Low-temperature Oil Reservoir Research Articles

Simultaneous detection of transcribed functional assA gene and the corresponding metabolites of linear alkanes (C4, C5, and C7) in production water of a low-temperature oil reservoir

Methanogenic hydrocarbon degradation is an important biogeochemical process in oil reservoirs; however, genomic DNA-based analysis of microorganisms and metabolite detection are not conclusive for identification of the ongoing nature of this bioprocess. In this study, a suite of analyses, involving the study of microbial community and selective gene quantification of both genomic DNA and RNA together with signature metabolites, were performed to comprehensively advance the understanding of the methanogenic biodegradation of hydrocarbons in a low-temperature oilfield. The fumarate addition products for alkanes—C4, C5, and C7-alkylsuccinates—and transcribed assA and mcrA genes were simultaneously detected in the production water sample, providing robust and convincing evidence for both the initial activation of n-alkanes and methane metabolism in this oilfield. The clone library of assA gene transcripts showed that Smithella was active and most likely responsible for the addition of fumarate to n-alkanes, whereas Methanoculleus and Methanothrix were the dominant and active methane-producers via CO2 reduction and acetoclastic pathways, respectively. Additionally, qPCR results of assA and mcrA genes and their transcribed gene copy numbers revealed a roughly similar transcriptional activity in both n-alkanes-degraders and methane producers, implying that they were the major participants in the methanogenic degradation of n-alkanes in this oilfield. To the best of our knowledge, this is the first report presenting sufficient speculation, through detection of signature intermediates, corresponding gene quantification at transcriptional levels, and microbial community analysis, of methanogenic degradation of n-alkanes in production water of an oil reservoir.

Methanogenic biodegradation of C9 to C12n-alkanes initiated by Smithella via fumarate addition mechanism

In the present study, a methanogenic alkane-degrading (a mixture of C9 to C12n-alkanes) culture enriched from production water of a low-temperature oil reservoir was established and assessed. Significant methane production was detected in the alkane-amended enrichment cultures compared with alkane-free controls over an incubation period of 1 year. At the end of the incubation, fumarate addition metabolites (C9 to C12 alkylsuccinates) and assA genes (encoding the alpha subunit of alkylsuccinate synthase) were detected only in the alkane-amended enrichment cultures. Microbial community analysis showed that putative syntrophic n-alkane degraders (Smithella) capable of initiating n-alkanes by fumarate addition mechanism were enriched in the alkane-amended enrichment cultures. In addition, both hydrogenotrophic (Methanocalculus) and acetoclastic (Methanothrix) methanogens were also observed. Our results provide further evidence that alkanes can be activated by addition to fumarate under methanogenic conditions.

Heat-Generating Expandable Foamed Gel Used for Water Plugging in Low-Temperature Oil Reservoirs

Polymers are often used for chemical water plugging. When the reservoir temperature is lower than 50 °C, the reaction between polymers and cross-linking agents is very slow, which extensively prolongs the gelation time and even leads to unsuccessful gelation. To overcome such problems, a foamed-gel system that is capable of spontaneous in situ heat generation was developed. The optimal system was identified through the orthogonal test using the gel strength, gelation time, and gel volume as indexes. The test shows that, when the ambient temperature is fixed at 30 °C and the pH value is 6.8, the system performs well. Under such circumstances, the gelation time is 40 h, the gel strength reaches the G grade, and the volumetric expansion ratio at 10 MPa exceeds 130%. Nuclear-magnetic-resonance-based T2 spectra indicate that the foamed gel injected into the rock can effectively plug large pores and, therefore, offset the heterogeneity. It is also found that the foamed gel has great capacity for volumetric expa...

A study on oxygen consumption mechanism of air-foam flooding in low-temperature oil reservoir

Air flooding/air-foam flooding recovery technology has been attracting attention in oilfield development. The key point to the application of this technology is to ensure that the oxygen injected with the air can be fully dissipated inside the reservoir to lower the explosion risk in production wells and reduce the oxidation of the pipeline. It is generally believed that the oxygen and crude oil can react more in low-temperature-oxidation (LTO) reactions above 80 °C. As a large amount of oxygen would be spontaneously consumed at this temperature, this technology is mainly used in reservoirs where the temperature is greater than 80 °C. In the fractured low-permeability reservoir of China's Yanchang oilfield, where the reservoir temperature is only approximately 30 °C, a wide range of crude oil LTO would not be triggered theoretically. However, we found that the oxygen content of the associated gas in the gas channeling wells is significantly lower than that in the air. This phenomenon suggests that, there are other factors that could consume oxygen apart from the LTO reaction. To verify this assumption, an oxygen dissipation core flooding experiment at 30 °C and 80 °C was carried out to evaluate the oxygen consumption at different temperatures. At the same time, in order to determine the main controlling factors and dissipation mechanisms of oxygen dissipation, theoretical analyses and experimental studies were carried out from the aspects of reductive mineral oxidation reaction in the rock surface and formation water, the physical absorption of the micropore and the dissolution of the reservoir fluid. The results show that at the same experimental conditions, the oxygen consumption at 30 °C is 38.14% lower than that at 80 °C. The oxygen consumption factors in the low-temperature reservoir mainly include the chemical reactions between the oxygen and reducing minerals, dissolution and retention of formation water and oil, adsorption of Van der Waals forces in the reservoir and physical electronic adsorption of the hydrogen-sulfur bonds and carbon-sulfur bonds. In Chang 6 Reservoir of Ganguyi oilfield, formation water could theoretically consume 56 mg/L (1.75 mmol/L) oxygen through dissolved Fe2+. The dissolution by formation water could absorb 11.85 mmol/L oxygen (experimental value). The dissolution by oil could absorb 69.25 mmol/L oxygen (experimental test value), and the oxygen physical adsorption rate of rock pore surface was 1.26 mmol/L. Together with the pilot test data, the results show that the LTO reaction of the stratum crude oil barely occurs in the reservoir at low temperature (30 °C). However, a large amount of oxygen dissipation could still be achieved. This study demonstrates that air-foam flooding could break the applicable limit temperature and could also be applied in low-temperature reservoirs.

Microbial Methane Production Associated with Carbon Steel Corrosion in a Nigerian Oil Field.

Microbially influenced corrosion (MIC) in oil field pipeline systems can be attributed to many different types of hydrogenotrophic microorganisms including sulfate reducers, methanogens and acetogens. Samples from a low temperature oil reservoir in Nigeria were analyzed using DNA pyrotag sequencing. The microbial community compositions of these samples revealed an abundance of anaerobic methanogenic archaea. Activity of methanogens was demonstrated by incubating samples anaerobically in a basal salts medium, in the presence of carbon steel and carbon dioxide. Methane formation was measured in all enrichments and correlated with metal weight loss. Methanogens were prominently represented in pipeline solids samples, scraped from the inside of a pipeline, comprising over 85% of all pyrosequencing reads. Methane production was only witnessed when carbon steel beads were added to these pipeline solids samples, indicating that no methane was formed as a result of degradation of the oil organics present in these samples. These results were compared to those obtained for samples taken from a low temperature oil field in Canada, which had been incubated with oil, either in the presence or in the absence of carbon steel. Again, methanogens present in these samples catalyzed methane production only when carbon steel was present. Moreover, acetate production was also found in these enrichments only in the presence of carbon steel. From these studies it appears that carbon steel, not oil organics, was the predominant electron donor for acetate production and methane formation in these low temperature oil fields, indicating that the methanogens and acetogens found may contribute significantly to MIC.

Selective inhibition of methanogenesis by sulfate in enrichment culture with production water from low-temperature oil reservoir

CO2-reducing, acetoclastic and methylotrophic methanogenesis are three main biochemical pathways for biogenic methane production in subsurface environments. In this study, methanol, formate and acetate were used as substrates amended with low-temperature oilfield production fluid to establish six anaerobic enrichment settings (three with addition of sulfate as exogenous electron acceptor, another three without), incubated at room temperature to monitor the biochemical processes involved in the biodegradation of them. Methane was analyzed in the headspace while acetate, propionate and butyrate were measured in the enrichment cultures during the incubation. Methane was produced in all microcosms regardless of whether sulfate was present or not and stoichiometric estimation indicated that accumulated methane accounted for 44%–76% of the expected in the microcosms without addition of sulfate, while in microcosms with sulfate, 11%–63% of methane was recovered. Methanosarcina and Methanomethylovorans were predominantly detected in enrichment cultures with methanol only, whereas Methanosaeta was the most encounter archaea in microcosms with sulfate and acetate addition. Members represented by Methanomassiliicoccus dominated in both settings amended with formate or methanol when sulfate was present. Our data showed that methanogenesis was selectively inhibited in the presence of sulfate. The high frequency of Methanomassiliicoccus and Methanosaeta in response to sulfate amendement yielded insights into the dynamics of the composition of potential functional microorganisms and also into the metabolic flexibility of methanogens residing in low temperature petroleum reservoirs. These results provide fundamental data on the biochemical process of methane formation, and the shift of methanogenic community through sulfate addition in low temperature oil reservoirs.

An Experimental Investigation of Foam for Gas Mobility Control in a Low-Temperature Fractured Carbonate Reservoir

This work concerns the experimental investigation of surfactant alternating CO2 injection in carbonate rocks. The core samples provided from a low-temperature fractured light oil reservoir, located in southwest Iran. The experiments were designed to observe the effect of CO2–foam injection on gas mobility and oil recovery at different surfactant concentrations. The core samples were initially saturated with synthetic/field brine, 5,000 ppm, and then flooded with live oil to reach connate water saturation at reservoir condition, 115°F and 1,700 psia. The commercial surfactant used was sodium lauryl sulfate as an anionic surfactant. The results of this work, along with field-scale simulation and/or economic considerations, could be helpful in making reliable decisions about optimum condition of foam-assisted water-alternating-gas (FAWAG) processes. Core flooding results demonstrated that macroscopic sweep efficiency increased due to foam generation inside the core. In addition, it led to an increase of between 5 and 12% in the recovery factor in comparison to a water-alternating-gas (WAG) process. Furthermore, the value of the critical micellar concentration for the surfactant–oil system was about 2,000 ppm. After primary and secondary recovery processes, there is a large amount of trapped oil in the reservoir. Extensive research has been directed toward enhancing the recovery of this oil, but limited success has been achieved. FAWAG injection appears to have more applicability to recover the trapped resources. However, little attention has been paid to experimental investigation of FAWAG processes in low-temperature oil reservoirs.

Microbial community succession in a bioreactor modeling a souring low-temperature oil reservoir subjected to nitrate injection

Injection of up-flow packed-bed bioreactors with excess volatile fatty acids and limiting concentrations of nitrate and sulfate gave complete reduction of nitrate from 0 to 5.5 cm and complete or near-complete reduction of sulfate from 3.2 to 11.5 cm along the bioreactor flow path. Most of the biomass (85%) and most of the genes for nitrate reduction (narG, 96%; napA, 99%) and for sulfate reduction (dsrB, 91%) were present near the inlet (0-5.5 cm) of the 37-cm-long bioreactor. Microbial community analysis by a combination of denaturing gradient gel electrophoresis and pyrosequencing of 16S rRNA amplicons indicated that nitrate-reducing Arcobacter and Pseudomonas species were located from 0 to 3.2 cm and throughout, respectively. Desulfobulbus species were the main sulfate reducers present and acetotrophic methanogens of the genus Methanosaeta predominated at 20-37 cm. Overall, the results indicated a succession of microbial communities along the bioreactor flow path. In the absence of nitrate, the sulfate reduction zone moved nearer to the bioreactor inlet. The sulfide concentration in the bioreactor effluent was temporarily lowered after nitrate injection was re-started. Hence, the bioreactor sulfide output could be disrupted by pulsed, not by constant nitrate injection, as demonstrated also previously in a low-temperature oil field.

A New Depletion-Performance Correlation for Gas-Condensate Reservoir Fluids

Abstract This paper presents the depletion-performance correlations developed using data from PVT studies and well-completion tests of 27 rich gas-condensate and volatile-oil reservoir fluids. The PVT behavior for each fluid was determined experimentally in the laboratory, and the depletion performance of each fluid was then calculated. Well-completion test data consisting of reservoir pressure and temperature, initial total gas-oil ratio, and stock-tank oil gravity were used with the depletion-performance data to develop general performance correlations. Three correlations are presented. The first equation shows the empirical relation of gas in place per barrel of hydrocarbon pore space with reservoir pressure and temperature, and initial total gas-oil ratio. The second equation is the correlation of stock-tank oil in place per barrel of hydrocarbon pore space. The third equation relates stock-tank oil production (to 500-psia depletion pressure) with pressure, temperature and oil gravity. All three correlations are also presented in convenient graphical form. Gas production to 500-psia depletion pressure is approximately 92.6 per cent of the gas in place for each fluid, so no correlation was developed for gas production. Introduction The rate of discovery of shallow, low-pressure and low-temperature oil reservoirs in the United States has been declining in recent years. For this reason, the search for oil gas gone to greater depths where high pressures and temperatures are encountered. A high-pressure and high-temperature reservoir is the environment in which many rich gas condensates and highly volatile oils are found. Because of this trend, the performance prediction of such reservoirs has become an important task of the reservoir engineer. The need and usefulness of a performance correlation based on field data that are available from the usual well-completion tests have previously been established.6 These earlier attempts have shown that the phase behavior of rich gas-condensate systems and volatile-oil and temperature.1 However, very little information on gas-condensate depletion performance5 was available at the time of these earlier attempts. The available of more recent data has made possible the development of an improved correlation. The data, as listed in Tables 1 and 2, are for 25 gas-condensate and two volatile-oil systems that were studied. The problem was to correlate the data for in-place fluids and the production data with the initial total gas-oil ratio, initial reservoir pressure and temperature, and stock-tank oil gravity. These correlations were obtained by multiple regression analysis, whereby the data were fitted to an equation by the method of least squared error.4 Basis of the Correlations The data used in the development of these correlations were obtained from depletion-performance calculations6 and well-test or production measurements. All of the independent variables used in the correlations are available from the usual well-test completion data. The reservoir volume considered is 1 bbl of hydrocarbon pore space (HCPS). The gas-oil ratio used is the total standard cubic feet of gas (at 14.7 psia, 60F) coming off the entire separator train per barrel of stock-tank oil. This minimizes the effect of separator conditions which might vary from system to system.