago-possessing multiple polyaromatic cores linked by alkyl chains. In this work, we study the influence of molecular architecture upon aggregation behavior and molecular folding of prototypical archipelago asphaltenes using coarse-grained molecular dynamics simulation and nonlinear manifold learning. The mechanistic details of aggregation depend sensitively on the molecular structure. Molecules possessing three polyaromatic cores show a higher aggregation propensity than those with two, and linear archipelago architectures more readily form a fractal network than ring topologies, although the resulting aggregates are more susceptible to disruption by chemical dispersants. The Yen-Mullins hierarchy of self-assembled aggregates is attenuated at high asphaltene mass fractions because of the dominance of promiscuous parallel stacking interactions within a percolating network rather than the formation of rodlike nanoaggregates and nanoaggregate clusters. The resulting spanning porous network possesses a fractal dimension of 1.0 on short length scales and 2.0 on long length scales regardless of the archipelago architecture. The incompatibility of the observed assembly behavior with the Yen-Mullins hierarchy lends support that high-molecular weight archipelago architectures do not occur at significant levels in natural crude oils. Low-dimensional free energy surfaces discovered by nonlinear dimensionality reduction reveal a rich diversity of metastable configurations and folding behavior reminiscent of protein folding and inform how intramolecular structures can be modulated by controlling asphaltene mass fraction and dispersant concentration.

Foaming of crude oil: Effect of acidic components and saturation gas

Foaming tendency of crude oil is diverse due to the variance in crude oil components. Crude oil viscosity, asphaltene, and resin contents have been studied in general, but the specific surfactant molecules that stabilize crude oil foam are not identified. Carbon dioxide oftentimes accompanies hydrocarbon gases in reservoirs rich in natural CO2 and fields where CO2 enhanced recovery is implemented. Crude oil acidic components and carbon dioxide may have considerable significance to foam formation and defoaming kinetics. We studied the effect of acidic components and saturation gas on foaming of crude oils with pneumatic and depressurization tests respectively. Foaminess, foamability and time to half collapse are used to characterize foaming of oils. We present a new piecewise model that can be divided into two segments with almost equal timescales to fit the collapse curve and we use parabola function and expdec1 function to model the two segments respectively. The t values at the intersection of two segments are approximately the same as time to half collapse. Model parameter A is close to foamability. Increase in the rigidity of the interface and viscosity of the bulk liquid may account for excellent foam stability of some acids/oil systems. Some acidic crude oil components, for example, naphthenic acids, long-chain fatty acids, increase foaminess significantly, suggesting that crude oil antifoams combined with reagents that will react with some acidic components may have better performance.

Composition of aquathermolysis catalysts forming in situ from oil-soluble catalyst precursor mixtures

Increasing efficiency of thermal recovery methods of heavy oil through injection of destructive hydrogenation catalysts precursors is a relevant task. In this paper the composition and properties of the active binary catalysts, which formed from a mixture of oil-soluble iron-, nickel- and copper-based precursors have been studied by XRD, Mössbauer spectroscopy, SEM, EDX-mapping methods. Moreover, the efficiency of a mixture of the corresponding metals in reducing the content of heavy components (SARA) of heavy crude oil from Ashal'cha field (Republic of Tatarstan) have been investigated. Mössbauer spectroscopy and EDX-mapping detected the formation of not only spinel ferrites MFe2O4 (where M = Ni or Cu), but also individual oxides. Moreover, in case of nickel-based catalyst three phases are formed: magnetite FeO⋅Fe2O3, spinel ferrite NiFe2O4 and superparamagnetic finely dispersed phase of iron oxides (and probably nickel oxides). According to XRD results, the oxides formed in the first stage transform into nonstoichiometric spinel ferrites Cu0.86Fe2.14O4 and Ni1.43Fe1.7O4 under hydrothermal influences. It is related with the thermodynamically beneficial processes of conversion of this specific phase composition. Based on the SARA and elemental analysis of the initial and converted oil, it was found that catalytic aquathermolysis decreases the content of asphaltenes and resins (about 45%) in the presence of catalyst nanoparticles, which indicates an improvement in the quality of the heavy oil.

BIOREMEDIATION STUDY: HYDROCARBON DEGRADING BACTERIA

Many microorganisms capable of degrading petroleum components have been isolated and few of them seem to be important for petroleum biodegradation in natural environments. To identify the bacteria that play a major role in degradation of petroleum polynuclear aromatic hydrocarbons (PAHs), bacteria were enriched from seawater by using Naphthalene, Phenanthrene, Trichlorodibenzofuran and Benzo[a]pyrene as a carbon and energy source. The result of study that members of the genus Alcanivorax and Thalassospira became predominant in the enrichment cultures. The strains isolated in this study could grow on crude oil and degraded PAH components of crude oil. The number of cells increased to 8.1x106 cells g-1 after 14 days in subculture. PAH degradation proceeded parallel with the growth of bacteria cells. This observation which has been conducted in Marine Biotechnology Institute, Kamaishi, Iwate-ken, Japan suggests that Alcanivorax and Thalassospira play an important role in the degradation of petroleum PAHs in marine environment.

Non-equilibrium 3-D simulator applied to air injection EOR in light oil reservoirs

In this paper, a 3-D simulator based on the assumption of both chemical and thermal non-equilibrium is presented for a more realistic view of non-equilibrium process for simulating air injection EOR (enhanced oil recovery). The mathematical model is based on a non-equilibrium stage with dynamic mass transfer process between different fluid phases and energy transfer process between fluid and matrix (solid). The light crude oil oxidation and air injection process are both simulated by this simulator, through which the pore blocking, gas breakthrough and gas override process are presented through the visualization of oil, gas saturation distributions. The crude oil oxidation is refined as in-phase and interphase reactions to investigate the effect of reactant dispersion caused by the gas-liquid mass transfer. The oxygen dissolution and the evaporation of light components in crude oil are considered in the non-equilibrium process. The results without mass transfer, with dynamic mass transfer and with instantaneous mass transfer between gas and liquid phases are compared through multiplying the gas-liquid interface area 0, 1 and 100 times respectively, and the influence of these different mass transfer conditions on the migration of temperature peak front and oil recovery is revealed. In addition, the effect of local chemical and thermal non-equilibrium in different matrix permeability as well as porosity scenarios is also investigated to obtain non-equilibrium behavior in different reservoir conditions when implementing air injection EOR. In the last section, we investigated the result with different simulation timescale to see if the non-equilibrium behavior can be captured in the simulator used in this paper. This study suggests that the non-equilibrium behavior would show obvious influence on oil recovery for air injection in the lower permeability reservoir. The non-equilibrium behavior is desirable to be considered for simulating air injection EOR in light oil reservoirs. The developed 3-D simulator is robust that can better reflect the mechanism of air injection EOR in light oil reservoirs. We hope this work encouraging more investigations for the simulation side of non-equilibrium phase behavior and energy transfer in the air injection process.

Effect of Asphaltene Characteristics on Its Solubility and Overall Stability

Complex molecular structure, high impurity content, and self-association tendency of asphaltenes make the determination of their phase behavior very difficult. Because asphaltene phase behavior is indicative of asphaltene stability within the bulk oil, it is very important to understand its stability. Various production and flow assurance challenges related to precipitation of unstable asphaltenes can be prevented by proper comprehension of asphaltene stability. This study provides a data set on 11 different asphaltenes, which helps us to understand the complicated nature of the components of asphaltenes and crude oils that play an important role in maintaining the stability of asphaltenes. In addition to the physical and chemical characterizations, elemental analysis and ΔPS parameter, which is the indication of the solubility of asphaltenes in different solvents of the bulk oil samples, were measured and evaluated. The results of this study show that the presence of paraffinic wax and water within the c...

Composition and distribution of NSO compounds in two different shales at the early maturity stage characterized by negative ion electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry

Gas chromatography/mass spectrometry (GC/MS) can only analyze volatile molecular compounds, and it has limitations when applied to determine the complex components of crude oils and hydrocarbon source rocks. Based on Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and GC/MS analyses, the molecular compositions of NSO compounds in extracts from the Permian Dalong Formation, Sichuan Basin and the Permian Lucaogou Formation, Junggar Basin in China were compared. Analyses of types of heteroatoms present (S1, S2, S3, OS, OS2, O2S, NS, and NOS compounds) suggest that marine shales from the Dalong Formation are mainly composed of carboxylic acids (O2 compounds) with a high abundance of fatty acids, indicating a marine phytoplankton organic source. However, lacustrine shales from the Lucaogou Formation are dominated by pyrrolic compounds (N1 compounds) with abundant dibenzocarbazole. It suggests that the organic source materials may be derived from lower aquatic organisms and lacustrine algae. Overall, FT-ICR-MS has potential for applications in analyses and determination of depositional environments and organic sources in petroleum geology.

Biodegradation-mediated alterations in acute toxicity of water-accommodated fraction and single crude oil components in cold seawater

Hydrocarbon biodegradation may be slower in cold Arctic than in temperate seawater, and this will affect the toxicity time window of the hydrocarbons. In this study, the acute toxicities of water-soluble phases of 1,3-dimethylnaphthalene, phenanthrene, fluoranthene, and low energy water-accommodated fractions (LE-WAFs) of an evaporated (200 °C+) crude oil, were screened by a Microtox bioassay during biodegradation in cold seawater (4–5 °C). The water-solubility of fluoranthene was too low to provoke a toxic response at any time, whereas the toxicity of 1,3-dimethylnaphthalene and phenanthrene decreased over time in relation to biotransformation of these compounds. In LE-WAFs, the Microtox EC50 was associated with biodegradation of the predominant hydrocarbons (naphthalenes, 2- to 3-ring PAH), as well as with phenol degradation products. The acute toxicities of single hydrocarbons and LE-WAFs persisted for a longer period in the cold seawater than previously shown at higher seawater temperatures. These results suggest implications for fate and effects assessment of hydrocarbons after oil spills in cold environments, like the Arctic. However, further biodegradation studies using Arctic seawater and relevant species for toxicity testing are needed for confirmation.

Estimating incident ultraviolet radiation exposure in the northern Gulf of Mexico during the Deepwater Horizon oil spill.

Millions of barrels of oil were released into the Gulf of Mexico following the 2010 explosion of the Deepwater Horizon oil rig. Polycyclic aromatic hydrocarbons (PAHs) are toxic components of crude oil, which may become more toxic in the presence of ultraviolet (UV) radiation, a phenomenon known as photo-induced toxicity. The Deepwater Horizon spill impacted offshore and estuarine sites, where biota may be co-exposed to UV and PAHs. Penetration of UV into the water column is affected by site-specific factors. Therefore, measurements and/or estimations of UV are necessary when one is assessing the risk to biota posed by photo-induced toxicity. We describe how estimates of incident UV were determined for the area impacted by the Deepwater Horizon oil spill, using monitoring data from radiometers near the spill, in conjunction with reference spectra characterizing the composition of solar radiation. Furthermore, we provide UV attenuation coefficients for both near- and offshore sites in the Gulf of Mexico. These estimates are specific to the time and location of the spill, and fall within the range of intensities utilized during photo-induced toxicity tests performed in support of the Deepwater Horizon Natural Resource Damage Assessment (NRDA). These data further validate the methodologies and findings of phototoxicity tests included in the Deepwater Horizon NRDA, while underscoring the importance of considering UV exposure when assessing possible risks following oil spills. Environ Toxicol Chem 2018;37:1679-1687. © 2018 SETAC.

Photo-induced toxicity following exposure to crude oil and ultraviolet radiation in 2 Australian fishes.

Some polycyclic aromatic hydrocarbons (PAHs), components of crude oil, are known to cause increased toxicity when organisms are co-exposed with ultraviolet radiation, resulting in photo-induced toxicity. The photodynamic characteristics of some PAHs are of particular concern to places like Australia with high ultraviolet radiation levels. The objective of the present study was to characterize the photo-induced toxicity of an Australian North West Shelf oil to early life stage yellowtail kingfish (Seriola lalandi) and black bream (Acanthopagrus butcheri). The fish were exposed to high-energy water accommodated fractions for 24 to 36 h. During the exposure, the fish were either co-exposed to full-intensity or filtered natural sunlight and then transferred to clean water. At 48 h, survival, cardiac effects, and spinal deformities were assessed. Yellowtail kingfish embryos co-exposed to oil and full-spectrum sunlight exhibited decreased hatching success and a higher incidence of cardiac arrhythmias, compared with filtered sunlight. A significant increase in the incidence of pericardial edema occurred in black bream embryos co-exposed to full-spectrum sunlight. These results highlight the need for more studies investigating the effects of PAHs and photo-induced toxicity under environmental conditions relevant to Australia. Environ Toxicol Chem 2018;37:1359-1366. © 2018 SETAC.

Characterization of the genome of a Nocardia strain isolated from soils in the Qinghai-Tibetan Plateau that specifically degrades crude oil and of this biodegradation

A strain of Nocardia isolated from crude oil-contaminated soils in the Qinghai-Tibetan Plateau degrades nearly all components of crude oil. This strain was identified as Nocardia soli Y48, and its growth conditions were determined. Complete genome sequencing showed that N. soli Y48 has a 7.3 Mb genome and many genes responsible for hydrocarbon degradation, biosurfactant synthesis, emulsification and other hydrocarbon degradation-related metabolisms. Analysis of the clusters of orthologous groups (COGs) and genomic islands (GIs) revealed that Y48 has undergone significant gene transfer events to adapt to changing environmental conditions (crude oil contamination). The structural features of the genome might provide a competitive edge for the survival of N. soli Y48 in oil-polluted environments and reflect the adaptation of coexisting bacteria to distinct nutritional niches.

Structural and Biochemical Characterization of BdsA from Bacillus subtilis WU-S2B, a Key Enzyme in the "4S" Desulfurization Pathway.

Dibenzothiophene (DBT) and their derivatives, accounting for the major part of the sulfur components in crude oil, make one of the most significant pollution sources. The DBT sulfone monooxygenase BdsA, one of the key enzymes in the “4S” desulfurization pathway, catalyzes the oxidation of DBT sulfone to 2′-hydroxybiphenyl 2-sulfonic acid (HBPSi). Here, we determined the crystal structure of BdsA from Bacillus subtilis WU-S2B, at the resolution of 2.2 Å, and the structure of the BdsA-FMN complex at 2.4 Å. BdsA and the BdsA-FMN complex exist as tetramers. DBT sulfone was placed into the active site by molecular docking. Seven residues (Phe12, His20, Phe56, Phe246, Val248, His316, and Val372) are found to be involved in the binding of DBT sulfone. The importance of these residues is supported by the study of the catalytic activity of the active site variants. Structural analysis and enzyme activity assay confirmed the importance of the right position and orientation of FMN and DBT sulfone, as well as the involvement of Ser139 as a nucleophile in catalysis. This work combined with our previous structure of DszC provides a systematic structural basis for the development of engineered desulfurization enzymes with higher efficiency and stability.

Surface wettability control of reservoir rocks by brine

Abstract Based on adhesion models between rock surface groups and organic molecules, the interactions between the chemical groups on the rock surface and the components of crude oil and the interactions of the electrical double layers at the rock surface and oil-water interface were analyzed to investigate the abilities and microscopic mechanisms of wettability control by H+, OH− and inorganic salt ions in brine, and a new method of wettability control for reservoir rocks was built. The results show that the interaction forces between rock surface groups and oil molecules are van der Waals forces, Coulomb forces, hydrogen bonds, and surface forces. By changing these forces, the control mechanisms of surface wettability of reservoir rocks by brine are: transformation of chemical groups, change of interfacial potential, pH variation of injected water, multicomponent ionic exchange, and salting-in or salting-out effect. For sandstone reservoirs, with the decrease of concentration and valence state of positive ions in brine or the increase of pH (increasing pH has a negligible impact on the brine salinity), the interaction between rock surface and oil becomes weak, thus resulting in increase of water wettability of rock surface. For carbonate reservoirs, CaSO4 or MgSO4 brine with high concentration is beneficial to increase water wettability of rock surface. Therefore, it is feasible to control rock wettability and improve oil recovery by adjusting the ion components of injected water.

Understanding Model Crude Oil Component Interactions on Kaolinite Silicate and Aluminol Surfaces: Toward Improved Understanding of Shale Oil Recovery

Shale oil is currently of interest for unconventional resource exploration and development. Understanding the mechanism of interaction between the complex mixture of organic compounds in shale oil and minerals making up the reservoir rock–oil interface will assist recovery. In this study, molecular dynamics simulation is used to study the adsorption characteristics of a model oil mixture within nanoscale intraparticle pores of kaolinite minerals, which form pore-filling structures in shale rock. To better understand the effects of oil composition, temperature, and pressure on the adsorption properties of the model oil mixture, a range of temperatures (298, 323, 348, and 373 K) and pressures (1, 50, 100, and 200 bar) representing up to reservoir conditions were used. This study shows that adsorption and arrangement of oil molecules is dependent on the surface of kaolinite and the distance away from it. The simulations show polar compounds are likely to be adsorbed on aluminol kaolinite basal surfaces, whil...

Microbial-Enhanced Heavy Oil Recovery under Laboratory Conditions by Bacillus firmus BG4 and Bacillus halodurans BG5 Isolated from Heavy Oil Fields

Microbial Enhanced Oil Recovery (MEOR) is one of the tertiary recovery methods. The high viscosity and low flow characteristics of heavy oil makes it difficult for the extraction from oil reservoirs. Many spore-forming bacteria were isolated from Oman oil fields, which can biotransform heavy crude oil by changing its viscosity by converting heavier components into lighter ones. Two of the isolates, Bacillus firmus BG4 and Bacillus halodurans BG5, which showed maximum growth in higher concentrations of heavy crude oil were selected for the study. Gas chromatography analysis of the heavy crude oil treated with the isolates for nine days showed 81.4% biotransformation for B. firmus and 81.9% for B. halodurans. In both cases, it was found that the aromatic components in the heavy crude oil were utilized by the isolates, converting them to aliphatic species. Core flooding experiments conducted at 50 °C, mimicking reservoir conditions to prove the efficiency of the isolates in MEOR, resulted in 10.4% and 7.7% for B. firmus and B. halodurans, respectively, after the nine-day shut-in period. These investigations demonstrated the potential of B. firmus BG4 and B. halodurans BG5 as an environmentally attractive approach for heavy oil recovery.

Multiphase flow modeling of asphaltene precipitation and deposition

Asphaltene precipitation in reservoirs during production and Enhanced Oil Recovery (EOR) can cause serious problems that lead to reduction of reservoir fluid production. In order to study asphaltene tendency to precipitate and change in flow rate as a function of distance from wellbore, an equation of state (Peng-Robinson) based model namely Nghiem et al.’s model has been employed in this study. The heaviest components of crude oil are separated into two parts: The first portion is considered as non-precipitating component (C31A+) and the second one is considered as precipitating component (C31B+) and the precipitated asphaltene is considered as pure solid. For determination of the acentric factor and critical properties, Lee-Kesler and Twu correlations are employed, respectively. In this study, a multiphase flow (oil, gas and asphaltene) model for an asphaltenic crude oil for which asphaltene is considered as solid particles (precipitated, flocculated and deposited particles), has been developed. Furthermore, effect of asphaltene precipitation on porosity and permeability reduction has been studied. Results of this study indicate that asphaltene tendency to precipitate increases and permeability of porous medium decreases by increasing oil flow rate in under-saturated oil reservoirs and dropping reservoir pressure under bubble point pressure. On the other hand, asphaltene tendency to precipitate decreases with pressure reduction to a level lower than bubble point pressure where asphaltene starts to dissolve back into oil phase. Moreover, it is observed that precipitation zone around the wellbore develops with time as pressure declines to bubble point pressure (production rate increases up). Also, there is an equilibrium area near wellbore region at which reservoir fluid properties such as UAOP (Upper Asphaltene Onset Pressure) and LAOP (Lower Asphaltene Onset Pressure) are constant and independent of the distance from wellbore.