Gas Exsolution Research Articles

Migration, trapping, and venting of gas in a soft granular material

Gas migration through a soft granular material involves a strong coupling between the motion of the gas and the deformation of the material. This process is relevant to a variety of natural phenomena, such as gas venting from sediments and gas exsolution from magma. Here, we study this process experimentally by injecting air into a quasi-2D packing of soft particles and measuring the morphology of the air as it invades and then rises due to buoyancy. We systematically increase the confining pre-stress in the packing by compressing it with a fluid-permeable piston, leading to a gradual transition in migration regime from fluidization to pathway opening to pore invasion. We find that mixed migration regimes emerge at intermediate confinement due to the spontaneous formation of a compaction layer at the top of the flow cell. By connecting these migration mechanisms with macroscopic invasion, trapping, and venting, we show that mixed regimes enable a sharp increase in the average amount of gas trapped within the packing, as well as much larger venting events. Our results suggest that the relationship between invasion, trapping, and venting could be controlled by modulating the confining stress.

Deep CO2 in the end-Triassic Central Atlantic Magmatic Province

Large Igneous Province eruptions coincide with many major Phanerozoic mass extinctions, suggesting a cause-effect relationship where volcanic degassing triggers global climatic changes. In order to fully understand this relationship, it is necessary to constrain the quantity and type of degassed magmatic volatiles, and to determine the depth of their source and the timing of eruption. Here we present direct evidence of abundant CO2 in basaltic rocks from the end-Triassic Central Atlantic Magmatic Province (CAMP), through investigation of gas exsolution bubbles preserved by melt inclusions. Our results indicate abundance of CO2 and a mantle and/or lower-middle crustal origin for at least part of the degassed carbon. The presence of deep carbon is a key control on the emplacement mode of CAMP magmas, favouring rapid eruption pulses (a few centuries each). Our estimates suggest that the amount of CO2 that each CAMP magmatic pulse injected into the end-Triassic atmosphere is comparable to the amount of anthropogenic emissions projected for the 21st century. Such large volumes of volcanic CO2 likely contributed to end-Triassic global warming and ocean acidification.

Vapor Transport and Deposition of Cu-Sn-Co-Ag Alloys in Vesicles in Mafic Volcanic Rocks

AbstractMetallic sublimates coated by sulfides and chlorides line the vesicle walls of mafic volcanic lava and bombs from Kīlauea, Vesuvius, Etna, and Stromboli. The metallic sublimates were morphologically and compositionally similar among the volcanoes. The highest concentrations of S and Cl occurred on the surface of the sublimates, while internally they had less than 1 wt % S and Cl in most cases, leading us to classify them as alloys. The major components of the alloys were Cu, Sn, Co, and Ag based on electron microprobe analyses and environmental scanning electron microscope element maps. Alloy element maps showed a covariance of Cu-Sn, while Co and Ag concentrations varied independently. Laser ablation-inductively coupled plasma-mass spectrometry analysis of matrix glass and melt inclusions in bombs from Stromboli showed appreciable amounts of Cu, Co, and Sn. We propose a model for the origin of the metallic grains, which involves syneruptive and posteruptive magma degassing and subsequent cooling of the basalt vesicles. During syneruptive vapor phase exsolution, volatile metals (Cu, Co, and Sn) partition into the vapor along with their ligands, S and Cl. The apparent oxygen fugacity (fO2) in these vapor bubbles is low because of the relative enrichment of the exsolved gas phase in H2 relative to H2O in silicate melts, due to the much higher diffusivity of the former in silicate melts. The high fH2 and low fO2 induces the precipitation of metal alloys from the vapor phase. Subsequently, the reducing environment in the vesicle dissipates as the cooling vapor oxidizes and as H2 diffuses away. Then, metal-rich sulfides (and chlorides) condense onto the outer surfaces of the metal alloy grains either due to a decrease in temperature or an increase in fO2. These alloys provide important insights into the partitioning of metals into a magmatic volatile phase at low pressure and high temperature.

Transition from stable column to partial collapse during the 79 cal CE P3 Plinian eruption of Mt. Pelée volcano (Lesser Antilles)

Explosive volcanic eruptions commonly form sustained Plinian columns that collapse at some stage producing dangerous pyroclastic density currents (PDC) on the ground. Numerical and laboratory models of volcanic plumes show that the conditions leading to total column collapse are strongly controlled by the amount of exsolved gas at the source and the mass eruption rate. However, column collapse is rarely total and the volcanic jet often separates in a dense collapsing part feeding PDC and a buoyant rising plume spreading volcanic gases and pyroclasts in the atmosphere. This transitional regime has been directly observed and/or inferred from the structure of the deposits for several past eruptions, but the number of cases for which the partial collapse regime is described in detail, including the 79 CE Vesuvius, and the 186 CE Taupo eruptions, remains too small to fully constrain physical models. Here, we present a detailed reconstruction of the time evolution of the P3 eruption at Mt. Pelée volcano (Martinique, Lesser Antilles) that underwent partial column collapse in order to discuss the mechanisms controlling the eruption dynamics and improve the volcanological database on transitional eruptions. The P3 eruptive succession consists of seven major phases that produced a total of 1 km3 dense rock equivalent (DRE) of deposits (i.e., VEI 5 event), starting with a thick pumice fall deposit (0.1 km3 DRE) overlain by alternating pyroclastic density current (PDC) (0.7 km3 DRE) and pumice fall deposits (0.2 km3 DRE). We use physical models together with field data on deposit dispersal, thickness, and grain-size distribution to reconstruct the dynamical evolution of the volcanic column. Our results show that the mass eruption rate (MER) increased from 1.2 × 108 to 1.7 × 108 kg s−1 during the initial phase producing a 28 to 30 km-high Plinian plume. The MER later reached up to 2.5 × 108 kg s−1 and the column entered the partial collapse regime characterized by the formation of a small (i.e., 12 to 17 km-high) ash plume and contemporaneous PDCs mainly channelized in three paleo-valleys. These estimates are used together with published data on magmatic water contents in glass inclusions to decipher the mechanisms leading to partial collapse. The P3 eruption column collapsed due to an increase in mass eruption rate and a decrease in gas content. A similar evolution was also inferred for the 1300 CE P1 and 280 CE P2 eruption deposits, revealing a systematic behavior in the recent Plinian eruptions of Mt. Pelée volcano. The comparison of model predictions of column collapse and field data reveals a good agreement for the P1, P2, P3 and Taupo eruptions, but not for the 79 CE Vesuvius eruption where thermal disequilibrium between gas and pyroclasts most likely strongly affected the column dynamics.

Sediment damage caused by gas exsolution: A key mechanism for mud volcano formation

Gassy sediments are common in marine environments and are characterized by a specific mechanical behavior significantly different from that of water-saturated sediments. It is shown that gas causes damage and initiates fractures in sediments. To define the controlling parameters dominating the damage process during gas exsolution and its consequences in terms of hydro-mechanical behavior, we developed a specific consolidation apparatus to test sediments collected from the active Absheron Mud Volcano. Indeed, mud volcano formation is initiated by gas exsolution and subsequent mud generation at depth from stratified sediments.Gas was generated in the samples by circulating carbonated water through the fine-grained sediments, then decreasing the total pressure. Particular attention was given to the impact of gas saturation and associated damage and fractures on P-wave velocity, sediment compressibility, permeability and preconsolidation pressure. Results show that fracture geometry is mainly controlled by the degree of gas saturation and preconsolidation pressure. The damage level increases with the degree of gas saturation while the elastic modulus of sediments is degraded. Experimental data show that sediments do not entirely recover their original mechanical behavior when gas dissipates. Finally, the experimental data confirm that gas exsolution and expansion is a key mechanism for mud generation.

Design of Groundwater Heat Pump Systems. Principles, Tools, and Strategies for Controlling Gas and Precipitation Problems

The utilization of groundwater heat pump systems is increasing in Norway, which are currently widely employed for heating and cooling applications in the town center of Melhus. The investigations of the Melhus installations are detecting gas exsolution as a possible trigger for precipitation reaction that causes incrustation of iron and manganese compounds in the systems. This paper discusses risks associated with gas exsolution and considers gas exsolution triggers in a typical Norwegian groundwater heat pump (GWHP) system configuration. The concept of the solubility grade line (SGL) is developed and suggested as a tool for optimizing the design. Based on SGL analysis and the intention of avoiding gas exsolution during heat production, an alternative system design in the same aquifer is presented and compared. The analyses show that the traditional system design is predisposed to gas clogging risks and prone to vacuum pressures in parts of the system. The alternative design mediates the risks by adjusting the well and piping configuration and by applying a backpressure technique. The results demonstrate how the groundwater heat pump system design can be customized according to local aquifer conditions to avoid gas exsolution during operation. It is recommended that the presented method of analysis should be utilized in dimensioning of systems and included in the monitoring scheme of the systems.

Experimental results on the influence of gas on the mechanical response of peats

Direct observation of gas in peat layers, generated by slow degradation in anoxic conditions, raised concern in the Netherlands about its potential impact on the geotechnical response of dykes founded on peat. To address this issue, an experimental investigation was initiated, aimed at quantifying the main consequences of the presence of gas on the mechanical response of peats. The results of a series of triaxial tests on natural peat samples flushed with carbonated water are presented and discussed. Controlled amounts of gas were exsolved by undrained isotropic unloading, and the samples were sheared under undrained conditions. During gas exsolution, the samples suffered volumetric expansion, at a rate which is ruled by the relative compressibility of the fluid and the soil skeleton. The gas in the pore fluid dominates the stress–strain response upon undrained shearing, causing lower excess pore pressure compared to fully saturated samples. The experimental results suggest that local fabric changes occur during gas exsolution. However, for the amounts of gas investigated, these fabric changes seem to be almost reversible upon compression. Although the ultimate shear strength is hardly affected by gas, the reduction in the mobilised shear strength at given axial strain thresholds is dramatic, compared to fully saturated samples. The study suggests that the presence of gas must be cautiously accounted for at low stresses, when a reference stiffness is chosen for serviceability limit states, and when operative shear strength definitions, based on mobilised strength for given strain thresholds, are chosen in the assessment of geotechnical structures on peats.

Effect of ebullition and groundwater temperature on estimated dinitrogen excess in contrasting agricultural environments

Denitrification is a key microbial-mediated reaction buffering the impact of agriculturally-derived nitrate loads. Groundwater denitrification capacity is often assessed by measuring the magnitude and patterns of dinitrogen excess, although this method can be biased by dissolved gasses exsolution and ebullition. To address this issue, shallow groundwater was sampled in two field sites via nested mini-wells on a monthly basis over an entire hydrological year and analysed for dissolved gasses, nitrate and physical parameters. Both sites are located in lowland areas of the Po River basin (Italy) and are characterized by intensive agriculture. The GUA site, a freshwater paleo-river environment, with a low content of organic matter (SOM) and oxic sub-oxic groundwater. The BAN site, a reclaimed brackish swamp environment, with abundant SOM and sulphidic-methanogenic groundwater. Groundwater samples evidenced a general deficit of dinitrogen and Argon concentrations, because of ebullition induced by a total dissolved gasses pressure exceeding the hydrostatic pressure. Ebullition was recorded only during winter at the reclaimed brackish soil and was triggered by methane exsolution. While in summer both sites were affected by ebullition because of the water table drawdown. Denitrification evaluated using dinitrogen excess via dinitrogen-Argon ratio technique, was not only affected by gas exsolution, but also by groundwater temperature fluctuations. In fact, the latter induced large biases in the calculated N2 excess even in the freshwater paleo-river environment. For these reasons, dinitrogen excess estimate with standard methods resulted to be unreliable in both lowland environments and a modified method is here presented to overcome this issue.

Quantification of gas exsolution of alkane solvents-CO2 mixture in heavy oil with consideration of individual interfacial resistance under nonequilibrium conditions

A novel and pragmatic technique has been developed and validated to quantify gas exsolution of alkane solvent(s)–CO2–heavy oil systems with consideration of interface mass transfer for each gas component under nonequilibrium conditions. Experimentally, constant composition expansion (CCE) tests of three alkane solvent(s)–CO2–heavy oil systems are conducted with a visualized PVT cell under equilibrium and nonequilibrium conditions. The liquid height and pressure of the system are continuously monitored and recorded during experiments to measure, respectively, the bubblepoint pressure, pseudo-bubblepoint pressure, and the entrained gas volume. With the assumption of instantaneous nucleation, a mathematical model which integrates Peng-Robinson equation of state (PR EOS), Fick's second law, and nonequilibrium boundary condition has been developed to quantify not only the amount of the evolved gas and entrained gas, but also the dynamic composition of gas phase as a function of time. Once the deviations between the measured gas volumes and the calculated ones have been minimized, the mass transfer Biot number, individual diffusion coefficient, and interface mass transfer coefficient of each gas component as well as the gas bubble number are determined. Increases in experimental temperature and pressure are found to impose opposite effects on diffusion coefficient during gas exsolution processes. The diffusion of each gas component is found to be faster when the temperature becomes higher or the initial pressure becomes lower. Either CO2 or C3H8 diffuses faster than CH4 in the liquid phase under the same condition. In addition, the interface mass transfer coefficients, with an order of CO2 > CH4 > C3H8, obtained in this study are much higher than those collected in the literature since the nonequilibrium conditions greatly facilitate gas exsolution. The determined mass transfer Biot numbers in this study are large, indicating that the bulk resistance due to molecular diffusion inside the heavy oil dominates the gas exsolution process compared to the interfacial resistance.

Petroleum charge history in the Baiyun depression and Panyu lower uplift in the Pearl River Mouth Basin, northern South China Sea: Constraints from integration of organic geochemical and fluid inclusion data

The source, thermal maturity, and charge history of oil and gas condensate in the Baiyun depression and the Panyu lower uplift were determined using organic geochemical and fluid inclusion data. Nearly all the gas condensates and oil in the study area originated from Eocene Enping Formation source rocks, whereas the condensate fluids in the eastern area originated from the Enping and Zhuhai Formations. The condensate fluids have a relatively low thermal maturity with a calculated vitrinite reflectance equivalent that typically ranges from 0.8% to 1.0% based on the C29 sterane isomerization ratios and aromatic hydrocarbon maturity parameters. Four types of oil inclusions with various fluorescence colors (yellow–blue) and API gravities (25°–44°) were recognized in the Oligocene and lower Miocene reservoir units, which suggest multiple oil charging events. The homogenization temperatures of the fluid inclusions recorded the oil charge events accompanying large-scale hot fluid movement, which was most likely caused by the charging of high-temperature natural gas from the deeper Wenchang Formation. The trapping pressures of the oil inclusions and the reconstructed burial histories were used to determine the timing of oil charge. The main oil charge occurred from 16–7 Ma throughout the Baiyun depression and Panyu lower uplift, whereas the Panyu lower uplift experienced an additional oil charge from 5–0 Ma. The natural gas charge occurred during the main oil charging period and may continue to the present day. Multiple physical processes, such as gas injection (without gas exsolution), gas washing (with gas exsolution), and migration fractionation, have played important roles in gas condensate formation and controlling the petroleum distribution in the Baiyun depression and Panyu lower uplift.

Degassing behavior of noble gases from groundwater during groundwater sampling

Noble gases dissolved in groundwater have been used to evaluate characteristics such as origin, residence time and recharge temperature. Although obtaining samples without degassing is highly desired, in certain cases, degassing during sampling is unavoidable. This study investigated the degassing behavior of noble gases during sampling and evaluated the correction methods that can be applied. Groundwater samples were obtained from an underground laboratory and a deep borehole. The amount of exsolved gas was estimated by comparing pressurized and non-pressurized samples. Henry's model, which assumes equilibrium between gas and water, and a diffusion model were applied to the results. In addition, the compositions of gases extracted into an evacuated vessel or hollow fiber contactor were also compared with those of pressurized groundwater samples. The results indicated that significant degassing occurred in unpressurized samples, but equilibrium was achieved between gas and water when the total dissolved gas pressure (TDGP) was relatively small (up to 4.2 atm). Therefore, the d’Ne value, defined as the difference between the Ne concentration of air-saturated water and a sample normalized by the Ne concentration of air-saturated water, could be used in Henry's model to correct exsolution. However, inclusion of the effects of excess air and contamination is problematic. However, when TDGP was high (32 atm in this study), the degassing behavior completely changed and could not be predicted by Henry's model. During extraction of noble gases using an evacuated vessel and hollow fiber contactor, degassing was observed to take place following the diffusion model. As a result, fractionation between Ne vs. Kr and Xe was significant. Thus, although we could extract dissolved Ne with a high recovery ratio (over 0.85), the ratios of Kr/Ne and Xe/Ne were found to be significantly smaller than those in groundwater. The gases extracted by an evacuated vessel or hollow fiber contactor appeared to be applicable only for evaluating He/Ne.

Evolution Model for the Absheron Mud Volcano: From In Situ Observations to Numerical Modeling

AbstractThe morphology of mud volcanoes (MVs) has been extensively studied over the last few decades. Although recent research has begun to focus on deep processes and structures, little is known about mud generation mechanisms. This study aims to investigate the feeder system and formation of an active kilometer‐scale MV by relying on a 3‐D seismic survey and an in situ data set on the Absheron anticline (South Caspian Basin). Seismic data show a depleted area in the Anhydritic Surakhany Formation (ASF), whose mineralogical composition fits with surface mud. Well data show that the ASF is a succession of evaporitic beds and low‐temperature shales near its fracture pressure. Biostratigraphic analysis confirms a Pliocene origin for the mud, suggesting that the ASF may be the source. Numerical modeling of sedimentation coupled with laboratory test results and well sonic logs fairly reproduces the observed in situ overpressure trend. Two‐dimensional methane diffusion coupled with overpressure caused by rapid sedimentation highlights the superposition of critical fracturing conditions with methane‐saturated sediments at the base of the studied MV. The present study demonstrates the predominant role of fluid overpressure due to sedimentation and gas saturation in the formation of the Absheron MV, and this is shown to occur as follows: (1) methane migration through the thrust‐related faults reaching the ASF, accompanied by (2) lateral overpressure, caused by rapid sedimentation, diffusing along the ASF leading to (3) hydro‐fracturing of overpressured and methane‐saturated sediments resulting in an important decrease in overpressure, causing (4) gas exsolution and expansion triggering sediment remobilization.

Relative Permeability Measurements during the Exsolution and Dissolution of Hydrogen Gas Produced by the Hydrolysis of Sodium Borohydride

Core Ideas Aqueous relative permeability experiments followed gas exsolution and trapping. Hydrogen gas was produced by the reaction of sodium borohydride and water. Internal drainage, external drainage and imbibition, and dissolution were compared. Trapped gas saturations were higher following internal drainage. Aqueous relative permeability was similar for internal and external displacement. The presence of trapped gas in otherwise water‐saturated porous media has a substantial effect on water flow, which is important to a wide variety of subsurface applications including groundwater remediation, CO2 sequestration, and the release of greenhouse gases associated with oil and gas development. Trapped gases can be created by external drainage and imbibition, where fluids invade from system boundaries, or by internal drainage (exsolution), where gas is produced at nucleation sites within a porous medium. A series of laboratory experiments were conducted in thin flow cells, in which H2 gas was produced by the reaction of sodium borohydride and water to produce a range of trapped gas saturations. Aqueous relative permeability was measured following H2 exsolution and during dissolution and compared with measurements in the presence of air trapped by external drainage and imbibition. The results showed that gas saturations higher than those produced by external displacement remained trapped (immobile) when produced by exsolution but that aqueous relative permeability was similar at the same gas saturations. The results of this study suggest that traditional aqueous relative permeability relationships based on external drainage may be suitable to describe the effects of internal drainage in uniform, loosely packed sands. However, additional investigation across a wider range of porous media and at higher gas saturations is required.

Effectiveness of supercritical-CO2 and N2 huff-and-puff methods of enhanced oil recovery in shale fracture networks using microfluidic experiments

Current oil recovery methods in hydraulically fractured shale reservoirs have a low recovery efficiency of about 10%. The objective of this work is to investigate the effectiveness of nitrogen and supercritical carbon dioxide in the huff-and-puff method for enhanced oil recovery as a means to re-energize reservoirs and improve recovery rates. We conduct direct visualization experiments with a microfluidic system to reveal the mechanisms and to quantify the recovery rates of oil from fracture networks. We compared the effectiveness of water, nitrogen and supercritical carbon dioxide at reservoir conditions in a process mimicking the huff-and-puff method in both dead-end and connected fracture systems. The microfluidic chips were made of glass and placed in a confining pressure system pressurized to 10 MPa, 50 °C. The system was allowed to equilibrate, and then depressurized to simulate huff-and-puff oil recovery. Fluorescence microscope images were continuously taken to visualize and calculate residual oil saturation as a function of pressure drawdown. As the system was depressurized from 10 MPa, gas exsolution from the oil liquid phase, including bubble nucleation, growth, and coalescence, appeared to be the main driver for mobilizing oil from the fracture networks. Injection of supercritical CO2 resulted in the highest recovery rate with an average end-point recovery of about 90% in the connected fracture network and 60% in the dead-end fracture network. N2 has lower solubility in oil and hence showed a lower recovery rate of 40% in the connected fracture network and 25% in the dead-end fracture network. Injection of water had no effect on oil mobilization since water is insoluble, immiscible and incompressible. The main mechanism of enhanced recovery was gas exsolution from the liquid phase as pressure was decreased below the bubble point pressure. Because the gas was distributed throughout the oil phase, bubble nucleation, growth, coalescence, and elongation occurred throughout the fracture network. Expansion of the gas forced oil out of the network through piston displacement in continuous oil areas and film flow in the dispersed oil areas. Bubbles began as spheres and grew until they touched the fracture walls where they elongated along the fracture length. The bubble growth rate depended on local mass transfer from liquid to gas phase and gas volume expansion due to pressure drop. The efficiency of the huff-and-puff process is dependent on the solubility and miscibility of the injection fluid with oil. High gas solubility allows for more bubble nucleation, growth and expansion during the depressurization cycle.

Pressure core analysis of geomechanical and fluid flow properties of seals associated with gas hydrate-bearing reservoirs in the Krishna-Godavari Basin, offshore India

Physical properties of the sediment directly overlying a gas hydrate reservoir provide important controls on the effectiveness of depressurizing that reservoir to extract methane from gas hydrate as an energy resource. The permeability of overlying sediment determines if a gas hydrate reservoir's upper contact will provide an effective seal that enables efficient reservoir depressurization. Compressibility, stiffness and strength indicate how overlying sediment will deform as the in situ stress changes during production, providing engineering data for well designs. Assessing these properties requires minimally-disturbed sediment. India's National Gas Hydrates Program Expedition 2 (NGHP-02) provided an opportunity to study these seal sediment properties, reducing disturbance from gas exsolution and bubble growth by collecting a pressure core from the seal sediment just above the primary gas hydrate reservoir at Site NGHP-02-08 in Area C of the Krishna-Godavari Basin. The effective stress chamber (ESC) and the direct shear chamber (DSC) devices in the suite of Pressure Core Characterization Tools (PCCTs) were used to measure permeability, compressibility, stiffness and shear strength at the in situ vertical stress. Geotechnical properties of the predominantly fine-grained seal layer at in situ vertical stress are in typical clay sediment ranges, with low measured permeability (0.02 mD), high compressibility (Cc = 0.26–0.33) and low shear strength (404 kPa). Though pressure and temperature were maintained throughout the collection and measurement process to stabilize gas hydrate, the lack of effective stress in the pressure core storage chamber and the chamber pressurization with methane-free water caused core expansion and gas hydrate in a thin coarser-grained layer to dissolve. The PCCTs can reapply in situ stress with incremental loading steps during a consolidation test to account for sediment compaction. Gas hydrate dissolution can be limited by storing cores just above freezing temperatures, and by using solid spacers to reduce the storage chamber's free volume.

Use of air to improve the efficiency of foamy flow and reservoir pressurization in heavy oil recovery

Primary recovery of heavy-oil is remarkably low due to high viscosity and low energy by solution gas exsolution to drive the oil. Gas injection to improve foamy flow and also to dilute the oil in such reservoirs has been proposed as a secondary recovery method. However, because of the high costs of injected gases, efforts are needed to optimize the process by selection of proper gas type (or gas combinations) and suitable injection scheme. To achieve this goal, an experimental procedure was followed with rigorous analyses of the output. A 1.5 m long and 5 cm diameter sand-pack was first saturated with brine, which was replaced with dead oil. Then, gas solvents were injected to dead-oil containing core-holder until nearly reaching 500 psi followed by a two-day soaking period. Pressures all along the sand-pack were recorded with eight pressure transducers. Different combinations of various gas solvents (methane, CO2, and air) aiming to select the most competitive and economic formula were tested with a certain set of pressure depletion rates.The physics of the foamy oil flow for different solvent mixtures and depletion conditions were analyzed using pressure profiles acquired, recorded oil/gas data with time, and gas chromatography and SARA analyses of the produced gas and oil. Three huff-n-puff cycles were applied. Compared with other light hydrocarbon solvents and carbon dioxide, air has a significant advantage in terms of accessibility and lowered cost. Hence, attention was given to air mainly used to pressurize the system and increase oil viscosity due to oxidation process with an expectation of better foam quality when injected with other gases such as CO2 and methane. Methane (CH4) yielded the quickest response in terms of gas drive but, in the long run, CO2 was observed to be more effective technically. Air was observed to be effective if mixed with CO2 or methane from an economics point of view. To sum up the results, air Huff-n-Puff (HnP) followed by 2-cycles of CH4 HnP yielded 36.21% recovery, while air HnP followed by 2-cycles of CO2 HnP delivered 30.36% oil. When the gases are co-injected, air 50%-CO2 50% and air 50%-CH4 50% recovered 29.85% and 23.74% of total oil-in-place, respectively.

Tracing enhanced oil recovery signatures in casing gases from the Lost Hills oil field using noble gases

Enhanced oil recovery (EOR) and hydraulic fracturing practices are commonly used methods to improve hydrocarbon extraction efficiency; however, the environmental effects of such practices remain poorly understood. EOR is particularly prevalent in oil fields throughout California where water resources are in high demand and the disposal of large volumes of produced water may affect groundwater quality. Consequently, it is essential to better understand the fate of injected (EOR) fluids in California, and other subsurface petroleum systems, as well as any potential effect on nearby aquifer systems. Noble gases can be used as tracers to understand hydrocarbon generation, migration, and storage conditions, as well as the relative proportions of oil and water present in the subsurface. In addition, a noble gas signature diagnostic of injected (EOR) fluids can be readily identified. We report noble gas isotope and concentration data in casing gases from oil production wells in the Lost Hills oil field, northwest of Bakersfield, California, and injectate gas data from the Fruitvale oil field, located within the city of Bakersfield. Casing and injectate gas data are used to: 1) establish pristine hydrocarbon noble-gas signatures and the processes controlling noble gas distributions, 2) characterize the noble gas signature of injectate fluids, 3) trace injectate fluids in the subsurface, and 4) construct a model to estimate EOR efficiency. Noble gas results range from pristine to significantly modified by EOR, and can be best explained using a solubility exchange model between oil and connate/formation fluids, followed by gas exsolution upon production. This model is sensitive to oil–water interaction during hydrocarbon expulsion, migration, and storage at reservoir conditions, as well as any subsequent modification by EOR.

Laboratory measurements of seismic attenuation and Young's modulus dispersion in a partially and fully water‐saturated porous sample made of sintered borosilicate glass

ABSTRACTWe measured the extensional‐mode attenuation and Young's modulus in a porous sample made of sintered borosilicate glass at microseismic to seismic frequencies (0.05–50 Hz) using the forced oscillation method. Partial saturation was achieved by water imbibition, varying the water saturation from an initial dry state up to ∼99%, and by gas exsolution from an initially fully water‐saturated state down to ∼99%. During forced oscillations of the sample effective stresses up to 10 MPa were applied. We observe frequency‐dependent attenuation, with a peak at 1–5 Hz, for ∼99% water saturation achieved both by imbibition and by gas exsolution. The magnitude of this attenuation peak is consistently reduced with increasing fluid pressure and is largely insensitive to changes in effective stress. Similar observations have recently been attributed to wave‐induced gas exsolution–dissolution. At full water saturation, the left‐hand side of an attenuation curve, with a peak beyond the highest measured frequency, is observed at 3 MPa effective stress, while at 10 MPa effective stress the measured attenuation is negligible. This observation is consistent with wave‐induced fluid flow associated with mesoscopic compressibility contrasts in the sample's frame. These variations in compressibility could be due to fractures and/or compaction bands that formed between separate sets of forced‐oscillation experiments in response to the applied stresses. The agreement of the measured frequency‐dependent attenuation and Young's modulus with the Kramers–Kronig relations and additional data analyses indicate the good quality of the measurements. Our observations point to the complex interplay between structural and fluid heterogeneities on the measured seismic attenuation and they illustrate how these heterogeneities can facilitate the dominance of one attenuation mechanism over another.

Ensemble-Based Assisted History Matching With 4D-Seismic Fluid-Front Parameterization

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 183901, “Ensemble-Based Assisted History Matching With 4D-Seismic Fluid-Front Parameterization,” by Mario Trani, Konrad Wojnar, Arthur Moncorgé, and Philippe Berthet, Total, prepared for the 2017 SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, 6–9 March. The paper has not been peer reviewed. An ensemble-based 4D-seismic history-matching case is presented in the complete paper. Seismic data are reparameterized as distance to a 4D anomaly front and assimilated with production data. This study shows that adding the 4D reparameterized seismic data in addition to the production data keeps a reasonable match with production data while constraining the overall gas distribution in the reservoir to the observed seismic data. Introduction The complex construction of a petro-elastic model renders the use of quantitative seismic data in history-matching work flows quite challenging. Several authors have investigated quantitative approaches for incorporating a large number of seismic data into history-matching work flows for production data. The work flows adopted in most of these studies require significant reduction of the uncertainty space or produce on a single history-matched model. In recent years, an algorithm that has gained increased popularity is the ensemble Kalman filter (EnKF). The EnKF and its derived algorithms are called ensemble methods, and their most notable characteristic is being computationally feasible for large systems and being relatively easy to implement. Several authors have investigated the effect of assimilating 3D- or 4D-seismic data with EnKF and have had to face the problem of building an accurate petroelastic model. To circumvent this problem, some authors have reparameterized 4D-seismic anomaly-front data into arrival times. Despite the advantage of eliminating seismic inversion, this method presents the disadvantage of at least doubling the simulation time for accurate ensemble arrival-time predictions. In the complete paper, the assisted history matching is performed on a large turbiditic field with the ensemble method to assimilate production and 4D-seismic data by use of the distance-to-front parameterization. The field is a large turbiditic body, with initial fluid pressure close to the bubblepoint. Oil production causes the pressure to fall below the bubblepoint in the very early life of the reservoir, leading to a widespread gas exsolution. The time-lapse change in gas saturation is considered the only factor responsible for the observed negative relative changes in seismic velocity seen over the entire reservoir. There is water injection occurring, but with a local effect, and it is therefore neglected. The innovation of this study is that the distance-to-front parameterization is applied to the gas phase, which can appear everywhere in the field, rather than coming from an injection source. Another innovation of this study is that the binarization of the simulated time-lapse anomaly is performed without use of a petroelastic model, which would be necessary to relate the measurements to fluid-property changes and to decide a threshold for binarizing observations and pressure. However, the effect of gas is so widespread and evident that the petroelastic model can be replaced by a clustering approach based on the gas-saturation change of the reservoir cells.

The role of substrate characteristics in producing anomalously young crater retention ages in volcanic deposits on the Moon: Morphology, topography, subresolution roughness, and mode of emplacement of the Sosigenes lunar irregular mare patch

AbstractLunar irregular mare patches (IMPs) comprise dozens of small, distinctive, and enigmatic lunar mare features. Characterized by their irregular shapes, well‐preserved state of relief, apparent optical immaturity, and few superposed impact craters, IMPs are interpreted to have been formed or modified geologically very recently (<~100 Ma; Braden et al. ). However, their apparent relatively recent formation/modification dates and emplacement mechanisms are debated. We focus in detail on one of the major IMPs, Sosigenes, located in western Mare Tranquillitatis, and dated by Braden et al. () at ~18 Ma. The Sosigenes IMP occurs on the floor of an elongate pit crater interpreted to represent the surface manifestation of magmatic dike propagation from the lunar mantle during the mare basalt emplacement era billions of years ago. The floor of the pit crater is characterized by three morphologic units typical of several other IMPs, i.e., (1) bulbous mounds 5–10 m higher than the adjacent floor units, with unusually young crater retention ages, meters thick regolith, and slightly smaller subresolution roughness than typical mature lunar regolith; (2) a lower hummocky unit mantled by a very thin regolith and significantly smaller subresolution roughness; and (3) a lower blocky unit composed of fresh boulder fields with individual meter‐scale boulders and rough subresolution surface texture. Using new volcanological interpretations for the ascent and eruption of magma in dikes, and dike degassing and extrusion behavior in the final stages of dike closure, we interpret the three units to be related to the late‐stage behavior of an ancient dike emplacement event. Following the initial dike emplacement and collapse of the pit crater, the floor of the pit crater was flooded by the latest‐stage magma. The low rise rate of the magma in the terminal stages of the dike emplacement event favored flooding of the pit crater floor to form a lava lake, and CO gas bubble coalescence initiated a strombolian phase disrupting the cooling lava lake surface. This phase produced a very rough and highly porous (with both vesicularity and macroporosity) lava lake surface as the lake surface cooled. In the terminal stage of the eruption, dike closure with no addition of magma from depth caused the last magma reaching shallow levels to produce viscous magmatic foam due to H2O gas exsolution. This magmatic foam was extruded through cracks in the lava lake crust to produce the bulbous mounds. We interpret all of these activities to have taken place in the terminal stages of the dike emplacement event billions of years ago. We attribute the unusual physical properties of the mounds and floor units (anomalously young ages, unusual morphology, relative immaturity, and blockiness) to be due to the unusual physical properties of the substrate produced during the waning stages of a dike emplacement event in a pit crater. The unique physical properties of the mounds (magmatic foams) and hummocky units (small vesicles and large void space) altered the nature of subsequent impact cratering, regolith development, and landscape evolution, inhibiting the typical formation and evolution of superposed impact craters, and maintaining the morphologic crispness and optical immaturity. Accounting for the effects of the reduced diameter of craters formed in magmatic foams results in a shift of the crater size–frequency distribution age from <100 Myr to billions of years, contemporaneous with the surrounding ancient mare basalts. We conclude that extremely young mare basalt eruptions, and resulting modification of lunar thermal evolution models to account for the apparent young ages of the IMPs, are not required. We suggest that other IMP occurrences, both those associated with pit craters atop dikes and those linked to fissure eruptions in the lunar maria, may have had similar ancient origins.