Gas Exsolution Research Articles

Influence of Exsolved Volatiles on Reheating Silicic Magmas by Recharge and Consequences for Eruptive Style at Volcán Quizapu (Chile)

AbstractThe two most recent eruptions of Volcán Quizapu (southern Andes, Chile), only 85 years apart, were both triggered by magma recharge and extruded the same volume (about 5 km3) of the same volatile‐rich dacitic magma, but showed a remarkable shift from effusive (1846–1847) to explosive (1932) behavior. We demonstrate, using a newly developed model, that the presence or absence of an exsolved volatile phase in the reservoir strongly influences its mechanical and thermal response to new inputs of magma. We propose that, prior to the 1846–1847 effusive eruption, gas bubbles damped the build‐up of excess pressure and allowed recharge of a significant volume of magma before triggering the 1846–1847 eruption. The strong temperature increase that resulted enhanced syneruptive outgassing leading to an effusive eruption. In contrast, during the repose period between the 1847 and 1932 eruptions, new recharges found a much less compressible host reservoir as the exsolved gas phase was largely removed in response to the prior eruption, yielding rapid pressurization, minor reheating, and comparatively less syneruptive outgassing. The combination of these effects culminated in an explosive eruption.

Unintentional contaminant transfer from groundwater to the vadose zone during source zone remediation of volatile organic compounds

Historical heavy use of chlorinated solvents in conjunction with improper disposal practices and accidental releases has resulted in widespread contamination of soils and groundwater in North America and worldwide. As a result, remediation of chlorinated solvents is required at many sites. For source zone treatment, common remediation strategies include in-situ chemical oxidation (ISCO) using potassium or sodium permanganate, and the enhancement of biodegradation by primary substrate addition. It is well known that these remediation methods tend to generate gas (carbon dioxide (CO2) in the case of ISCO using permanganate, CO2 and methane (CH4) in the case of bioremediation). Vigorous gas generation in the presence of chlorinated solvents, which are categorized as volatile organic contaminants (VOCs), may cause gas exsolution, ebullition and stripping of the contaminants from the treatment zone. This process may lead to unintentional ‘compartment transfer’, whereby VOCs are transported away from the contaminated zone into overlying clean sediments and into the vadose zone. To this extent, benchtop column experiments were conducted to quantify the effect of gas generation during remediation of the common chlorinated solvent trichloroethylene (TCE/C2Cl3H). Both ISCO and enhanced bioremediation were considered as treatment methods. Results show that gas exsolution and ebullition occurs for both remediation technologies. Facilitated by ebullition, TCE was transported from the source zone into overlying clean groundwater and was subsequently released into the column headspace. For the case of enhanced bioremediation, the intermediate degradation product vinyl chloride (VC) was also stripped from the treatment zone. The concentrations measured in the headspace of the columns (TCE ∼300ppm in the ISCO column, TCE ∼500ppm and VC ∼1380ppm in the bioremediation column) indicate that substantial transfer of VOCs to the vadose zone is possible. These findings provide direct evidence for the unintended spreading of contaminants as a result of remediation efforts, which can, under some circumstances, result in enhanced risks for soil vapour intrusion.

Crystal-rich lava dome extrusion during vesiculation: An experimental study

Lava dome-forming eruptions represent a common eruptive style and a major hazard at numerous active volcanoes worldwide. The extrusion mechanics of crystal-rich lava domes and the influence of volatiles on the transition from viscous to brittle behaviour during lava dome extrusion remain unclear. Understanding how gas exsolution and crystallinity control effusive versus explosive eruption behaviour is essential. Here, we present new experimental results on the rheology of synthesised, crystal-rich (50 to 80vol% quartz crystals), hydrous (4.2wt% H2O in the glass) dacite samples, which vesiculate from 5 to 27vol% gas bubbles at high temperatures (from glass transition temperature to 797°C) during deformation conducted in a parallel plate viscometer (constant stress at 0.63–0.64MPa, and variable strain-rates ranging from 8.32·10−8 to 3.58·10−5s−1). The experiments reproduce certain aspects of lava dome deformation in volcanic conduits during vesiculation of the residual melt, instigated in the experiments by increasing temperature. During gas exsolution (i.e. nucleation and growth of gas-pressurised bubbles) and volume inflation, we find that the rheological lubrication of the system during deformation is strongly dictated by the initial crystallinity. At crystal contents <60vol%, gas bubbles form and coalesce during expansion and viscous deformation, favouring strain localisation and gas permeability within shear bands, which control the overall sample rheology. At crystallinities of 60 to 70vol%, gas exsolution generates pressurisation (i.e. pore pressure increase) within the bubbles trapped in the solid crystal clusters, and embryonic formation of microscopic fractures through melt and crystals drives the system to a brittle behaviour. At higher crystallinity (80vol%) vesiculation leads to large pressurisation, which then triggers extensive brittle fragmentation. Through macroscopic fractures, outgassing determines the rheological stalling of the system. In the light of these results we propose a rheological description of crystal-rich lava dome mechanics. The contrasting experimental behaviours at different crystallinities have implications for the style of slow-ascending dome-forming eruptions. All other factors being equal, our experiments suggest that crystal-poor magmas will undergo efficient outgassing, reducing the potential for an explosive eruption. Conversely, crystal-rich magmas may experience limited outgassing and larger gas overpressures during vesiculation, therefore increasing the potential for an explosive eruption.

Modeling of Gas Exsolution for CO2/brine Surface Dissolution Strategy

CO2/brine surface dissolution strategy is perceived as one of the safest technologies for geological carbon sequestration (GCS) as it emphasizes the total dissolution of capture CO2 in the extracted brine and eliminates the risks associated with buoyant escape of free CO2 gas. Keeping CO2 in solution would require terminating the storage operation when the fluid front of injected CO2-saturated brine arrives at the contour of bubble point pressure in the storage reservoir. This complicates the operation and reduces the storage efficiency. To assess whether allowing a free gas phase to form would pose unacceptable risk, we conducted simulations of continuous injection until the injected CO2-saturated brine breaks through at the production well. We quantify the gas saturation profile by combining fractional flow theory and fluid flow model. We use two limiting cases for the mobility of free gas, one assumes exsolved gas remains immobile at all saturations and the other assumes gas flow with the maximum mobility. Both limits yield similar gas saturations formed within the two-phase fluid region, which is small (about 1%) in our case study, thus the risk associated with this small gas saturation is low, regardless of different relative permeability models. The accumulation time has a first-order effect on gas saturation, and it can be reduced by increasing the flow rate or methods that make the location of bubble point pressure close to the extraction well.

A new kinetic model for non-equilibrium dissolved gas ex-solution from static heavy oil

A new multi-component kinetic model was developed to simulate the non-equilibrium gas ex-solution process in the heavy oil bulk volume. The model assumes that bubble formation starts by populating microbubbles in the oil phase when depressurization starts. Microbubbles have flexibility to form either bigger bubbles or free gas phase depending on depletion rate. When the pressure depletion rate is extremely low, microbubbles directly move to the gas phase. When the depletion rate is fast, the microbubbles aggregate into bigger bubbles. As a result, the model has flexibility to shift from non-equilibrium to equilibrium behavior simulating the full spectrum of foam formation behavior. The kinetic model was tuned by using a thermal reservoir simulator. The equilibrium K-values for the kinetic model were generated by tuning the Peng-Robinson equation of state. In the numerical simulator, the variables for the kinetic model were adjusted to match the experimental data for a heavy oil. In the proposed kinetic model, dispersed bubbles in the oil phase increase its compressibility and reduce its density, however, in the new model, there is no significant change to the oil phase viscosity. The new model is able to represent non-equilibrium gas-exsolution laboratory data and reveals that there must be a minimum excess concentration of the dissolved gas in the oil phase to drive the bubble formation reaction.

Monitoring crustal changes at volcanoes by seismic noise interferometry: Mt. Etna case of study

In this work, we analysed the seismic noise recorded at Mt. Etna by 18 stations during the interval 2007–2015 in the frequency band 0.1–0.3Hz, chosen to avoid contamination from volcanic tremor. Variations in time of medium seismic velocity in the range −0.8 to 0.8% were found, mostly affecting the stations located on the volcano summit and flanks. Based on the investigated frequency content, the Δv/v changes took place from the surface to a depth of ~4.5–6.5km. To identify the source mechanism of the observed medium changes, the variations were quantitatively compared by wavelet transform coherence with volcano-tectonic and meteorological parameters. A significant relationship with meteorological parameters with seasonal periodicity (especially air temperature and snow loading) was found, probably caused by thermo-elastic strain and increasing-decreasing surface loading cycles. Moreover, a sharp medium velocity decrease, taking place in mid-December 2009 and clearly time-related to the largest volcano-tectonic strain release phenomenon of the investigated period, was also found. Such a velocity decrease was interpreted as resulting from ascent of fluids and gas exsolution taking place at the same time as the volcano-tectonic swarm.

Sand volcano generated by a violent degassing from methane-saturated aquifers: The case study of Medolla (Modena, Italy)

Sand volcano is an unusual and remarkable geological feature which forms when water-saturated sand deposits are set in motion by liquefaction and are ejected onto the surface. Commonly it is generated during earthquakes, as a result of liquefaction of waterlogged bodies at shallow depth, often causing wide damages. The generation of a sand volcano however can also occur unrelated to a seismic event. On the 10th of October 2014, near Medolla (Italy) during a Cone Penetration Test, a large amount of natural gas (CO2 and CH4) together with a mixture of water and sand were erupted, creating a sand volcano. The study of this event gives the possibility of understanding the dynamics of sand volcano generation, may enable to prevent other anomalies during future CPT tests and, more importantly, underlines the role that natural gas, stored in a sand aquifer, may play in triggering a liquefaction phenomenon. Our results suggest that episodes of gas eruptions require the onset of very peculiar conditions within the reservoir that feeds the emission. The simulations suggest that a geyser discharging a mixture of gas and water, capable of building a sand volcano, requires the presence of a shallow pressurized reservoir (1.2MPa) where water coexists with a small amount of exsolved gas (a volume fraction of 0.05).The violent degassing occurred in Medolla confirms the role that a free gas phase may have in favoring the mobilization of liquid water and loose deposits, even in the absence of a seismic event.

Re-saturation targets identified from 4D seismic softening responses in the Forties Field

Abstract A novel 4D methodology by targeting softening responses, a decrease in impedance over time, is described from the Forties Field. It is demonstrated that these anomalies identified areas where oil had re-saturated previously swept rock. When first observed in some of the older vintages of 4D data, softening responses were dismissed as side lobes of underlying water-swept reservoirs. With improved 4D data quality, softening responses were seen to be clearly responding to an acoustic softening effect isolated in the reservoir. Pressure measurements ruled out hypotheses of gas exsolution or geomechanically induced dilatation of the overlying shale. At the time of writing 22 re-saturation targets had been drilled at Forties with an average water saturation ( S w ) of 28%, close to virgin conditions. These results indicated that a previously swept sand can be re-saturated to near initial oil conditions. The driving mechanism behind the re-saturation is understood to be related to changes in both production offtake and water injection. The resulting complex hydrodynamic interactions appeared to be moving banks of oil around the field. From 2011 to year-end of 2015 re-saturation targets produced 14.6 MMBO and were a key target type at the Forties Field.

Unraveling the Formation of Large Amounts of Calcite Scaling in Geothermal Wells in the Bavarian Molasse Basin: A Reactive Transport Modeling Approach

Results from reactive transport simulations performed for the geothermal plant in Kirchstockach, located in the Bavarian Molasse Basin in southern Germany, are presented to unravel the formation of calcite scaling. Simulation results successfully predict the calcite scaling profile observed along the production well if supersaturation with respect to calcite is specified for the model water leaving the pump at 800 m depth. This observation is in good agreement with a previous study suggesting that gas exsolution (i.e., boiling) occurring at the pump due to an unwanted pressure drop is the most likely driver for the formation of those calcite scalings.

Nonequilibrium degassing, regassing, and vapor fluxing in magmatic feeder systems

Magma degassing models typically invoke volatile depletion of a single parental melt, with permeable loss of exsolved gas having served for many years as the paradigm for the transition from volatile-rich, explosive eruptions to volatile-depleted lava flows. These degassing models are guided by measurements of H2O, CO2, and hydrogen isotope variations retained in melt that quenched to glass, but the existing models are not uniquely constrained by the data. There also remains uncertainty surrounding the origin and significance of volcanic glass fragments. We show that individual obsidian pyroclasts from Mono Craters, California (USA), are heterogeneous in dissolved H2O and CO2, suggesting that clasts are assembled from juvenile melt and rewelded ash during magma ascent. This is in contrast to the conventional view that clasts are chemically homogeneous and sample the chilled, glassy margins of conduit walls. The new measurements of dissolved H2O and CO2 help reconcile existing open-system degassing models used to explain elevated CO2/H2O ratios, provide time scales based on diffusion modeling for pyroclast formation, and show that magma does not necessarily lose volatiles monotonically during ascent-driven decompression.

Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics.

Timescales of magma ascent in conduit models are typically assumed to be much longer than crystallization and gas exsolution for basaltic eruptions. However, it is now recognized that basaltic magmas may rise fast enough for disequilibrium processes to play a key role on the ascent dynamics. The quantification of the characteristic times for crystallization and exsolution processes are fundamental to our understanding of such disequilibria and ascent dynamics. Here we use observations from Mount Etna's 2001 eruption and a magma ascent model to constrain timescales for crystallization and exsolution processes. Our results show that plagioclase reaches equilibrium in 1–2 h, whereas ascent times were <1 h. Using these new constraints on disequilibrium plagioclase crystallization we also reproduce observed crystal abundances for different basaltic eruptions. The strong relation between magma ascent rate and disequilibrium crystallization and exsolution plays a key role in controlling eruption dynamics in basaltic volcanism.

Frequency scaling of seismic attenuation in rocks saturated with two fluid phases

Seismic wave attenuation is frequency dependent in rocks saturated by two fluid phases and the corresponding scaling behaviour is controlled primarily by the spatial fluid distribution. We experimentally investigate the frequency scaling of seismic attenuation in Berea sandstone saturated with two fluid phases: a liquid phase, water, and a gas phase, air, carbon dioxide or nitrogen. By changing from a heterogeneous distribution of mesoscopic gas patches to a homogeneous distribution of pore scale gas bubbles, we observe a significant steepening of the high-frequency asymptote of the attenuation. A transition from one dominant attenuation mechanism to another, from mesoscopic wave-induced fluid flow to wave-induced gas exsolution dissolution (WIGED), may explain this change in scaling. We observe that the high-frequency asymptote, for a homogenous pore scale gas bubble distribution, scales in accord with WIGED.

Pore-scale multiphase flow modeling and imaging of CO2 exsolution in Sandstone

This study utilizes synchrotron X-ray micro-tomography and pore scale modeling to investigate the process of gas exsolution and how it affects non-wetting phase relative permeability. Exsolved gas distributions are measured on Domengine and Boise sandstone samples using synchrotron X-ray micro-tomography. Observed gas phase distributions are compared to a new model that simulates the growth and distribution of exsolved gas phase at the pore-scale. Water relative permeability curves are calculated using a Stokes flow simulator with modeled and observed gas distributions, under various conditions, such as rock geometry, and pressure depletion rates. By comparing the actual bubble distributions with modeled distributions, we conclude that exsolved gas is more likely to form and accumulate at locations with higher water velocities. This suggests that convective delivery of CO2 to the gas bubble is a primary mechanism for bubble growth, as compared to diffusive transport through the aqueous phase. For carbonated brine flowing up a fault at half a meter per day, with 5% exsolved gas, the water relative permeability is estimated to be 0.6∼0.8 for various sandstones. The reduction of water mobility reduces upward brine migration when even a small amount of exsolution occurs.

Tidally controlled gas bubble emissions: A comprehensive study using long-term monitoring data from the NEPTUNE cabled observatory offshore Vancouver Island

Long-term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity-temperature-depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day-long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant sub-surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans.

Binary 4D seismic history matching, a metric study

This paper explores 4D seismic history matching and it specifically focuses on the objective function used during the optimisation with seismic data. The objective function is calculated by using binary maps, where one map is obtained from the observed seismic data and the other is from one realisation of the optimisation algorithm from the simulation model. In order to decide which set of parameters is a relevant update for the simulation model, an efficient way is required to measure how similar these two binary images are, during their evaluation within the objective function. Behind this aspect of quantification of the similarities or dissimilarities lies the metric notion, or the art of measuring distances. Four metrics are proposed with this study, the well-known Hamming distance, two widely used metrics, the Hausdorff distance and Mutual Information and a recent metric, called the Current Measure Metric. These metrics will be tested and compared on different case scenarios, designed in accordance to a real field case (gas exsolution) before being used in the second part of the paper. Despite its simplicity, the Hamming distance gives positive results, but the Current Measure Metric appears to be a more efficient choice to cover a wider range of scenarios, these conclusions remain true when tested on synthetic and real dataset in a history matching exercise. Some practical aspects of binary map processes will be examined through the paper, as it is shown that it is more proper to use a derivative free optimisation algorithm and a proper metric should be more inclined to capture global features than local features.

Excitation and resonance of acoustic-gravity waves in a column of stratified, bubbly magma

Oscillations of magma in volcanic conduits are thought to be the source of certain seismic and infrasonic signals observed near active volcanoes. However, the multiphase and stratified nature of magma within the conduit complicates the calculation of resonant modes that is required to interpret observations. Here we present a linearized mathematical framework to describe small-amplitude oscillations and waves in a stably stratified column of two-phase magma (liquid melt and gas bubbles) with a traction-free upper surface (a lava lake). We explore the role of time-dependent mass exchange between the phases, depth-varying fluid properties and gravity on the modes of oscillation of inviscid magma within an axisymmetric, vertical conduit. Non-equilibrium phase exchange, which we refer to as bubble growth and resorption (BGR), is parameterized by introduction of a kinetic time scale quantifying mass exchange between the liquid and gas phases that evolves the mixture towards a state of thermodynamic equilibrium. Using a provably stable finite difference method, we solve the eigenvalue problem for the resonance frequencies, decay rates, and spatial structure of the conduit eigenmodes. The numerical method is then extended to time-domain simulations of waves excited by internal volumetric sources in the conduit or forces applied to the surface of the lava lake. We connect time-dependent wave propagation simulations to the modal analysis by identifying the primary modes that are excited by representative excitation processes. Waves propagating through bubbly magma are dispersive, and their behaviour is determined by three dimensionless parameters. One quantifies the importance of buoyancy and gravitational restoring forces relative to compressibility, the second quantifies differences between fluid properties (e.g. mixture compressibility) under equilibrium and non-equilibrium conditions, and the third compares the wave period to the BGR time scale. Pronounced depth variations in background fluid properties, such as the transition from liquid melt with dissolved volatiles at the high pressures at depth to bubbly magma above the gas exsolution depth, segment the conduit into distinct regions. The longest-period modes, which are expressed with the largest amplitudes for typical excitation processes, are most sensitive to the length of the bubbly region and properties of the bubbly magma within it. While the boundary condition at the bottom of the conduit determines whether the fundamental mode is affected by the total conduit length, modes localized above the exsolution depth are remarkably insensitive to the overall conduit length. Our analysis suggests that parameters affecting eruption style, such as total volatile content and kinetic time scales of BGR, along with excitation source characteristics, are imprinted on long-period seismic and infrasonic signals at active volcanoes.

Mathematical models for quantifying eruption velocity in degassing pipes based on exsolution of a single gas and simultaneous exsolution of multiple gases

After the limnic eruptions at Nyos and Monoun in the 1980s, degassing pipes were installed to reduce the continuous increase of CO2 at the bottom of these lakes. The degassing system consists of a vertical pipe from the lake bottom to the surface and a small pump located near the top of the pipe, which raises water in the pipe up to a level where it becomes saturated with gas, which in turn leads to volume expansion and eruption. This paper describes two new mathematical models for predicting eruption velocity in degassing pipes based on exsolution of a single gas and the simultaneous exsolution of multiple gases. The models were applied to the degassing system of lakes Nyos and Monoun, which contain two main gases, namely CO2 and CH4. Because the volume proportion of CH4 is significant only in Lake Monoun, the Lake Nyos test case considered the CO2 gas only, while as the Lake Monoun test case considered the simultaneous exsolution of CO2 and CH4. Good agreement between the results of the models and observed data is found for both test cases. The results for the eruption in Lake Monoun considering the two main gases measured in this lake (CO2 and CH4) were found to have a better agreement with the measurements compared to the model results obtained considering the main gas only (CO2).

Exophthalmia in wild-caught cod (Gadus morhua L.): development of a secondary barotrauma effect in captivity.

Capture-based aquaculture (CBA) of Atlantic cod (Gadus morhua) has become increasingly important in recent years, and increased attention is being paid to animal welfare issues linked to these activities. Earlier studies showed that some cod develop secondary exophthalmia in captivity. This study investigated the development of secondary exophthalmia in two groups of wild-caught cod, one of which was exposed to rapid decompression causing acute barotrauma (treatment group) while the other was not (control group). Photographs and radiographs before and up to 33days after barotrauma revealed a significant increase in overall eye protrusion caused by an accumulation of gas in the orbita in the treatment group, first observed on day 9 after decompression, while no protrusions were observed in the control group. Barotrauma was thus identified as an important trigger for the development of secondary uni- or bilateral exophthalmia of wild-caught cod. Two underlying mechanisms are suggested, where the more likely is residual swim bladder gas taking the route of least resistance, while the less likely is the exsolution of gas from the blood. Our results have implications for a wide range of contexts in which cod are rapidly brought to the surface from great depth.

Heat of solution: A new source of thermal energy in the subsurface of cometary nuclei and the gas-exsolution mechanism driving outbursts of Comet 29P/Schwassmann‒Wachmann and other comets

This paper is a continuation of Miles et al. (2015) [Icarus] and Miles (2015b) [Icarus], which detail new observations of Comet 29P/Schwassmann‒Wachmann, characterise its rotational period (∼57d), and identify the presence of discrete sources of outburst on its nucleus: the latter ruling out amorphous-to-crystalline H2O ice transitions as the cause of its outbursts. Summary data are presented for 29P and a further 16 non-fragmenting comets which exhibit outbursts of >2 magnitudes. A comprehensive physicochemical mechanism is postulated to account for major outbursts based on melting of cometary ices and the exothermic dissolution of gases, especially CO and CO2 at pressures of 10‒200kPa. The thermodynamics of enthalpy heating are described and heats of solution are calculated from gas–liquid solubility data yielding −6kJmol−1 for CO in CH4, and −15kJmol−1 for CO2 in CH3OH close to their freezing point. Heats of solution are ∼6 times greater (per mole) than the enthalpy of fusion of the pure CH4 and CH3OH ices, enabling gas pressures of >∼80kPa to continually melt these ices. Supervolatile O2 and N2 gases may also participate by dissolving exothermically in liquid CH4 and other hydrocarbons potentially reaching high mixing ratios. H2S and NH3 gases dissolve exothermically in CH3OH liberating up to 20kJmol−1 and 13kJmol−1, respectively, and all three hydrophilic species facilitate sintering of H2O ice in the near-surface of comets. Localised melting and consolidation is favoured in slowly-rotating cometary nuclei of intermediate dust/gas ratios, at pressures of ∼1kPa, and temperatures as low as 50‒65K where O2 and N2 are abundant. Nyctogenic processes on the night-time side of the nucleus restock desiccated surface layers, reseal the crust, enabling fractionation of solutes in sub-crustal liquid phases via fractional sublimation/distillation of non-polar, hydrophobic CH4 and other hydrocarbons; and by fractional crystallisation of polar, hydrophilic phases rich in aqueous CH3OH and other organic oxygenates, e.g. CH2O, able to form low melting point eutectic mixtures. A generalised outburst mechanism is described involving the containment of gases as solutes in cryomagma beneath consolidated surface crustal regions. Disruption of the crust and associated pressure loss render the cryomagma supersaturated, and the concomitant explosive exsolution of gases provokes a cometary outburst. The CO gas-exsolution mechanism operates at ∼65 to 95K and accounts for activity of 29P and other distant comets up to rh=∼15AU. A similar mechanism can operate at ∼150 to 200K driven by CO2 in aqueous CH3OH and may account for rare outbursts of Jupiter-family comets such as 17P/Holmes. At least 10–15% of all periodic comets may be subject to gas-exsolution outbursts, the majority of which are weak and go undetected. Possible surface morphologies of the nucleus of Comet 29P are discussed. The mechanism may also explain the phenomenon of strong cometary outbursts triggering secondary events, as observed for 17P, 29P and 41P.

On fingering of steam chambers in steam‐assisted heavy oil recovery

Western Canadian oil sands reservoirs are among the largest petroleum accumulations in the world. Given original oil viscosity up to 5,000,000 mPa‐s, these oils are currently recovered from these reservoirs using steam which heats the oil to ∼250°C with reduced viscosities &lt;10 mPa‐s. A key issue faced by thermal recovery processes is the uniformity of the steam chamber within the reservoir. Nonuniformities of the chamber arise from multiphase flow instabilities in the porous media where fingering has been explained by penetration of low viscosity steam into viscous oil. Here, fine‐grid thermal reservoir simulation reveals that fingering takes place in the gas phase beyond the chamber edge in a zone created by gas exsolution due to elevated temperature beyond the edge of the steam chamber. The results suggest that nonuniform chambers will occur in perfectly homogeneous reservoirs which implies that uniform chambers along wells may be impossible to achieve. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1364–1381, 2016