Understanding the kinetics of CO2 hydrate formation and the resulting saturation of the hydrate in porous rocks is crucial for processes such as the storage of CO2 in underground formations. Nevertheless, to date, there is no established procedure that utilizes a medical CT scanner for quantifying gas hydrate saturation in core samples during the growth stage. This study proposes a methodology for estimating hydrate saturation using a medical CT scanner during the injection of CO2 into porous media. This method uses the mean area obtained from the image analysis to calculate the dynamic profile of water and the CO2 hydrate along the length of the sandstone core. To demonstrate the technique, core flooding experiments were conducted to form gas hydrates in semibrine-saturated sandstone cores.@en

The efficiency of oil processes depends on the product of volumetric sweep and microscopic sweep. In oil recovery by steam injection the microscopic sweep is generally good; however, obtaining a good volumetric sweep can be challenging. This is caused by low density and viscosity of the injected steam combined with the reservoir heterogeneity, in particular existence of thief zone. Consequently, the steam utilization factor measured by steam-to-oil ratio (SOR, kg steam/bbl of oil) for many steam-flooding projects becomes poor. All these issues can be addressed by a successful application of steam foam technology. In steam foam applications, steam (plus a non-condensing gas) is injected simulateneously with a surfactant solution. Under the favorable injection conditions a foam is formed inside the reservoir leading to significant reduction of steam mobility and can eventually improve sweep efficiency. In the literature many successful steam foam pilots have been reported. However, most of these applications are at relatively shallow reservoirs with low pressures and thus low temperatures. In our paper we investigate if steam foam can also be effectively used for applications at high steam temperatures, significantly exceeding 200°C. To test the viability of steam foam technology at high temperatures, we have tested the stability of multiple surfactants at reservoir conditions. For those surfactants that showed good stability, core flood tests have been carried out to test the ability to form foam and to assess the resulting foam strength. Steam foam tests have also been carried out at temperature up to 240°C.

@en

An understanding of how CO2 foam flows through a reservoir rock is useful for many subsurface applications, including enhanced oil recovery and CO2 storage. There are economic and environmental benefits in identifying surfactants that exhibit good foaming behavior with CO2 at both low concentrations and high foam qualities. Core flood experiments have been carried out to investigate the behavior of supercritical CO2 foams flowing through a high-permeability Indiana Limestone. The foaming behavior and concentration response of two surfactants, a betaine and a sultaine, were investigated. For the two surfactants, the transition foam quality and the maximum apparent foam viscosity both decreased with reducing surfactant concentration. A comparison between the foaming behaviors of these surfactants with CO2 and N2 was also carried out. It was found that the N2 generated stronger foam at low foam qualities, but the CO2 was better at maintaining good foaming behavior at high foam qualities.

@en

The efficiency of the dimethyl ether (DME) enhanced oil recovery (EOR) technique in a fractured chalk reservoir core plug was investigated. The coreflood experiment showed that DME EOR could lead to 44.2% additional oil recovery, amounting to 80.6% of the ultimate oil recovery. A comprehensive set of laboratory experiments, including density measurements of miscible fluids, DME-induced oil swelling factor, and partition coefficient of DME between the aqueous and oleic phase, were performed. The experimental results show that the partition coefficient of DME for the mixture of DME-brine-oil can reach up to 18.3. The oil swelling factor for such a system can reach up to 2.7 under realistic reservoir conditions. Comparing this data set to the available data for other mutually soluble solvent-based EOR techniques shows that the oil swelling caused by DME is far stronger than for other common solvents. Due to the strong partitioning of DME between the phases, the DME from the DME-brine solution rapidly partitions into the bypassed oil in the low permeability matrix, which leads to strong oil swelling and production.

@en

Foam structure evolves with time due to gas diffusion between bubbles (coarsening). In a bulk foam, coarsening behaviour is well defined, but there is less understanding of coarsening in confined geometries such as porous media. Previous predictions suggest that coarsening will cause foam lamellae to move to low energy configurations in the pore throats, resulting in greater capillary resistance when restarting flow. Foam coarsening experiments were conducted in both a model-porous-media micromodel and in a sandstone core. In both cases, foam was generated by coinjecting surfactant solution and nitrogen. Once steady state flow had been achieved, the injection was stopped and the system sealed off. In the micromodel, the foam coarsening was recorded using time-lapse photography. In the core flood, the additional driving pressure required to reinitiate flow after coarsening was measured. In the micromodel the bubbles coarsened rapidly to the pore size. At the completion of coarsening the lamellae were located in minimum energy configurations in the pore throats. The wall effect meant that the coarsening did not conform to the unconstricted growth laws. The coreflood tests also showed coarsening to be a rapid process. The additional driving pressure to restart flow reached a maximum after just 2 minutes.

@en

Gas trapping is an important mechanism in both Water or Surfactant Alternating Gas (WAG/SAG) and foam injection processes in porous media. Foams for enhanced oil recovery (EOR) can increase sweep efficiency as they decrease the gas relative permeability, and this is mainly due to gas trapping. However, gas trapping mechanisms are poorly understood. Some studies have been performed during corefloods, but little work has been carried out to describe the bubble trapping behaviour at the pore scale. We have carried out foam flow tests in a micromodel etched with an irregular hexagonal pattern. Image analysis of the foam flow allowed the bubble centres to be tracked and local velocities to be obtained. It was found that the flow in the micromodel is dominated by intermittency and localized zones of trapped gas. The quantity of trapped gas was measured both by considering the fraction of bubbles that were trapped (via velocity thresholding) and by measuring the area fraction containing immobile gas (via image analysis). A decrease in the quantity of trapped gas was observed for both increasing total velocity and increasing foam quality. Calculations of the gas relative permeability were made with the Brooks Corey equation, using the measured trapped gas saturations. The results showed a decrease in gas relative permeabilities, and gas mobility, for increasing fractions of trapped gas. It is suggested that the shear thinning behaviour of foam could be coupled to the saturation of trapped gas.

@en

Gas injection was introduced to the petroleum industry in the early 1950s. Nevertheless, the process efficiency is impacted by the low density and viscosity of the gas, which decrease sweep efficiency. Foam for Enhanced Oil Recovery (EOR) can overcome the downside of the viscous fingering by increasing the apparent viscosity of the gas. Importantly, the structure of the foam evolves with time due to gas diffusion between bubbles (coarsening). In a bulk foam, the coarsening behaviour is well defined, but there is a lack of understanding of coarsening behaviour in confined geometries, especially in porous media. Nonnekes et al [2014] predicted numerically and analytically that coarsening will cause the foam lamellae to move to low energy configurations in the pore throats, resulting in greater capillary resistance when trying to restart flow. This study describes foam coarsening in a porous medium and the implications for foam propagation. Foam coarsening experiments have been conducted in both a micromodel and in a rock core. The micromodel is etched with an irregular hexagonal pattern, with a Gaussian distribution of pore diameters. Foam was generated by coinjecting surfactant solution and nitrogen gas into the micromodel. Once steady state flow had been achieved, the flow was stopped. The coarsening behaviour of the foam was recorded using time-lapse photography. The core flood coarsening experiments were carried out using a Bentheimer Sandstone core. Foam was produced by coinjecting surfactant solution and nitrogen at the base of the core. Once a steady state flow was achieved, the flow was stopped and the core sealed off. When flow restarted, the additional driving pressure required to reinitiate flow was measured, and this could be attributed to the stable configuration of the coarsened foam. The microfluidic results found that the bubbles coarsened rapidly (t < 10 minutes) to the size of the pores. At the completion of coarsening the majority of the lamellae were located in the pore throats with minimum length. Because of the effect of the walls, the behaviour did not conform to the unconstricted coarsening growth laws. Furthermore, results on coreflood showed that coarsening is a rapid process, in agreement with microfluidic results. An increase in the additional pressure required to re-initiate flow was observed for the first 1 – 5 minutes of flow stoppages, while the pressure peaks did not increase for durations above 5 min. The implications of this behaviour for the field scale are also discussed. @en

Foams for enhanced oil recovery can increase sweep efficiency, as they decrease the gas relative permeability, mainly due to gas trapping. However, gas trapping mechanisms are poorly understood. Some studies have been performed during corefloods, but little work has been carried out to describe the bubble trapping behaviour at the pore scale. Microfluidic experiments are a useful tool for studying the foam flow behavior at the pore scale. We have carried out foam flow tests in a model porous media glass micromodel. Image analysis of the foam flow allowed local velocities to be obtained. The quantity of trapped gas was measured both by considering the fraction of bubbles that were trapped (via velocity thresholding) and by measuring the area fraction containing immobile gas (via image analysis). A decrease in the trapped gas fraction was observed both for increasing total velocity and for increasing foam quality. Calculations of the gas relative permeability were made with the Brooks Corey equation, using the measured trapped gas saturations. The results showed a decrease in gas relative permeabilities for increasing fractions of trapped gas. It is suggested that the shear thinning behaviour of foam could be coupled to the saturation of trapped gas. @en

We present a comparative study of foam coreflood experiments with various surfactant concentrations. Plots of apparent viscosity vs. injected gas fraction were obtained for surfactant concentrations at the critical micelle concentration and above. Bulk foam stability was measured for all concentrations and compared with the coreflood results. There were different responses to surfactant concentration in bulk and in corefloods. The coreflood results were matched with an implicit-texture foam model, and the dependency of the model parameters on the surfactant concentration is discussed. Fitting the data requires relating the surfactant concentration to the dry-out function or the limiting capillary pressure. @en

Aqueous foams play an important role in many industrial processes, from ore separation by froth flotation to enhanced oil recovery (EOR), where the foam is used as a means of increasing sweep efficiency through oil-bearing rock. The complex, structure-dependent, flow behavior of the foam gives improved penetration of lower-permeability regions. Foam is stabilized by surfactant molecules, and the foam strength is influenced by the surfactant concentration in the water phase. It is therefore of great importance to understand the effect of surfactant concentration on foam processes. Implicit Texture (IT) foam models eg STARS account for the surfactant effect with functions that depend on surfactant concentration in the water and a few other parameters. However, there is no evidence that these functions are able to capture adequately the effect of surfactant concentration effect. We present a comparative study of foam core-flood experiments with various surfactant concentrations. Core-flood tests were conducted in rock cores with a diameter of 1 cm and length of 17cm, significantly smaller than typical cores. Plots of apparent viscosity vs. injected gas fraction were obtained for surfactant concentrations at the critical micellar concentration (CMC) and above. Bulk foam stability and surface tension were measured for all concentrations, in order to define the CMC and to compare with coreflood results. The experimental results have been matched with the STARS IT foam model and the dependency of model parameters on the surfactant concentration is discussed. This work found that the IT model is not able to predict the decrease of the foam strength with decreasing surfactant concentration. Instead, the study shows that the effect of surfactant concentration can be correlated with the dry-out function of the IT model, and specifically to the limiting capillary pressure.@en

tAqueous foams play an important role in many industrial processes, from ore separation by froth flotationto enhanced oil recovery (EOR). In the latter case, the foam is used as a means of increasing the sweepefficiency through the oil bearing rock – the complex, structure dependent, flow behavior of the foammeans that it has improved penetration of lower permeability regions than would be obtained with aNewtonian fluid. An understanding of how foam behaves when flowing through a rock is therefore of greatimportance when selecting suitable surfactants for EOR processes. Previous tests have suggested thatthere is no reliable correlation between bulk foam behavior and foam behavior in a rock core, especially inthe presence of oil. We present a comparative study of bulk stability tests and core floods with foam, bothwith and without oil. Core-flood tests were conducted in rock cores with a diameter of 1 cm, significantlysmaller than typical cores. Apparent viscosity/injected gas fraction response curves were obtained, bothwith and without oil in the system.The current work finds that, in the absence of oil, there is a positive correlation between bulk foamstability and core-flood performance. Bulk foam experiments can therefore be a useful screening tool togive a good indication of the surfactant performance in the core flood. However, in the presence of oil,although there was a general trend of increasing maximum apparent viscosity with increasing bulk foamstability, no strong correlation was found between bulk foam stability and the performance in the corefor the experiments performed.@en