Dissolved Hydrocarbon Concentrations Research Articles

What Can Groundwater Monitoring Tell Us About Gas Migration? A Numerical Modeling Study.

Groundwater monitoring to measure a variety of indicator parameters including dissolved gas concentrations, total dissolved gas pressure (TDGP), and redox indicators is commonly used to evaluate the impacts of gas migration (GM) from energy development in shallow aquifer systems. However, these parameters can be challenging to interpret due to complex free-phase gas source architecture, multicomponent partitioning, and biogeochemical reactions. A series of numerical simulations using a gas flow model and a reactive transport model were conducted to delineate the anticipated evolution of indicator parameters following GM in an aquifer under a variety of physical and biogeochemical conditions. The simulations illustrate how multicomponent mass transfer processes and biogeochemical reactions create unexpected spatial and temporal variations in several analytes. The results indicate that care must be taken when interpreting measured indicator parameters including dissolved hydrocarbon concentrations and TDGP, as the presence of dissolved gases in background groundwater and biogeochemical processes can cause potentially misleading conclusions about the impact of GM. Based on the consideration of multicomponent gas partitioning in this study, it is suggested that dissolved background gases such as N2 and Ar can provide valuable insights on the presence, longevity and fate of free-phase natural gas in aquifer systems. Overall, these results contribute to developing a better understanding of indicators for GM in groundwater, which will aid the planning of future monitoring networks and subsequent data interpretation.

A comparison of crude oil hydrocarbon mobilization by vaporization gas drive into methane, ethane, and carbon dioxide at 15.6 MPa and 42 °C

An innovative sampling method was developed that allowed crude oil hydrocarbons dissolved in the upper gas-rich phase to be collected and quantitatively measured while maintaining constant reservoir conditions of 15.6 MPa and 42 °C. Crude oil was allowed to come to equilibrium with the injected gas phase (ca. 1/1 vol ratio) prior to collecting the dissolved hydrocarbons via a heated flow restrictor and analyzing them using high-resolution gas chromatography. As solvents, the three fluids’ efficacy parallel their minimum miscibility pressures (MMPs) for the test crude oil. Methane (MMP = 28.1 MPa) was a very poor hydrocarbon solvent, and was only able to dissolve ca. 18 mg oil/g methane. In contrast, CO2 (MMP = 9.68 MPa) dissolved up to 133 mg oil/g CO2 and ethane (MMP = 5.27 MPa) dissolved 722 mg oil/g ethane. Sequential collections of the dissolved oil hydrocarbons in the gas-dominated phase showed progressively lower concentrations for ethane and CO2, which demonstrated that liquid/liquid partitioning (and not saturation solubility) between the gas-dominated and oil-dominated phases controls the amounts of dissolved hydrocarbons. Thus, the lower the ratio of injected ethane or CO2 to the bulk crude oil, the higher the dissolved hydrocarbon concentrations. Both methane and CO2 showed a bias against dissolving heavier hydrocarbons. Methane was only capable of dissolving the lightest hydrocarbons (up to about C12), while CO2 preferentially dissolved light and middle hydrocarbons (up to about C16), leaving a residual oil rich in higher molecular weight hydrocarbons with significantly higher viscosity and density (lower API gravity) than the original unexposed crude oil. In contrast, ethane was capable of efficiently dissolving the entire range of hydrocarbons, with only a small bias against the heavier hydrocarbons. These lab studies show that, at the constant reservoir pressure and temperature used, ethane is a far superior hydrocarbon solvent for crude oil hydrocarbons than CO2, while CO2 is far superior to methane. Thus, ethane may ultimately yield higher oil recovery efficiencies (used either instead of CO2 or after a CO2 flood), while reducing the deposition of paraffinic and other higher molecular weight hydrocarbons in the reservoir.

Special Issue: Diagnostic Tools to Assess In Situ Remediation System Performance

Special Issue: Diagnostic Tools to Assess In Situ Remediation System Performance

Biodiesel presence in the source zone hinders aromatic hydrocarbons attenuation in a B20-contaminated groundwater

The behavior of biodiesel blend spills have received limited attention in spite of the increasing and widespread introduction of biodiesel to the transportation fuel matrix. In this work, a controlled field release of biodiesel B20 (100L of 20:80 v/v soybean biodiesel and diesel) was monitored over 6.2years to assess the behavior and natural attenuation of constituents of major concern (e.g., BTEX (benzene, toluene, ethyl-benzene and xylenes) and PAHs (polycyclic aromatic hydrocarbons)) in a sandy aquifer material. Biodiesel was preferentially biodegraded compared to diesel aromatic compounds with a concomitant increase in acetate, methane (near saturation limit (≈22mgL−1)) and dissolved BTEX and PAH concentrations in the source zone during the first 1.5 to 2.0years after the release. Benzene and benzo(a)pyrene concentrations remained above regulatory limits in the source zone until the end of the experiment (6.2years after the release). Compared to a previous adjacent 100-L release of ethanol-amended gasoline, biodiesel/diesel blend release resulted in a shorter BTEX plume, but with higher residual dissolved hydrocarbon concentrations near the source zone. This was attributed to greater persistence of viscous (and less mobile) biodiesel than the highly-soluble and mobile ethanol in the source zone. This persistence of biodiesel/diesel NAPL at the source zone slowed BTEX and PAH biodegradation (by the establishment of an anaerobic zone) but reduced the plume length by reducing mobility. This is the first field study to assess biodiesel/diesel blend (B20) behavior in groundwater and its effects on the biodegradation and plume length of priority groundwater pollutants.

Observing methane hydrate dissolution rates under sediment cover

Dissolution rates of naturally occurring gas hydrates vary by orders of magnitude across studies suggesting that environmental factors may influence hydrate dissolution. To determine the role that sediment cover plays in hydrate dissolution, we used a mini-pore fluid array sampler (mPFA) to continuously collect sediment porewater adjacent to a hydrate outcrop and maintain it at in situ pressure for later analysis. This allowed us to measure in situ dissolved hydrocarbon concentrations in the porewater over time without sample loss due to degassing. We deployed the mPFA at a hydrate outcrop at Barkley Canyon on the Cascadia Margin for nine months. This novel approach yielded concentration data that were used to determine the steady-state dissolution rate of the hydrate outcrop and test predictions of the diffusion-control model for dissolution in the field. In the lab, we conducted a series of experiments with artificial hydrate to directly compare dissolution rates between exposed and sediment-covered hydrate. The dissolution rate of the natural hydrate outcrop covered with sediment was 0.06cmy−1. The laboratory experiments of sediment-covered hydrate yielded dissolution rates of 0.6±0.5cmy−1 (n=5). In both laboratory and field observations, the dissolution rate of hydrates exposed directly to bulk water (3.9±1.7cmy−1 and 3.5cmy−1 respectively) was at least an order of magnitude faster than the dissolution rate of sediment covered hydrate. These results are consistent with expectations of diffusion-control and support this model of hydrate dissolution. In nature, sediment may account for the persistence of hydrate in otherwise methane-depleted environments by increasing the diffusive boundary layer and slowing the rate of molecular diffusion via porosity/tortuosity effects. We provide a number of “Lessons Learned” for improving the instrument design and for consideration during future studies.

In situ biostimulation of petroleum hydrocarbon degradation by nitrate and phosphate injection using a dipole well configuration

The main aim of this study was to explore the feasibility of source zone bioremediation by nitrate and nutrient injection in a crude-oil contaminated aquifer using a recirculating well dipole. Groundwater pumped from a downgradient well at a rate of 2.5m3h−1 was enriched with bromide (tracer), nitrate and ammonium phosphate and injected in a well 40m upgradient. The test was run for 49days with solute injection, followed by 65days of dipole operation without solute addition. The resulting bromide breakthrough curve allowed quantifying a first-order leakage coefficient of 0.017day−1 from the dipole, whereas from the nitrate data a first-order nitrate consumption rate of 0.075day−1 was determined. Dissolved hydrocarbon concentrations including benzene decreased to non-detect in 84days but experienced important rebounds after ending circulation. Nitrite accumulated temporarily but was consumed entirely when solute injection stopped. The mass balance calculations revealed that about 83% of the nitrate was used for hydrocarbon degradation, with the remaining being used for oxidation of reduced sulfur. A reactive transport model was used for the delineation of the treated zone. This model suggested that denitrification influenced flow and transport in the dipole. It is concluded that successful promotion of denitrifying hydrocarbon degradation is easily obtained in this aquifer and enables to abate dissolved concentrations, and that dipole configuration is a good option.

A reassessment of the potential for reduction of dissolved CO2 to hydrocarbons during serpentinization of olivine

The concept that aqueous CO2 can be reduced to hydrocarbons abiotically during serpentinization of olivine has become widespread in the earth and planetary sciences. This process has been invoked to explain the occurrence of hydrocarbons in crystalline igneous rocks and proposed as a source of prebiotic organic compounds for the origin of life. We reevaluate this scenario through an experimental study of the reaction of dissolved CO2 in the presence of olivine under hydrothermal conditions (300°C, 350 bar). Reduction of CO2 to formate (HCOO−) was found to proceed rapidly, with H2 generated from hydrothermal alteration of olivine serving as the reductant. The reverse reaction, decomposition of formic acid to CO2 and H2, was also found to proceed rapidly. Although dissolved hydrocarbon concentrations increased throughout the experiments, isotopic labeling of dissolved CO2 with 13C indicated that these compounds were primarily generated from reduced carbon compounds already present in olivine at the beginning of the experiment rather than by reduction of CO2. The only hydrocarbon product from reduction of CO2 observed in the experiments was a small amount of methane (<0.04% conversion of dissolved CO2 in more than 2500 h of heating). Comparison of the reaction products with thermodynamic data indicates that reactions between dissolved CO2 and formate rapidly achieved metastable equilibrium at the experimental conditions, suggesting that similar reactions could control the concentration of formate in geologic fluids. The results indicate that the potential for abiotic formation of hydrocarbons during serpentinization may be much more limited than previously believed, and other mineral catalysts or vapor phase reactions may be required to explain many occurrences of abiotic hydrocarbons in serpentinites and igneous rocks.

Evaluation of groundwater extraction for remediation of petroleum‐contaminated aquifers

ABSTRACT: Past experience has shown that restoration of an aquifer to drinking water quality by groundwater extraction and surface treatment may require many years of pumping. In this research, a mathematical model of hydrocarbon dissolution is developed to aid in the evaluation of groundwater remediation systems. The model assumes that residual hydrocarbon is distributed between two fractions: (1) a fast fraction in equilibrium with the aqueous phase and (2) a slow fraction in which mass transfer is limited. Relationships for simulating equilibrium partitioning are based on classical liquid‐liquid equilibrium theory. Column experiments were performed to test the model and examine the kinetics of aromatic hydrocarbon dissolution as residual hydrocarbon ages. The hydrocarbon dissolution process included an initial equilibrium period during which dissolved hydrocarbon concentrations were high and roughly constant followed by a rapid drop‐off period and an asymptotic period during which dissolved hydrocarbon concentrations were low and declined slowly. After passing more than 700 pore volumes of water through the column, between 8 and 10% of the original toluene and xylene isomers remained in the column. Model simulations were conducted to evaluate the effect of pulsed pumping on groundwater cleanup time.

Transport and fate of hydrocarbons in rivers and their estuaries. Partitioning between dissolved and particulate phases: Case studies of the Rhône, France, and the Huanghe and the Changjiang, China

In order to study the biogeochemistry of organic matter in estuaries at the freshwater/seawater interface, three rivers were selected for study: the Rhône and its Mediterranean delta, France (several cruises each year, since 1985), the Huanghe, China (two cruises, spring and summer 1985), and the Changjiang, also in China (two cruises, winter and summer 1986). These rivers were characterized by different contaminant levels and sources and different suspended matter loads, an important factor in determining partitioning between dissolved and particulate phases. During these cruises, water was sampled at key stations chosen as a function of salinity and turbidity, filtered and analyzed for both dissolved and particulate non-volatile hydrocarbons. Hydrocarbons, after separation from other lipids, were analyzed by gas chromatography, high pressure liquid chromatography and gas chromatography/mass spectrometry. Particulate non-aromatic hydrocarbon and n-alkane concentrations are compared with suspended matter concentrations and with the organic carbon content of the suspended matter (percent organic carbon, POC). A good correlation was found between hydrocarbon concentrations and POC, but no such correlation was systematically observed between hydrocarbon and suspended matter concentrations. The ratio between particulate and dissolved hydrocarbon concentrations varies with suspended matter load and season. The ratio is probably also modified by the biological activity of the waters, which influences the transport processes through the freshwater/seawater interface and the budget of hydrocarbon inputs carried by rivers to the ocean.

The water solubility of crude oils and petroleum products

Solubilities are reported for 42 crude oil and petroleum products in water as a function of temperature, salinity, oil weathering and water-to-oil volume ratio. The applicability of several analytical techniques (purge-and-trap gas chromatography, high pressure liquid chromatography, and fluorescence) for the determination of dissolved hydrocarbon concentrations is discussed critically. The effect of water-to-oil volume ratio on the apparent solubility of oils in water is discussed in detail.

Hydrocarbons in waters and particulate material from hydrothermal environments at the East Pacific Rise, 13°N

The fast spreading East Pacific Rise at 13°N consists of an active circulating hydrothermal system, with associated massive sulfide deposits and large communities of animals clustered around hot smokers. The main food source of these benthic organisms is supplied by symbiotic chemosynthetic activity of sulfur oxidizing bacteria. Biomarker lipids such as hydrocarbons, were analyzed to describe their sources and composition and also the relationships among microorganisms, biological communities, and dissolved and particulate organic materials in this productive area. The organic matter of vent waters is both quantitatively and qualitatively different from the non-vent bottom water. Dissolved hydrocarbon concentrations in warm water ( T ∼ 15°C) from fissures in basalts and hot clear water ( T > 250°C) from smokers are 100 and 400 times more concentrated, respectively, than the water outside active areas. The biomarker compounds from hot clear water are characteristic of thermally mature organic matter as indicated by the 17 α(H)- > 17 β(H)-hopane series, and the dominance of 5α(H), 14β(H), 17β(H)-steranes over the 5α(H), 14α(H), 17α(H), 20R-steranes, i.e. the biologically inherited configuration. The dissolved hydrocarbons from the warm waters may be correlated with bacterial lipid residues as indicated by the presence of isoprenoid hydrocarbons (pristane > phytane) and low molecular weight cyclic hydrocarbons. The water column samples collected near hydrothermally active areas contain bicyclic alkanes with drimane and rearranged drimane skeletons and mature biomarkers (17α(H)-hopanes and αββ-steranes). These components probably derived from thermally-altered microbial sources. The dissolved lipid matter is found to be degraded more rapidly by microbial activity and thermal processes, especially in the warm water effluent and in the water column around hydrothermal vents, than the lipids of the particulate matter.

An exploratory study of the behaviour of crude oil spills under ice

AbstractAn experimental spill of 0.38 m3 of warm (55°C) crude oil under the ice cover of a fresh water pond with an underwater viewing port is described. Temperatures, dissolved hydrocarbon concentrations and oil slick behaviour and areas were determined. Some oil recovery and disposal methods were tested. An attempt has been made to quantify the rates of heat and mass transfer from the oil using reported values for transfer coefficients. The results indicate that the oil slick adopted a thickness of about 2 cm under the ice, that heat transfer from the oil was relatively fast and complete, that dissolution mass transfer is slow resulting in oil concentrations in the water of only 18 to 287 μg/l in the vicinity of the spill. Some recommendations are made for further studies to elucidate the nature and rates of the relevant physical processes and thus assist in assessing the environmental impact of under‐ice oil spills.