A scenario for the creation of H2S heterogeneities in acid gas reservoirs in contact with an active aquifer: a simulation study

International audienceCompositional heterogeneities of H2S have been noticed in many sour gas reservoirs. Its occurrence is an important factor of economic depreciation. Thus, the knowledge of its content and distribution is a critical parameter when planning field development. The paper aims at exploring the role of an active aquifer in the creation of H2S heterogeneities in high H2S-bearing gas reservoir. Indeed, under conditions of pressure and temperature of typical reservoirs, H2S is far more soluble than hydrocarbons and other gases. A preferential leaching of H2S (e.g. versus CH4) over time is thus possible. This mechanism is controlled by: (1) Differential solubility of gases, which change the relative amounts of each gas near the gas-water contact (GWC); (2) Contact with an active aquifer, which can export the dissolved gases thus enhancing dissolution on the long-term; (3) Diffusional transport in the gas phase, which transfers the compositional anomalies farther from the gas-water contact; (4) Geological parameters (type of aquifer, permeability heterogeneities) which can modify the transport scenario. To illustrate and quantify this process, we show the results of numerical simulations, performed with the two-phase transport and geochemical software Hytec. First, a very schematic reservoir with a composition considered uniformly distributed within the reservoir, has been simulated to quantify the leaching of H2S. The results highlight the potential role of the active aquifer, which can leach the gases and export them outside the reservoir. In a second phase, the effect of geological parameters on the H2S heterogeneity development was studied: additional simulations were performed on geometries closer to natural cases. The amount of leached H2S depends strongly on the geometry: the larger the GWC area, the larger the amounts leached

Inhibiting Effect of Tetralin on the Pyrolytic Decomposition of Hexadecane. Comparison with Toluene

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Up to what temperature is petroleum stable? New insights from a 5200 free radical reactions model

International audienceThe discovery of crude oils and condensates at ever higher temperatures casts doubt on the validity of the usual geochemical modelling approach, that uses empirical reactions and rate constants. The solution used to account for such a high thermal stability is presently to adjust the rate parameters, but the physical meaning and scientific value of such a strategy can be questioned. We have developed a mechanistic model consisting of 5200 lumped free radical reactions to describe the thermal evolution of a mixture of 52 organic species meant to represent light petroleum. Rate constants used are those available in the literature or estimated using well established thermochemistry-reactivity correlations. Chemical structures included in the model are linear, branched and cyclic hydrocarbons, hydro-and alkyl-aromatics, PAHs, and three heteroatomic compounds. Reactions include cracking and alkylation chains and inhibiting and accelerating reactions from the various reactants. This model has been applied to several mixtures with various proportions of reaction inhibitors and accelerators, and to a composition representing a light mature oil. From the results obtained, we conclude that mature oils will be stable up to 240-260 C, depending on their composition, and that the thermal cracking of oil to gas is not possible under reasonable basin conditions. The kinetics of petroleum cracking are thus much slower than generally recognized

Role of hydrodynamism in compositional heterogeneities in acid gas reservoir

International audienceAcid gases (H2S and CO2) compositional heterogeneities have been noticed in many sour gas reservoirs. Their occurrence is an important factor of economic depreciation. Thus, the knowledge of the acid gases distribution is a critical parameter for the design of field development. The mechanisms to explain compositional heterogeneities of acid gas in a reservoir are various. The paper aims at exploring the role of an active aquifer in contact with an initial high H2S content reservoir. The major mechanisms may be controlled by: * Differential solubility of gases which can change the relative amounts of each gas near the contact; * Active aquifer solubilization and transport which can export dissolved gases thus enhancing dissolution on the long-term; * Diffusional transport in the gas phase which can transfer the compositional anomalies farther from the gas-water contact. To test the influence of several parameters on the efficiency of the acid gases leaching, simulations on basic geometries have been performed with the diphasic transport and geochemical software Hytec. The simulation results show a major role of the occurrence of horizontal impermeable barriers yields to sharp heterogeneities, including a decrease in acid gas near the contact, while farther areas H2S concentration remain unaffected

Modeling of hydrocarbons pyrolysis at low temperature. Automatic generation of free radicals mechanisms

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Comparative studies of a high resolution sampling of the different facies of the organic-rich Orbagnoux Deposit (upper Kimmeridgian, Jura); petrographic and bulk geochemical approach; extent and origin of interfacies and intrafacies variations

A large set of high resolution samples (thickness down to a few millimetres), corresponding to the five facies of the Orbagnoux deposit, were examined for their compositional (major and trace elements, organic matter (OM) content), petrographic (transmitted-and reflected-light microscopy, scanning and transmission electron microscopy), pyrolytic (Rock-Eval, Geoelf Sulphur Analyser and Pyromat pyrolyses) and spectroscopic (Fourier transform infrared) features. A number of similarities were thus observed although, as previously shown, the formation of the light undulated laminae was associated with the development of cyanobacterial mats on sediment surface, whereas the other four facies originated from coccolith settling. Nevertheless, three substantial differences were also noted :- Lower OM contents in the light undulated laminae likely due to the extensive mineralization of mat biomass before sulphurization. - Relatively less reducing conditions during the deposition of the light undulated laminae and massive limestones. - Large semi-quantitative differences in FTIR spectra (the kerogens from the dark-coloured samples tend to be richer in C = O groups while those from the light-coloured ones tend to be richer in OH groups and C = C bonds). Furthermore, the FTIR spectra show, for each facies, large intrafacies differences that may reflect, in part, differences in the degree of post-depositional oxidative alteration during emergences

New Bio-Indicators for Long Term Natural Attenuation of Monoaromatic Compounds in Deep Terrestrial Aquifers

International audienceDeep subsurface aquifers despite difficult access, represent important water resources and, at the same time, are key locations for subsurface engineering activities for the oil and gas industries, geothermal energy, and CO 2 or energy storage. Formation water originating from a 760 m-deep geological gas storage aquifer was sampled and microcosms were set up to test the biodegradation potential of BTEX by indigenous microorganisms. The microbial community diversity was studied using molecular approaches based on 16S rRNA genes. After a long incubation period, with several subcultures, a sulfate-reducing consortium composed of only two Desulfotomaculum populations was observed able to degrade benzene, toluene, and ethylbenzene, extending the number of hydrocarbonoclastic-related species among the Desulfotomaculum genus. Furthermore, we were able to couple specific carbon and hydrogen isotopic fractionation during benzene removal and the results obtained by dual compound specific isotope analysis (ε C = −2.4 ± 0.3 ; ε H = −57 ± 0.98 ; AKIE C : 1.0146 ± 0.0009, and AKIE H : 1.5184 ± 0.0283) were close to those obtained previously in sulfate-reducing conditions: this finding could confirm the existence of a common enzymatic reaction involving sulfate-reducers to activate benzene anaerobically. Although we cannot assign the role of each population of Desulfotomaculum in the mono-aromatic hydrocarbon degradation, this study suggests an important role of the genus Desulfotomaculum as potential biodegrader among indigenous populations in subsurface habitats. This community represents the simplest model of benzene-degrading anaerobes originating from the deepest subterranean settings ever described. As Desulfotomaculum species are often encountered in subsurface environments, this study provides some interesting results for assessing the natural response of these specific hydrologic systems in response to BTEX contamination during remediation projects