Oil-water Transition Zone Research Articles

Study of electrical resistivity vertical changes in carbonate oil-bearing reservoirs to determine the depth of free water level

Abstract The purpose of the study is to establish the depth of the free water level (FWL), the zone of oxidized oil, the oil-water transition zone, the zone of maximum and decreasing resistivity. Correct understanding of initial oil saturation zonation in carbonate oil-bearing reservoirs is very important for development planning. The article proposes a method for express assessment of the FWL depth and initial oil saturation zonation in carbonate oil-bearing reservoirs based on electrical resistivity study. The values of electrical resistivity determined from induction logging curves in more than 200 wells were used as initial data. The vertical change in the electrical properties is considered using the resistivity values of oil-bearing reservoirs averaged over all wells. The initial oil saturation zonation was analysed using the graphs of electrical resistivity against depth. As a result of the work done, the following conclusions were made regarding the zonation of initial oil saturation and the possible geological characteristics of carbonate oil-bearing reservoirs: 1) based on the graphs of electrical resistivity against depth it is possible to distinguish 3 zones of initial oil saturation: oil-water transition zone above FWL, zone of maximum resistivity in the middle part of the deposit, and zone of decreasing resistivity in the upper part of the deposit; 2) in general, the initial oil saturation in the reservoir (as a function of the electrical resistivity) upward from the initial FWL depth does not grow exponentially, as in the Leverett function, but linearly in each zone. Proposed express method for initial oil saturation zonation in carbonate reservoirs provides novel information for formation testing and perforation planning. In oil-water transition zone it is recommended to use the gentlest methods of perforation and production intensification to preserve the integrity of the oxidized oil zone, since this zone is water confining and can prevent premature water flooding of the product.

3D-MODELING TECHNOLOGY OF INITIAL OIL SATURATION IN TRANSITIONAL OIL-WATER ZONE BY COMPLEX OF CAPILLARIMETRY AND ELECTRIC LOGGING METHODS

Link for citation: Potekhin D.V., Shiryaev E.O., Galkin S.V. 3D-modeling technology of initial oil saturation in transitional oil-water zone by complex of capillarimetry and electric logging methods. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 10, рр. 98-107. In Rus. The relevance. The 3D distribution of the initial oil saturation in the reservoir volume is one of the key elements of geological and technological modeling, largely determining the quality of subsequent development design solutions. The publication presents an analysis of the current state of the problem of studying the oil-water transition zone and the distribution of oil saturation. For terrigenous hydrophilic reservoirs, electric lateral logging quite reliably controls the fluid saturation of the void space of the oil part of the deposit, which suggests the possibility of its integration with widely used capillarimetry methods. Aim: to evaluate the possibility of reliably determining the oil saturation of terrigenous hydrophilic reservoirs by combining the data of electrical lateral logging, filtration-capacitance properties of reservoirs and capillarimetric core studies, to build a 3D model of the distribution of the initial oil saturation of reservoirs in oil deposits, taking into account the allocation of the oil-water transition zone. Object: oil-saturated reservoirs of the Visean oil deposits of the Perm region. Method: methods of multidimensional mathematical modeling in the development of a methodology for determining the initial oil saturation of reservoirs based on a complex of well logging and capillarimetric studies of the core, construction of 3D distribution of the initial oil saturation of the reservoir, taking into account the parameters of the oil-water transition zone. Results. The authors, based on the developed methodology for the deposit of the reservoir Bb of the Aspinskoe field, built a 3D model of the distribution of the initial oil saturation. For the studied reservoir, they carried out the analysis of distribution of zones of free saturation, undersaturation, transition zone and limiting oil saturation. The resulting 3D model can be used in geological and technological modeling of the development of an oil deposit.

Progress and prospect of carbon dioxide capture, utilization and storage in CNPC oilfields

The development history of carbon capture, utilization and storage for enhanced oil recovery (CCUS-EOR) in China is comprehensively reviewed, which consists of three stages: research and exploration, field test and industrial application. The breakthrough understanding of CO2 flooding mechanism and field practice in recent years and the corresponding supporting technical achievements of CCUS-EOR project are systematically described. The future development prospects are also pointed out. After nearly 60 years of exploration, the theory of CO2 flooding and storage suitable for continental sedimentary reservoirs in China has been innovatively developed. It is suggested that C7–C15 are also important components affecting miscibility of CO2 and crude oil. The mechanism of rapid recovery of formation energy by CO2 and significant improvement of block productivity and recovery factor has been verified in field tests. The CCUS-EOR reservoir engineering design technology for continental sedimentary reservoir is established. The technology of reservoir engineering parameter design and well spacing optimization has been developed, which focuses on maintaining miscibility to improve oil displacement efficiency and uniform displacement to improve sweep efficiency. The technology of CO2 capture, injection and production process, whole-system anticorrosion, storage monitoring and other whole-process supporting technologies have been initially formed. In order to realize the efficient utilization and permanent storage of CO2, it is necessary to take the oil reservoir in the oil-water transition zone into consideration, realize the large-scale CO2 flooding and storage in the area from single reservoir to the overall structural control system. The oil reservoir in the oil-water transition zone is developed by stable gravity flooding of injecting CO2 from structural highs. The research on the storage technology such as the conversion of residual oil and CO2 into methane needs to be carried out.

Erratum: High temperature susceptibility measurements: A potential tool for the identification of oil water transition zone in petroleum reservoirs

An erratum on: Abdulkarim MA, Muxworthy AR and Fraser A (2022) High temperature susceptibility measurements: A potential tool for the identification of oil-water transition zone in petroleum reservoirs. Front. Earth Sci. 10:973385. doi: 10.3389/feart.2022.973385 Due to a production error, an incorrect version of Figure 5 was included in the published article. The correct version of Figure 5 appears below. The publisher apologizes for this mistake. FIGURE 5. Example results from the HT-χ measurements carried out on the Bonneville samples. The magnetic minerology of the ‘main’ oil leg is dominated by siderite. The oil-water region presents a diminished siderite signature and the presence of hexagonal pyrrhotite. Note that the oil-water contact (OWC) marked is as obtained via well logs and pressure analysis. The original version of this article has been updated.

High temperature susceptibility measurements: A potential tool for the identification of oil-water transition zone in petroleum reservoirs

Determining the position and thickness of the oil-water transition zone (OWTZ) in hydrocarbon reservoirs is important to reserve estimation and production planning. This paper describes a magnetic method of identifying this zone. High temperature susceptibility (HT-χ) measurements on core samples from Paleogene reservoirs of the UK Central North Sea revealed distinct signatures around the oil water interface. Rapid increases in susceptibilities at temperatures <250°C were observed for samples around the oil water interface unlike the main oil leg where alteration involving increase in susceptibility occurred at significantly slower rates and higher temperatures. The HT-χ data together with Mössbauer measurements revealed that the variation in alteration characteristics is due to the increasing concentration of hexagonal pyrrhotite and/or lepidocrocite around the oil water interface. Hexagonal pyrrhotite was identified in reservoirs existing at temperatures of <80°C, while lepidocrocite dominated the signature around the contact of deeper reservoirs. These observations suggest that the precipitation of hexagonal pyrrhotite is related to OWTZ centred biogenic activities i.e., biodegradation. The dominance of lepidocrocite in deeper diagenetic settings has been related to hydrolysis of hydrocarbon at the oil water interface, together with cessation of biogenic activities.

Effect of Hydrocarbon Presence and Properties on the Magnetic Signature of the Reservoir Sediments of the Catcher Area Development Region, UK North Sea

This paper presents a detailed study investigating the effect of hydrocarbon presence on magnetic mineral diagenesis in sediments from the Catcher Area Development (CAD) region, UK North Sea, between 1,000 and 1,500 m (True Vertical Depth Sub-Sea). Magnetic analysis of core samples from hydrocarbon fields of the region and nearby dry-well sandstones (background) was carried out to determine if their signatures can serve as a proxy for understanding petroleum reservoir systems. From the background samples, nanometric and micron-sized magnetite, hematite and titano-iron oxides, were identified. Hydrocarbon presence in the reservoir sediments was found to diminish the iron-oxide signature and favour the precipitation of hexagonal pyrrhotite, siderite and potentially vivianite, lepidocrocite, greigite and paramagnetic iron sulphides. Hexagonal pyrrhotite was found at the oil-water transition zones. This relationship is possibly related to biodegradation at this interface. Siderite was found in increased abundance at shallower depths within the reservoir, which we attribute to hydrocarbon vertical migration and biodegradation. The interbedded shales also experienced significant magnetic mineral diagenesis that depended on its proximity to the hydrocarbon plume. These findings suggest that mineral magnetism can be applied to the identification of oil-water transition zones, reserve estimation, production planning and the determination of hydrocarbon migration pathways. It also suggests that mineral magnetic methods can be used to estimate the timing of hydrocarbon migration.

Experimental investigation on plugging performance of nanospheres in low-permeability reservoir with bottom water

The oil production rate decreases rapidly after a short period of high yield from acidizing or fracturing in low-permeability reservoirs. In this paper, nanospheres are applied before the fracturing step, which possess the ability to absorb water and expand in the water layer, reducing the flow capacity of bottom water and finally enhancing the oil recovery. The plugging performance is investigated by nanosphere displacement experiments in cores and sand-packs, which explores the plugging effect in the oil layer, the oil-water transition zones, the water layer and the fracturing zones. In addition, a nuclear magnetic resonance experiment is conducted to study the flow mechanism of nanospheres and determine the plugging rates, which can characterize the plugging performance of nanospheres in porous media. The results show that the plugging rate is 85.84% and 78.65% on the water layer and oil-water transition zone, respectively, and 94.36% in the fracturing zone. Meanwhile, the nanospheres cannot plug the oil layer. The formation pressure has a less considerable effect on the plugging performance of nanospheres. The nanospheres have good injectivity, and the intensity variations in small, medium and large pores account for 34.46%, 13.22% and 52.32%, respectively. Overall, this paper explores the feasibility of applying nanospheres for water plugging and enhanced oil recovery. Cited as: Tang, M., Wang, C., Deng, X., Yang, H., Lu, J., Yu, H. Experimental investigation on plugging performance of nanospheres in low-permeability reservoir with bottom water. Advances in Geo-Energy Research, 2022, 6(2): 95-103. https://doi.org/10.46690/ager.2022.02.02

Study on oil-water interface of Budate fracture reservoir in each block of Suderte Oilfield

It is well known that in addition to bound water in the reservoir, formation water mainly exists in the form of edge water and bottom water. Fractured buried hill metamorphic rock reservoirs are generally bottom-water reservoirs. In fractured reservoirs, rock blocks are cut by fractures and oil and water are distributed in fractures. Due to the low capillary pressure of fractures, the oil-water transition zone is also small. In the same fault block, there is relatively uniform oil-water interface. And oil-water interface to judge correctly or not will directly related to the determination of oil-bearing area and effective thickness division and well placement, thus correctly determine the position of the oil-water interface of oil field development is of great significance, through the variation of oil-water interface also presumably or some guidance on the seismic section can't precisely divide the small fault.

Densely Populated Water Droplets in Heavy-Oil Seeps.

Most of the microbial degradation in oil reservoirs is believed to take place at the oil-water transition zone (OWTZ). However, a recent study indicates that there is microbial life enclosed in microliter-sized water droplets dispersed in heavy oil of Pitch Lake in Trinidad and Tobago. This life in oil suggests that microbial degradation of oil also takes place in water pockets in the oil-bearing rock of an oil leg independent of the OWTZ. However, it is unknown whether microbial life in water droplets dispersed in oil is a generic property of oil reservoirs rather than an exotic exception. Hence, we took samples from three heavy-oil seeps, Pitch Lake (Trinidad and Tobago), the La Brea Tar Pits (California, USA), and an oil seep on the McKittrick oil field (California, USA). All three tested oil seeps contained dispersed water droplets. Larger droplets between 1 and 10 μl revealed high cell densities of up to 109 cells ml-1 Testing for ATP content and LIVE/DEAD staining showed that these populations consist of active and viable microbial cells with an average of 60% membrane-intact cells and ATP concentrations comparable to those of other subsurface ecosystems. Microbial community analyses based on 16S rRNA gene amplicon sequencing revealed the presence of known anaerobic oil-degrading microorganisms. Surprisingly, the community analyses showed similarities between all three oil seeps, revealing common OTUs, although the sampling sites were thousands of kilometers apart. Our results indicate that small water inclusions are densely populated microhabitats in heavy oil and possibly a generic trait of degraded-oil reservoirs.IMPORTANCE Our results confirmed that small water droplets in oil are densely populated microhabitats containing active microbial communities. Since these microhabitats occurred in three tested oil seeps which are located thousands of kilometers away from each other, such populated water droplets might be a generic trait of biodegraded oil reservoirs and might be involved in the overall oil degradation process. Microbial degradation might thus also take place in water pockets in the oil-bearing oil legs of the reservoir rock rather than only at the oil-water transition zone.

Articles of Significant Interest in This Issue

Most of the microbial degradation in oil reservoirs takes place at the oil-water transition zone between the oil leg and the underlying water. However, Pannekens et al. (e00164-20) proved that microliter-sized water droplets dispersed in biodegraded heavy-oil seeps act as densely populated microhabitats, containing actively living microbial communities. Since these microhabitats occurred in all three tested oil seeps, which are located thousands of kilometers from each other, such populated water droplets might be a generic trait of biodegraded oil reservoirs and possibly be involved in the overall oil degradation process.

Borehole resistivity simulations of oil-water transition zones with a 1.5D numerical solver

When simulating borehole resistivity measurements in a reservoir, it is common to consider an oil-water contact (OWC) planar interface. However, this consideration can lead to an unrealistic model since in the presence of capillary actions, the mix of two immiscible fluids (oil and water) often appears as an oil-water transition (OWT) zone. These transition zones may be significant in the vertical direction (20 m or above), and in context of geosteering, an efficient method to simulate the OWT zone can maximize the production of an oil reservoir. Herein, we propose an efficient one and a half-dimensional (1.5D) numerical solver to accurately simulate the OWT zone in an oil reservoir. Using this method, we can easily consider arbitrary resistivity distributions in the vertical direction, as it occurs in an OWT zone. Numerical results on synthetic examples demonstrate significant differences between the results recorded by a geosteering device when considering a realistic OWT zone vs an OWC sharp interface.

Coupled mineral alteration and oil degradation in thermal oil-water-feldspar systems and implications for organic-inorganic interactions in hydrocarbon reservoirs

Abstract Organic-inorganic interactions after oil charging are critical for determining the ongoing evolution of hydrocarbons and rock quality in water-wet siliciclastic reservoirs. It is the conceptual approach of this study to simulate and decipher these interactions by using quantitative analyses of the interrelated changes of minerals, water, hydrocarbons, gases, and organic acids in heated oil-water-rock systems. The experimental results show that organic-inorganic interactions occur between the organic oil and inorganic feldspar in the presence of water. Water promotes the oil degradation by an extra supply of H+ and OH− ions. In the oil-water-rock systems, mutual exchanges of H+ and OH− ions among minerals, water, and hydrocarbons probably result in the mutual interactions between oil degradation and mineral alteration, with water serving as a matrix for the ion exchange. In the oil-water-feldspar system, feldspar alteration does not cease in the oil zone in the presence of some water and, inversely, oil degradation can even accelerate the alterations of the aluminosilicate minerals. The mineral alterations from feldspar to boehmite, illite, and muscovite promote the oil degradation in the oil-deionized water-feldspar systems with the mutual ion exchange. Due to a possible competition between the Bronsted acid sites in the minerals and the halide anions or the direct replacement of K-feldspar by albite, the mineral alterations retard the oil degradation in the oil-NaCl water-feldspar systems. The experimental results also indicate that oil degradation via free radical cross-linking and free radical thermal cracking is extensive in anhydrous oil systems and may lead to blocking of pores and throats by the generated pyrobitumen in the oil zone of a reservoir. In contrast, in the oil-water transition zone of a reservoir, oil degradation via thermal cracking and oxidative decomposition may dominate the oil degradation and may lead to the generation of secondary pores with leaching of minerals by the generated CO2 and organic acids. This study demonstrates that organic-inorganic interactions are complex and proceed in diverse pathways in different hydrogeochemical systems. Thus, further quantitative investigations of the reaction pathways and reaction kinetics of coupled mineral alteration and oil degradation in oil-water-rock systems are warrented. Meanwhile, the incorporation of such organic-inorganic reactions into geochemical models will improve the prediction of the evolution of organic and inorganic species in petroleum reservoirs.

Oil reservoirs, an exceptional habitat for microorganisms

Microorganisms are present in oil reservoirs around the world where they degrade oil and lead to changes in oil quality. Unfortunately, our knowledge about processes in deep oil reservoirs is limited due to the lack of undisturbed samples. In this review, we discuss the distribution of microorganisms at the oil-water transition zone as well as in water saturated parts of the oil leg and their possible physiological adaptations to abiotic and biotic ecological factors such as temperature, salinity and viruses. We show the importance of studying the water phase within the oil, because small water inclusions and pockets within the oil leg provide an exceptional habitat for microorganisms within a natural oil reservoir and concurrently enlarge the zone of oil biodegradation. Environmental factors such as temperature and salinity control oil biodegradation. Temperature determines the type of microorganisms which are able to inhabit the reservoir. Proteobacteria and Euryarchaeota, are ubiquitous in oil reservoirs over all temperature ranges, whereas some others are tied to specific temperatures. It is proposed that biofilm formation is the dominant way of life within oil reservoirs, enhancing nutrient uptake, syntrophic interactions and protection against environmental stress. Literature shows that viruses are abundant in oil reservoirs and the possible impact on microbial community composition due to control of microbial activity and function is discussed.

Laboratory simulation and CO2 flooding efficiency of oil-water transition zones in a low permeability reservoir in the Jilin Oilfield

Geological reserves of oil-water transition zone (OWTZ) in low-permeability reservoirs have been considered as uneconomical resources because of high water cut and low abundance. Though the OWTZ may account for 30 %–50% of a reservoir, it has not been paid more attentions yet. The average oil saturation of the OWTZ is about 35%, which is equal to that of a reservoir after water flooding. Currently, CO2 flooding is an effective technique for residual oil recovery after water flooding, which could reduce the residual oil saturation greatly. Therefore, it is of significance for the EOR of the low-permeability reservoir if the CO2 flooding could be successfully applied in the OWTZ. In this study, a method based on a long core to simulate distribution of oil saturation in OWTZs is set up in the laboratory using bidirectional saturation. In order to investigate CO2 flooding characteristics in OWTZs, experiments are carried out on 3 sets of initial oil saturation, and the recoverable reserves of the OWTZ and its contribution to the EOR are calculated based on the WJ reservoir in the Jilin Oilfield, China.

Microfacies, diagenesis and oil emplacement of the Upper Jurassic Arab-D carbonate reservoir in an oil field in central Saudi Arabia (Khurais Complex)

The Upper Jurassic (Kimmeridgian) Arab-D reservoir of the Arab Formation in the Arabian Peninsula is considered to be the largest hydrocarbon carbonate reservoir in the world. Despite a relative large number of studies dealing with lithofacies and depositional sequences, there are still few studies dealing with diagenesis of this supergiant reservoir. In this study, petrographic, cathodoluminescence and ultra-violet light fluorescence microscopy analyses of the Arab-D reservoir in an oil field of Saudi Arabia (Khurais Complex) have been combined with isotope geochemistry (δ13C, δ18O and 87Sr/86Sr) to decipher the sequence of diagenetic events affecting the reservoir before and during oil emplacements, and the impact of depositional mineralogies and their modifications during the diagenesis on the reservoir quality and heterogeneity. All the analyzed samples correspond to lithofacies that can be ascribed to nine microfacies types (MFT-1 to MFT-9). Three stages of carbonate dissolution (d1 to d3), four stages of calcite cements (C1 to C4) and 5 zones of replacive dolomite and dolomite cement (D1 to D5) have been documented from diagenetic environments ranging from marine to late burial. Abundant oil fluid inclusions have been recognized in calcite C3 and C4 and dolomite D5 from wells located in the flanks of the structure (Group 2 of cements). Cements from wells located in the center of the structure are largely oil-inclusions free (Group 1 of cements). This distribution suggests that Group 2 of cements formed within the oil-water transition zone of the reservoir during the oil infilling of the structure, allowing to constrain the timing of oil emplacement and suggesting at least two phases of oil charging. Porosity enhancement by corrosive dissolution (d3) occurred during late burial and took place just before precipitation of dolomite D5 and calcite C4, and just before the second stage of hydrocarbon arrival, suggesting the presence of aggressive fluids preceding the late phase of hydrocarbon migration. Cross-plot of carbon vs. oxygen stable isotopes shows a general trend toward decreasing δ18O values with increasing burial depth, whereas all δ13C values remain positive except for Group 2 of C3 and C4 calcites, which exhibit negative (up to −4.6‰) δ13C values. The later suggest precipitation from pore waters with increased volumes of 12C as a by-product of hydrocarbon maturation. Except for C4, all the 87Sr/86Sr values are within the range of the Late Jurassic seawater, suggesting precipitation from Jurassic marine or evolved marine pore waters. In contrast, the more radiogenic calcite C4 suggests a source from fluids that interacted with siliciclastics or clay-rich sequences. Depositional mineralogy of the grainy lithofacies is interpreted to have had an impact on the effectiveness of cementation and dissolution, controlling the reservoir heterogeneity. Due to excess of cementation, reservoir quality is worse in grainstone microfacies rich in aragonitic components and better in grainstone and packstone microfacies poorer in aragonitic grains but affected by late dissolution of micrite. The integration of all these data resulted in a more precise understanding of the reservoir quality and heterogeneity.

Diagenesis of the palaeo-oil-water transition zone in a Lower Pennsylvanian carbonate reservoir: Constraints from cathodoluminescence microscopy, microthermometry, and isotope geochemistry

Oil-water transition zones in carbonate reservoirs represent important but rarely studied diagenetic environments that are now increasingly re-evaluated because of their potentially large effects on reservoir economics. Here, data from cathodoluminescence and fluorescence microscopy, isotope geochemistry, microthermometry, and X-ray tomography are combined to decipher the diagenetic history of a 5-m-long core interval comprising the oil-water transition zone in a Lower Pennsylvanian carbonate reservoir. The aim is to document the cementation dynamics prior, during, and after oil emplacement in its context of changing fluid parameters. Intergrain porosity mean values of 7% are present in the upper two sub-zones of the oil-water transitions zone but values sharply increase to a mean of 14% in the lower sub-zone grading into the water-saturated portions of the reservoir and a very similar pattern is observed for permeability values. In the top of the water-filled zone, cavernous porosity with mean values of about 24% is found. Carbonate cements formed from the earliest marine to the late burial stage. Five calcite (Ca-1 through 5) and one dolomite (Dol) phase are recognized with phase Ca-4b recording the onset of hydrocarbon migration. Carbon and oxygen cross-plots clearly delineate different paragenetic phases with Ca-4 representing the most depleted δ13C ratios with mean values of about −21‰. During the main phase of oil emplacement, arguably triggered by far-field Alpine tectonics, carbonate cementation was slowed down and eventually ceased in the presence of hydrocarbons and corrosive fluids with temperatures of 110–140 °C and a micro-hiatal surface formed in the paragenetic sequence. These observations support the “oil-inhibits-diagenesis” model. The presence an earlier corrosion surface between phase Ca-3 and 4 is best assigned to initial pulses of ascending corrosive fluids in advance of hydrocarbons. The short-lived nature of the oil migration event found here is rather uncommon when compared to other carbonate reservoirs. The study is relevant as it clearly documents the strengths of a combined petrographic and geochemical study in order to document the timing of oil migration in carbonate reservoirs and its related cementation dynamics.

Life in the slow lane; biogeochemistry of biodegraded petroleum containing reservoirs and implications for energy recovery and carbon management.

Our understanding of the processes underlying the formation of heavy oil has been transformed in the last decade. The process was once thought to be driven by oxygen delivered to deep petroleum reservoirs by meteoric water. This paradigm has been replaced by a view that the process is anaerobic and frequently associated with methanogenic hydrocarbon degradation. The thermal history of a reservoir exerts a fundamental control on the occurrence of biodegraded petroleum, and microbial activity is focused at the base of the oil column in the oil water transition zone, that represents a hotspot in the petroleum reservoir biome. Here we present a synthesis of new and existing microbiological, geochemical, and biogeochemical data that expands our view of the processes that regulate deep life in petroleum reservoir ecosystems and highlights interactions of a range of biotic and abiotic factors that determine whether petroleum is likely to be biodegraded in situ, with important consequences for oil exploration and production. Specifically we propose that the salinity of reservoir formation waters exerts a key control on the occurrence of biodegraded heavy oil reservoirs and introduce the concept of palaeopickling. We also evaluate the interaction between temperature and salinity to explain the occurrence of non-degraded oil in reservoirs where the temperature has not reached the 80–90°C required for palaeopasteurization. In addition we evaluate several hypotheses that might explain the occurrence of organisms conventionally considered to be aerobic, in nominally anoxic petroleum reservoir habitats. Finally we discuss the role of microbial processes for energy recovery as we make the transition from fossil fuel reliance, and how these fit within the broader socioeconomic landscape of energy futures.

Simulation Study of Steam-Flooding Mechanisms and Influence Factors in Light-Oil Reservoirs after Water-Flooding

Steam-flooding, the most successful among enhanced recovery methods, has been applied mainly to heavy-oil reservoirs. And it is still in its infancy on light-oil reservoirs at present. This paper will take the oil-water transition zone in Saertu oilfield for example, presents a comprehensive simulation study on the use of steam-flooding after water-flood in light-oil reservoirs. Some important observations are made on this new application of the process. Relative importance of key mechanisms to oil recovery is also discussed. Guidelines are developed not only for selecting reservoir candidates for steam-flooding, but also for the factors which will effect the oil recovery in steam-flooding performance.

Oil Saturation Boundary for Partial Oil and Partial Water Recognition in the Oil-Water Transition Zone

With the development of oilfield, the oil reserves in oil-water transition zone has become a significant part of comprehensive reserves gradually. Especially the partial oil layer of oil-water transition zone has potential exploitation. But how to identify partial oil layer has become a difficulty in the development planning of the oil-water transition zone. Over the years, there has been little research on the oil-water transition. The oil saturation boundaries for partial oil and partial water recognition are mainly studied in this paper. Two major approaches, theoretical calculation methods and cumulative probability curve have been applied to the study. That will provide the basis for further perforation development and dynamic adjustment. Key words: The oil-water transition zone; Partial oil layer; Oil saturation; Theoretical calculation methods; Cumulative probability curve

The controls on the composition of biodegraded oils in the deep subsurface – Part 3. The impact of microorganism distribution on petroleum geochemical gradients in biodegraded petroleum reservoirs

A combined geochemical, geological and microbiological analysis of an actively biodegrading 24.5m thick oil column in a Canadian heavy oil reservoir has been carried out. The reservoir properties associated with the cored vertical well are characterised by a 15.75m thick oil column and an 8.75m zone of steadily decreasing oil saturation below the oil column, referred to as the oil–water transition zone (OWTZ), grading down into a thin water leg. The oil column exhibits systematic gradients in oil physical properties and hydrocarbon composition and shows variations in biodegradation level throughout the reservoir consistent with the notion that the biodegradation of oil is focussed in a bioreactor zone at the base of the oil column. Through the oil column, the dead oil viscosity measured at 20°C ranged from 50,000cP (0.05McP) at the top of the oil column to 1.4McP at the oil–OWTZ contact, and continued to increase to 10.5McP within the OWTZ. The saturated and aromatic hydrocarbons are characterised by systematically decreasing bulk fraction and component concentrations down through the oil column. Different compound classes decreased to levels below their detection limit at different depths within the OWTZ, defining a likely bioreactor extent of over 5m in depth with, for example, n-alkanes being reduced to their detection limit concentration at the bottom of the oil column/top of the OWTZ, while branched isoprenoid alkanes were not completely degraded until well into the OWTZ.Core samples from the oil column and the lower part of the OWTZ were estimated to contain ca. 104–105 bacterial cells/g, based on qPCR of bacterial 16S rRNA genes, while samples from a narrow interval in the OWTZ immediately below the oil column contained on the order of 106–107cells/g of sediment. Interestingly, these latter numbers are typical of those observed in active deep subsurface biosphere systems with the notion that microbial activity and abundance in the deep subsurface is elevated at geochemical interfaces. The numbers of organisms are not constant throughout the OWTZ. The highest bacterial abundance and geochemical gradients of, for example, methylphenanthrene biodegradation define a zone near the oil–water contact as likely the most active in terms of biodegradation. The largest bacterial abundances in the upper part of the OWTZ are in line with the trend of bacterial abundance with depth that has emerged from extensive analysis of microbial cells in deep subsurface sediments, implying that in terms of deep biosphere cell abundance, oil reservoirs are similar to other deep subsurface microbial environments. This is puzzling, given the atypical abundance of organic carbon in petroleum reservoirs and may imply a common large scale control on microbial abundance and activity in the deep biosphere, including in oilfields.