Physical Properties Of Crude Oil Research Articles

Numerical study of elbow corrosion in the presence of sodium chloride, calcium chloride, naphthenic acids, and sulfur in crude oil

During the crude oil refining process, the piping system suffers a lot of corrosion damage at the elbows due to complex chemical reactions. This work aimed to investigate the effects of sodium chloride (NaCl), calcium chloride (CaCl2), naphthenic acids, and sulfur compounds, which can be responsible for corrosion in oil refineries due to the rheological and physical properties of crude oils. The corrosion occurs by a cracking operation that depends on the solid–fluid interaction, the temperature, and the crude oil nature. Computational fluid dynamics (CFD) has been used to predict the corrosion behavior based on the velocity profiles, shear stress, the friction factor, and the solid deposition rate of iron(II) chloride (FeCl2), iron(II) acetate (Fe(CH3COO)2), and iron sulfide (FeS) along the elbow. The numerical results proved that the corrosion locations are focused at certain regions, especially the elbow extrados wall outlet. The numerical results provide more solutions to avoid corrosion and improve the operational safety of the refinery.

Origin and migration pathway of biodegraded oils pooled in multiple-reservoirs of the Chepaizi Uplift, Junggar Basin, NW China: Insights from geochemical characterization and chemometrics methods

The Chepaizi Uplift possesses three sets of oil-bearing reservoirs, six sets of source intervals, multiple episodes of hydrocarbon charging, and variable physical properties of crude oils. In this study, based on geochemical characterization source intervals and the biodegradation rank of the crude oils, four oil populations named as group A, B, C, and D (D1, D2) were defined. The combined results of Q-cluster analysis (QCA) and discriminant analysis (DA) indicate that group A occurred mainly in Lower Neogene Shawan Formation (N1s) in the western Chepaizi Uplift and were primarily derived from the Middle-Lower Jurassic Xishanyao, Badaowan, Sangonghe formations (J2x-J1b-J1s) and a little of Paleogene Anjihaihe Formation (E2-3a) of the Sikeshu Sag. Group B occurred essentially in Upper Carboniferous Xibeikulasi Formation (C2x) of the western Chepaizi, and originated mainly from the J1b mudstone of the Shawan Sag. In addition, the crude oils of group C occurred mainly in the Lower Cretaceous Tugulu Group (K1tg) of the eastern Chepaizi and were primarily derived from Carboniferous-sourced oil from the Chepaizi Uplift. The crude oils of groups D (D1 and D2) are mixed with Permian (P1f and P2w) of Shawan Sag and Carboniferous-sourced oil from the Chepaizi Uplift. Based on the reconstruction of the migration history, the oils of group A in the N1s Formation of western Chepaizi migrated predominantly from the south (Sikeshu Sag) to the north and northeast, with wells Su1-2 and C27 as the initial filling points. The oils of group B in the N1s Formation of the western Chepaizi travelled predominantly from the southeast (Shawan Sag) to the north and northwest. The oils of group D1 in the C2x Formation of eastern Chepaizi migrated mainly from the northeast and southeast (Shawan Sag) to the west, with well P661 as the initial filling point. Wells SM011 and C64 are the initial filling points of the K1tg Group in the Hongche Fault Belt. Crude oils of group C in the K1tg Group travelled predominantly from the southeast (Shawan Sag) and northeast to the west into the eastern Chepaizi and from well P68 to the southwest. According to the paleo-oil preferential filling orientations and paleogeomorphology, some favourable targets for petroleum exploration of the Chepaizi Uplift are forecast in this study.

Research and development of viscosity reducer for oil well production fluid in cold transportation

In view of the situation that heavy oil well production fluid needs to be gathered and transferred by heating or with heat water to ensure the normal operation, a kind of cold transportation viscosity reducer was developed through the compounding of different types of surfactants and the screening of functional additives, which adapt to the oil well produced fluid without heating or watering gathering process, for saving energy, reducing consumption and increasing efficiency, while meeting the environmental needs. After on-site practical applications of 30 pipelines with different physical properties of crude oil, the average pressure of gathering pipelines dropped 1.3 Mpa, which has achieved good results. Application results show that the cold transportation viscosity reducer has strong adaptability and less usage, and it has no influence on demulsification, while having a high value of applications and a broad market prospect.

Effect of N2O produced by indigenous denitrifiers in oil reservoir on the physical properties of crude oil

The success of microbial enhanced oil recovery (MEOR) relies on complex microbial processes. Nevertheless, the contribution and mechanism of in-situ denitrification to microbial oil recovery remain unclear. In this study, eight denitrifying bacterial strains, designated T1, D1, D44, D46, D15, S1, S2 and S6, were isolated from the produced water of Xinjiang Oilfield, China, by a double layered plate method. The16S rDNA gene sequences of these denitrifying strains shared 100% similarity with Pseudomonas stutzeri (T1, D1, and D44), Pseudomonas putida (D46 and D15), and Pseudomonas aeruginosa (S1, S2, S6), respectively. The N2O production effects of these strains on the physical properties of crude oil were evaluated with batch experiment. Results showed that the highest total gas yield was observed with sucrose as carbon source, and the maximal concentration of N2O occurred with glycerol as carbon source. The denitrification process by these bacterial strains led to volume expansion and viscosity reduction of crude oil. Crude oil expansion rate was positively correlated with the concentration of N2O, with a correlation coefficient of 0.983, but not correlated with the volume of total gas production. Strain S1, S2, and S6 produced 530-730 mg·L-1 of surfactant using glycerol as ole carbon source, which could reduce surface tension and emulsify crude oil. However, these surfactant-producing strains produced less N2O, exhibited weaker effects on oil swelling and viscosity reduction, compared to the none-surfactant-producing denitrifying strains. Our results suggested that more attention should be paid to the ability of N2O production by denitrifying bacteria when exploiting microbial resources towards enhancing oil recovery.

Effects of location and extraction solvent on physico chemical properties of Moringa stenopetala seed oil

This research was initiated to evaluate the physicochemical property of crude oil from Moringa stenopetala (M. Stenopetala) seed collected from three locations (Damba Gofa, Shelle and Konso) which wasextracted with two solvents (hexane and petroleum ether). Physical properties of crude oil varied from 34.8- 44.3%, 0.8–0.9, 0.8–0.9gcm3and 1.4–1.5 at 40 °C for yield, specific gravity, relative density and refractive index, respectively. The chemical properties of the crude oil varied from 0.1 – 0.3mg KOHg−1, 76.5–91.6g I2 100g−1, 17.6–20.6 mEqKg−1, 154.0–199.3mg KOHg−1 oil and 0.2–0.4mgg−1for Free fatty acid(FFA), iodine value, peroxide value, saponification value and acid value respectively. Two-way interaction effect of all physical and chemical properties showed a significant difference (p < 0.05) except the refractive index. The finding indicates that oil extracted with Hexane from Konso had a better quality. The extraction of oil using hexane from M. stenopetala seed could be of great importance in edible crude oil production. Further investigation should be done on oxidative stability of crude oilof M. Stenopetala.

Mixing of Crude Oil with Organic ZnO Nano-Particles from Rice Bran to Improve Physical Properties of Crude Oil: A Novel Agent for Enhanced Oil Recovery

In this study, ZnO nanoparticles were modified with rice bran. Synthesis and production of ZnO nano-particles is highly important due to the use of rice bran. In addition, XRD and SEM analyses were used to ensure the production quality of the nano-particles. The images clearly showed surface uniformity of the synthesized organic ZnO nano-particles. Afterward, the modified nano-particles were injected into crude oil in different weight percentages according to their properties (heavy and light crude oil). The injection was done at a temperature range of 30–150 °C with operating pressures varying from 10 bar to 300 bar. The adhesion force created between heavy or light crude oil molecules and organic ZnO nano-particles was modified with rice bran. Furthermore, an increase in the operating temperature increased the thermal conductivity of oil samples from 0.21 to 2.54 W/m °C for the light crude oil sample and 1.56–6.3 W/m °C for the heavy crude oil sample. The results showed that the percentage of the asphaltene precipitation decreased with the increased API of the crude oil. In addition, the percentage of asphaltene precipitation for nano-light and heavy crude oil was considerably better than the simple light and heavy crude oil samples, respectively. The nano-particles improved the crude oil recovery from reservoirs. The results indicate that the probability of asphaltene precipitation in the case of light crude oil nano-particles is less than the simple light crude oil by 28.3%. This is 8.1% for the heavy crude oil compared to the simple heavy crude oil.

A new empirical model for estimation of crude oil/brine interfacial tension using genetic programming approach

Detailed understanding of the behavior of crude oils and their interactions with reservoir formations and other in-situ fluids can help the engineers to make better decisions about the future of oil reservoirs. As an important property, interfacial tension (IFT) between crude oil and brine has great impacts on the oil production efficiency in different recovery stages due to its effects on the capillary number and residual oil saturation. In the present work, a new mathematical model has been developed to estimate IFT between crude oil and brine on the basis of a number of physical properties of crude oil (i.e., specific gravity, and total acid number) and the brine (i.e., pH, NaCl equivalent salinity), temperature, and pressure. Genetic programming (GP) methodology has been implemented on a data set including 560 experimental data to develop the IFT correlation. The correlation coefficient (R2 = 0.9745), root mean square deviation (RMSD = 1.8606 mN/m), and average absolute relative deviation (AARD = 3.3932%) confirm the acceptable accuracy of the developed correlation for the prediction of IFT.

Prediction of Physical Properties of Crude Oil Based on Ensemble Random Weights Neural Network

Prediction of physical properties of crude oil plays a key role in the petroleum refining industry, therefore, it is of great significance to establish the prediction model of physical properties of crude oil. In this paper, we propose an ensemble random weights neural network based prediction model whose inputs are nuclear magnetic resonance (NMR) spectra and outputs are carbon residual and asphaltene of crude oil. The model uses random vector functional link (RVFL) networks as the basic components and employs the regularized negative correlation learning strategy to build neural network ensemble and the online method to learn the new data. The experiment using the practical data collected from a refinery is carried out and compared with the decorrelated neural network ensembles with random weights (DNNE), least squares support vector machine (LS-SVM), partial least squares regression (PLS) and multiple linear regression (MLR). The results indicate the effectiveness of the proposed approach.

The properties and kinetics characteristics of oxidised ultra-heavy oil in the coke deposition process

High pressure air injection (HPAI) is generally considered an efficient method to drive crude oil. In this paper, the results showed that the oxidation of heavy oil was able to reduce the oxygen content. After the high pressure oxidation, the physical properties of crude oil changed, and it became inflammable. Due to the formation of coke, the viscosity increased sharply and the hydrogen/carbon (H/C) atomic ratio decreased. By using an environmental scanning electron microscope, it was observed that the structures of the original oil were reconstructed and the surface was covered by a large number of inflammable globular structure with a diameter of 500 nm that were believed to effectively reduce the flame point of the oxidised oil. Based on the kinetic analysis, under HTO conditions, deposited coke caused the reaction to be more exothermic, activating the subsequent deposition and forming a stable oxidation front. [Received: January 14, 2016; Accepted: September 18, 2016]

Experimental research on phase behaviour and vapour/liquid equilibrium of N&lt;sub align="right"&gt;2 and heavy oil system

In the process of N2 injection, the dissolution and diffusion of N2 into oil will change the physical properties of crude oil and enhance oil recovery. In this paper, two experimental procedures are investigated to measure phase behaviour and equilibrium features of crude oil and injected N2. The dynamic miscibility experiment was conducted to obtain N2-oil phase behaviour and physical property change data, while the static miscibility experiment was conducted to simulate the vapour/liquid equilibrium process features. The result of dynamic miscibility experiment showed that it is difficult to achieve miscible displacement for N2 injection due to the low solubility and low extraction capability of N2; however, the solution of N2 can reduce oil viscosity and IFT, and the high compressibility coefficient of N2 can implement the displacement energy to obtain higher oil producing rate. The static miscibility result indicated that N2 dissolved in crude oil through a contacting interface and then diffused in deeper part of oil, contacting time and equilibrium pressure are key influence parameters to vapour/liquid equilibrium features.

The properties and kinetics characteristics of oxidised ultra-heavy oil in the coke deposition process

High pressure air injection (HPAI) is generally considered an efficient method to drive crude oil. In this paper, the results showed that the oxidation of heavy oil was able to reduce the oxygen content. After the high pressure oxidation, the physical properties of crude oil changed, and it became inflammable. Due to the formation of coke, the viscosity increased sharply and the hydrogen/carbon (H/C) atomic ratio decreased. By using an environmental scanning electron microscope, it was observed that the structures of the original oil were reconstructed and the surface was covered by a large number of inflammable globular structure with a diameter of 500 nm that were believed to effectively reduce the flame point of the oxidised oil. Based on the kinetic analysis, under HTO conditions, deposited coke caused the reaction to be more exothermic, activating the subsequent deposition and forming a stable oxidation front. [Received: January 14, 2016; Accepted: September 18, 2016]

Applying SVM framework for modeling of CO2 solubility in oil during CO2 flooding

CO2 solubility is one of the most important parameters that affects CO2 flooding, because gas dissolution into crude oil results in oil swelling, viscosity reduction, IFT reduction, oil mobilization, and oil recovery improvement. Therefore, a better understanding of CO2 solubility mechanisms and its influence on physical properties of crude oil are essential to any effective CO2 flooding process. In this study, Least-Square Support Vector Machine (LSSVM) as a newly established soft computing algorithm is applied for developing a new correlative model for CO2 solubility in both dead and live oil systems. CO2 solubility in dead oil is basically affected by the oil saturation pressure (Ps), oil specific gravity (γ), oil molecular weight (MW), and reservoir temperature (T). Moreover, the impact of bubble point pressure is considered in constructing the LSSVM model for the live oil. A number of statistical quality measures are utilized to assess and demonstrate the superior capability of the newly developed LSSVM model in comparison with the previous empirically derived correlations. The average absolute relative deviation (AARD) and coefficient of determination (R2) of 2.2783% and 0.9933 for the dead oil system, and 1.7432% and 0.9958 for the live oil system, respectively, verify the acceptable accuracy and efficient performance of the proposed LSSVM model over a wide range of dataset used in this study within the range of the used databank. However, the impact of CO2 liquefaction pressure is ignored, the LSSVM model gives the best result. In conclusion, it is worth mentioning that the proposed LSSVM model can serve as an accurate correlative tool for fast and effective estimation of CO2 solubility in both dead and live crude oils.

Genesis and distribution of hydrogen sulfide in deep heavy oil of the Halahatang area in the Tarim Basin, China

As the largest oil-and-gas-bearing basin in China, the Tarim Basin contains rich oil and gas resources buried deep underground. In recent years, large oil fields have been discovered in the Halahatang area of the northern Tarim Basin. The reservoir is buried 6000–7300 m underground. This reservoir is dominated by the Ordovician carbonate rocks, and the crude oil is mainly heavy oil. As a crude oil-associated gas, the natural gas generally contains hydrogen sulfide (H2S). The heavy oil in this region is the deepest buried heavy oil found in the world. H2S is also associated with the deepest buried natural gas. The burial, preservation and degree of biodegradation of a paleo-reservoir can be used to predict the distribution of H2S. According to research findings, there is a clear planar distribution pattern of H2S content: high in the east and north, and low in the west and south. We compared the physical properties of crude oil and the analysis of the composition of natural gas and isotopes, biomarker compounds of crude oil and groundwater. We find that the content of H2S in natural gas bears some relation to the physical properties and degree of biodegradation of crude oil. Crude oil density, sulfur content, colloid, and asphaltene have positive correlations with H2S content in natural gas. The formation of H2S is controlled by the degradation and densification of crude oil. Crude oil densification can lead to an increase of the sulfur content. The rise in the temperature of the reservoir resulting from the depth of burial causes the thermal decomposition of sulfur compounds to produce H2S. The generation of H2S by the thermal decomposition of sulfur compounds is confirmed by data on sulfur isotopes. The distribution of H2S can then be predicted based on the burial conditions of the paleo-reservoir and the degree of biodegradation. In the south Rewapu of the Halahatang area, the thick cap rock of the Ordovician oil reservoir was preserved well in the late Hercynian Period, without undergoing biodegradation. The oil is mainly normal oil and light oil. Sulfur content in the crude oil is quite low, making it impossible to generate a large amount of H2S in the later stages of deep burial.

Computational Fluid Dynamics Simulation of a Crude Oil Transport Pipeline: Effect of Crude Oil Properties

Transporting crude oil inside a pipeline is a common process in the petroleum industry. Crude oil from different sources has different properties due to terrains and climates which cause the transport profile to change during operations. In this study, the computational fluid dynamics model was developed using computer language code. The governing equations were employed to study the effect of crude oil properties such as crude oil density and viscosity on the transport profile using the 24 factorial experimental design. A good agreement between the developed numerical model and commercial software suggests that the proposed numerical scheme is suitable for simulating the transport profile of crude oil in a pipeline and predicting the phenomena affecting conditions. The result showed that the heat capacity and density of crude oil had statistical significance to the transport profile at 95% confidence level. The heat capacity had the most significant effect on wax appearance. Therefore, the physical properties of crude oil should be modified to prevent the occurrence of wax inside the crude oil pipeline.

邦戈尔盆地北部斜坡油气成藏期次研究

利用原油物性、色谱–质谱及包裹体等资料，通过地球化学、流体包裹体等方法，结合主生烃期和圈闭形成，对乍得邦戈尔盆地北部斜坡油藏成藏期次及时间进行了分析。结果显示，原油样品中具有正构烷烃系列与25-降藿烷系列同时并存及正常原油与降解原油的倒置分布等特征，表明研究区油气经历多期次的成藏。包裹体镜下鉴定可见烃类包裹体和盐水溶液包裹体特征整体相似，表明原油组分及成熟度相近。成藏综合分析表明，邦戈尔盆地北部斜坡存在一个主要的成藏期为晚白垩世早期，而晚白垩世晚期和古近纪晚期的2次构造反转运动期为研究区2个重要的油气成藏调整期。 Based on the analysis data of physical properties of crude oil, GC-MS, fluid inclusion and so on, by using the organic geochemistry method, fluid inclusion analysis method, combined with the main hydrocarbon generation period method and trap forming method, the period of the oil and gas accumulation on the northern slope of Bongor Basin was researched. The results show that there are probably more periods of oil and gas accumulation in the study area according to the characteristics of the coexistence of n-alkanes series and 25-norhopance series, the inversion distribution of normal oils and degraded oils, and so on. The result of microscopic identification of the fluid inclusion indicates that the characteristics of hydrocarbon inclusions and saline inclusions are similar. Comprehensive analysis of the oil and gas accumulation shows that the early period of late Cretaceous is the main period of oil and gas accumulation on the northern slope of Bongor Basin, while the late period of late Cretaceous and late Paleogene are the 2 important adjustment periods of hydrocarbon accumulation in the 2 times of tectonic inversion movement.

Heavy oil viscosity and density prediction at normal and elevated temperatures

Viscosity and density are important physical properties of crude oil. However, no practical theory exists for the calculation of these properties for heavy oil at elevated temperatures. The principal objective of this paper is to obtain exact models that can successfully predict these two important fluid properties covering a wide range of temperatures. In this study, heavy oil density was predicted from API and temperature, and then the predicted values of the densities were used in the second step to develop the viscosity correlation. A total of 30 heavy oil samples of different API gravities ranging from 11.7 to 18.8 were tested. Viscosity and density were measured in the temperature range from 20 to 160 °C. The accuracy of the experimental density data was determined using Standing and Katz method. Published correlations were also used to evaluate the experimental viscosity data. The comparison between the experimental data and the predicted values indicated that the proposed model successfully predicted the experimental data with an average absolute relative error of less than 8 % and correlation coefficients (R 2) of 0.97 and 0.92 at normal and high temperatures, respectively. The proposed model and the literature models were tested on heavy oil samples. It was found that it is not possible to generalize a correlation for the heavy oil viscosity using only API and temperature. However, the proposed model significantly minimizes the relative error and increases the correlation between the predicted and experimental data compared with other published methods.

The impact of severe biodegradation on the molecular and stable (C, H, N, S) isotopic compositions of oils in the Alberta Basin, Canada

Although the effects of biodegradation on the composition and physical properties of crude oil have been well studied, effects of in-reservoir petroleum biodegradation on molecular and isotopic compositions of crude oils are not yet clearly understood. The Alberta Basin, in western Canada, is one of the world’s largest petroleum accumulations and constitutes an ideal example of a natural suite of sequentially biodegraded oils. The basin hosts moderately to severely biodegraded petroleum, regionally distributed and in single, more or less continuous, oil columns. In this study, a series of oil samples from the Alberta heavy oil and oil sands provinces, with varying degrees of biodegradation, were analyzed to assess the impact of progressive biodegradation on the molecular and C, H, N, and S isotopic compositions of oils. The results of the molecular characterization of the hydrocarbon fraction of the studied oils show that the oils have suffered biodegradation levels from 2 to 10+ (toward the Alberta–Saskatchewan border) on the Peters and Moldowan scale of biodegradation (abbreviated PM 2 to PM 10) and from tens to hundreds on the Manco scale. Within single reservoirs, increasing biodegradation was observed from top to bottom of the oil columns at all sites studied. The whole oil stable isotopic compositions of the samples varied in the ranges δ13C=−31.2‰ to −29.0‰, δ2H=−147‰ to −133‰, δ15N=0.3–4.7‰ and δ34S=0.4–6.4‰. The maximum differences between δ values of samples (Δ) within single oil columns were Δ13C=1.4‰, Δ2H=7‰, Δ15N=1.7‰ and Δ34S=1.0‰. Regional variations in the isotopic compositions of oil samples from different wells (averaged values from top to bottom) were 1.2‰ for δ13C, 12‰ for δ2H, 4.1‰ for δ15N and 5.5‰ for δ34S and hence generally significantly larger variations were seen than variations observed within single oil columns, especially for N and S. It appears that even severe levels of biodegradation do not cause observable systematic variations in carbon, nitrogen or sulfur isotope composition of whole oils. This indicates that sulfur and nitrogen isotopic compositions may be used in very degraded oils as indicators for oil charge from different source rock facies.

Multivariate Analysis of Crude Oil Composition and Fluid Properties Used in Multiphase Flow Metering (MFM)

Crude oil characterization by infrared (IR) spectroscopy and whole oil gas chromatography (GC) has been used to provide data for establishing multivariate prediction models for physical properties of crude oils. The parameters of interest are used in multiphase flowmeters (MFMs) for monitoring production and transport of petroleum fluids, and permittivity parameters are of special interest. Data for 20 crude oils and condensates has been acquired and modeled using partial least squares (PLS) modeling. Good quality predictions were obtained for density, velocity of sound, and static and high frequency permittivity. Biodegradation of crude oil is the main cause of variation in the modeled variables. The data required are obtained in standard analytical procedures, and thus the approach has a considerable potential for use in on-site quality assurance.

Correlation of FT-ICR Mass Spectra with the Chemical and Physical Properties of Associated Crude Oils

In this study, the peaks observed using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were correlated with properties of crude oils. The correlations were statistically analyzed and graphically presented using Circos diagrams. Numerous peaks with statistical significance (p < 0.05) correlated strongly with elemental sulfur, nitrogen, nickel, and vanadium contents. In addition, a number of peaks correlated with properties such as acidity, gravity, and weight percent of residue after atmospheric residue distillation of crude oils. The correlation agreed well with generally accepted ideas, thereby validating this approach. For example, sulfur-containing classes such as S1, S2, and NS correlated positively with sulfur content. Positive correlation denotes that the relative abundance of the peaks containing S1, S2, and NS heteroatoms increased as bulk concentrations of sulfur in the samples increased. The O2 and O4 classes of compounds, presumably with COOH functional groups, had a strong correlation with total acid number. Subsequent analyses showed some correlations had carbon number and double-bond equivalence dependence. This study clearly shows the correlation between the chemical compositions determined using FT-ICR MS and the chemical and physical properties of crude oils.

Steam Flooding of Naturally Fractured Reservoirs: Basic Concepts and Recovery Mechanisms

Abstract A review of important issues in steam injection in naturally fractured reservoirs (NFRs) is presented. The effect of temperature on physical properties of crude oils and rocks and the thermo-chemical alteration of crude oil are discussed. The recovery of oil from NFRs can be modelled as a two step process: first the oil is expelled from the matrix blocks through mechanisms that impose a pressure gradient within each matrix block and then it is swept through the fracture network to a production well by mechanisms that impose a pressure gradient within the fracture network. The recovery mechanisms associated with steam injection in NFRs and their characteristic times are presented. The most important recovery mechanism in matrix blocks is differential thermal expansion between oil and the matrix pore volume and the strongest mechanism in fracture network is the reduction of viscosity ratio (µo/µw). The matrix oil recovery mechanisms are relatively independent of oil gravity, making steam an equally attractive recovery process in fractured light and heavy oil reservoirs. The mechanism and impact of CO2 generation during steam injection in carbonate reservoirs are discussed. The rate of CO2 generation is controlled by the rate of heat conduction from fracture into the matrix. For a specific reservoir the rate of heat conduction is a function of temperature and injection rate of steam and these can be optimized to make use of the in situ generated CO2. Introduction Heavy oil in naturally fractured carbonate reservoirs is an important resource, which accounts for one-third of total heavy oil worldwide. Many fractured reservoirs in the Middle East, former Soviet Union and Canada are candidates for thermal heavy oil recovery. Steam injection processes, which have been used extensively to recover heavy oil from non-fractured reservoirs, were not applied to fractured reservoirs until recently. This was primarily based on the belief that the injected steam would bypass the oil through the fractures and would be ineffective in recovering the oil. However, the results of experimental, theoretical and pilot tests which have appeared in the literature since early 1980s, show the feasibility of heavy oil recovery from fractured reservoirs using steam injection.