In this paper, we present a dynamic microfluidic tensiometer able to perform measurements over more than four decades and which is suitable for high throughput experimentations. This tensiometer is able to withstand hard conditions such as high pressure, high temperature, high salinity, and crude oil. It is made of two coaxial capillaries in which two immiscible fluids are injected. Depending on the flow rate of each phase, either droplets or jetting will be obtained. The transition between these two regimes relies on the Rayleigh-Plateau instability. This transition can be theoretically computed thanks to a linear analysis based on the convective and absolute instabilities theory. From this model, the interfacial tension between the two phases can be calculated.

Abstract In the case of surfactant EOR, an optimum formulation of surfactant has to be injected in the reservoir. This so-called optimum formulation corresponds to a minimum in the interfacial tension and a maximum in oil recovery and may be obtained with an appropriate balance of the hydrophobic and hydrophilic affinities of the surfactant. Salinity—scan tests are generally used to screen phase behavior of surfactant formulations before conducting time-consuming coreflood tests. The objective of this study was to develop a high-throughput dynamic microfluidic tensiometer, with the aim of studying interfacial phenomena between EOR injected formulations and crude oils and of optimizing chemical EOR processes for pilot or field applications. We have selected a method based on the Rayleigh-Plateau instability and the analysis of the droplets to jetting transition in a coaxial flow of two fluids. In fact, in coaxial flows, the transition between a droplet and a jetting regime depends on the velocities of each phase, the viscosity ratio, the confinement and the interfacial tension (IFT). As the three first parameters are known, the dynamic interfacial tension can be calculated. This microfluidic device has been specifically designed to support high temperatures (up to 150°C), high pressures (up to 150 bars) and is compatible with complex fluids such as crude oils and solutions of surfactants and polymers. The method was first developed and validated on a microfluidic device on model fluids at ambient temperature and atmospheric pressure for IFTs higher than 1 mN/m. It was then successfully applied for the measurement of IFTs over more than four decades. Measurements were also performed with a crude oil and a typical surfactant formulation. The validation of the HP/HT assembly, which has been designed with the aim to work in reservoir conditions, is currently under progress. By using this tensiometer, it would be quite easy to perform in short time numerous salinity scans on real systems in order to get the evolution of IFT and determine the optimal salinity S*.

In this paper, we report an experimental study of foam flow in different channel geometries using microfluidic devices in the framework of Enhanced Oil Recovery (EOR). Two different processes of foam formation are studied. The first corresponds to co-injection of gas and water through a cross junction which gives rise to a monodisperse foam. The second one corresponds to the fragmentation of large bubbles by a porous media, a foam formation process simulating multiphase flows in rocks. The foam formation is completely controlled and characterized varying both the water and gas pressure applied. We also use a microdevice with two permeabilities that permits to highlight the diversion of the continuous phase in the low permeability channels. The observations are important for a better understanding of the implied phenomena in EOR as well as to determine pertinent data to feed flow simulators.

Foam injection is one of the processes that may be used for enhanced oil recovery, in particular for gas mobility control. The work we present is focussed on understanding foam formation and foam flow at small scale in very simple model geometries. To this purpose, a microfluidic device is used to accurately control diphasic flows confined at 10 to 100 micrometer scale. The microfluidic device is made of polydimethylsiloxane (PDMS), a transparent elastomer allowing simple and fast prototyping and easy observation of flows with optical microscopy. The experimental method is based on a microsystem that first permits formation of a dispersion of very monodisperse gas bubbles in water and surfactant at a flow-focusing geometry. The flow and the behaviour of the bubbles downstream is as well observed and measured in the model geometries including chamber, channels, etc. Complete phase diagram of the foam formed is shown, from very wet to very dry (small or high gas volume fraction) and with small or large bubbles according to pressures applied of both the gas and the aqueous phase. Image analysis is used to characterize the foam structure (quality, bubble size, bubble monodispersity). A simple analysis is done to give a criterion for the foam to be formed according to geometry, surface tension and pressures applied to the fluids. First results show that in a geometry modelling two permeabilities with simple large and small channels, fluid flow may be redirected from high permeabilities areas to small permeabilities ones. Even if obtained without any oil, this observation may be compared to what happen to foam flows in fractured rocks and may explain part of the complex phenomena involved for increase oil recovery. To conclude, microfluidic tool appears as an interesting technique to characterize the behaviour of foam at the micrometric scale.

We studied the displacement of the interface between a viscoelastic fluid pushing a simple liquid through a rectangular contraction by following the front interface deformation in time. The progressive deformation of the interface until apparent stabilization is followed, which makes it possible to identify a transient and a stationary regime. For low Weissenberg number the shape of the interface is essentially similar to that between two simple liquids. For sufficiently large Weissenberg number the shape of the interface is different: it is narrower before the entrance and wider just after the exit. The characteristics of this shape are qualitatively analogous to those of the interface between the vortices and the convected regions for the flow of a single viscoelastic fluid through a contraction-expansion. This suggests that the entrance effect is due to extensional effects and the exit effect is due to normal stress effects. (C) 2013 Elsevier B.V. All rights reserved.

We consider the displacement, in a rectangular channel, of a Newtonian oil pushed by different types of liquids (Newtonian, shear-thinning, viscoelastic) of slightly higher apparent viscosity. In the absence of viscoelastic effects the interface between the two fluids becomes sharper at larger velocities, so that the thickness of the lateral film left behind increases with the flow rate. On the contrary, with a viscoelastic fluid, the shape of the interface is almost independent of the velocity so that the thickness of the lateral film is approximately constant. Moreover this thickness decreases when the ratio of normal to tangential stresses increases, suggesting that this effect can be attributed to normal stress differences. A heuristic theoretical approach tends to confirm this statement.

Based on molecular-dynamics simulations and experimental data, a new coarse-grained forcefield is proposed for the polyacrylamide (PAM)-water system that allows to study dynamical properties of chains at several concentrations with molecular weight up to 17000 g/mol. Non-equilibrium simulations were used to compute relative viscosities, enabling a direct comparison with experimental values. High-shear-rate measurements for low-molecular-weight PAM (10000 g/mol) were done using a microfluidic rheometer Rheosense to decrease the gap between experimental and simulated shear rates. DPD simulations reproduced qualitatively and quantitatively structural properties as well as rheological properties in the dilute regime and qualitatively in the semi-dilute regime. Copyright (C) EPLA, 2012

One of chemical Enhanced Oil Recovery (EOR) methods consists in injecting aqueous solutions of polymers into the reservoir in order to improve mobility ratio between the injected fluid and the remaining oil. This “polymer flooding” process is usually only characterized with the low shear viscosity of the injected fluid, even if these aqueous solutions are strongly shear thinning and may show high elastic properties evidenced by normal stresses appearance. In order to study the mechanisms at the interface level, we develop simple model experimentations with the goal of quantifying the influence of viscoelastic properties on fluid displacement in a simple geometry. For this purpose, we propose and characterize a model fluid formulation, for which elastic and viscous effects can be tuned systematically. We study then the displacement of a viscous oil by a Newtonian non elastic, a viscoelastic or a purely shear thinning fluid in a two dimensional flow cell. Observing the shape of the interface between aqueous fluids and displaced oil permits to appreciate viscoelasticity effects on the displacement. Using model geometries and controlled rheology fluids, we show that viscoelastic fluids tend to better displace immiscible liquids than Newtonian fluids and that those effects are closely related to the apparitions of normal stresses independently of shear thinning property or variation of interfacial tension as soon as viscous effects govern the flow.

Summary Gas-well productivity in tight reservoirs is greatly impeded by fracturing-fluid interactions with the formation. New simulators introduce formation-damage mechanisms to calculate gas-well productivity. However, equations describing formation damage must be supported by experimental data obtained in conditions representative of fracturing operations. The purpose of this work is to derive absolute-permeability and multiphase-flow damages upon return gas permeability after core invasion by a fracturing fluid by methods used in the Special Core Analysis Laboratory (SCAL). The core permeability is in the microdarcy range with significant illitic content. Absolute-permeability damages caused by fracturing-fluid filtration and water sensitivity are measured. Water-saturation profiles recorded by X-ray in two-phase-flow experiments are interpreted. The methodology of interpretation provides the petrophysical data specific to the rock/fluid system: absolute permeability, relative permeability damage caused by hysteresis, and capillary pressure. In addition, simulations are presented for the evaluation of the effect of various operational parameters, such as pressure drawdown, on gas productivity. It is shown that permeability hysteresis is the determinant factor to explain low gas recoveries at short term. In the long term, the natural cleanup is very slow. The results, derived from a real rock/fluid system, are used to provide recommendations for improving backflow procedures. This methodology can be applied to any case of damage caused by the alteration of rock/fluid properties.

Modeling Aging and Yielding of Complex Fluids: Application to an Industrial Material - Complex fluids either natural or encountered in numerous industrial processes are often composed of several phases constituting emulsions, suspensions, foams or other colloidal dispersions. Many of these complex fluids may be described in a general manner as "soft-jammed systems" which have the ability to undergo a solid-liquid transition when submitted to a sufficient stress. The description of this transition from a solid state to a flowing situation is essential to understand for example the dynamic of flow stoppage or restart of natural processes (snow avalanches, ground sliding, etc.) or of industrial processes (self placement concrete, glues, cosmetic formulations, mud circulation, flow assurance, etc.). In this study, we have interest in the link between the microstructure of a complex fluid and its macroscopic rheological behavior, especially regarding the solid liquid transition characteristic of these systems. By coupling conventional rheometry giving macroscopic properties, and IRM velocimetry giving access to local properties, we can identify the structural origin of the major rheological properties as the yield stress, the aging at rest and the viscosity bifurcation in the liquid regime. We show that the progressive stoppage of the material, induced by the growing of aggregates under a critical stress explains some peculiar characteristics of the flow curves. We show then how the transient and stationary behavior of the fluid may be described by a unique thixotropic model involving a structural time and shear dependant parameter. A practical application of this model is proposed, showing how the parameters of the model may be deduced from simple experiments, and how the model may be used to predict restart conditions after rest for a fluid flowing in a pipe. This work allows to propose elements of microscopic modeling of the thixotropy of these systems in relation with their structure, and show the applicability of this modeling work to practical situations.

Summary It has been widely recognized that formation damage caused by drilling fluid has a huge impact on well productivities. The degree of formation damage can be studied using a numerical model. However, uncertainties in the prediction of productivity loss need to be investigated. To quantify these uncertainties, experimental designs combined with the Response Surface Methodology (RSM) are used to assess the impact of uncertain parameters on formation damage. This approach allows to identify the most influential parameters on well productivity loss and to estimate the risks of formation damage. A good control of the most sensitive parameters can limit productivity loss. This approach provides key recommendations for the selection of drilling fluid to maximize well performance.

When drilling underbalanced, the pressure of the drilling fluid is maintained at a value below the formation pressure. In order to lower the well pressure, specific low density fluids are needed, i.e., gas, aerated mud or foam. Foam is particularly beneficial for drilling mainly due to its low density coupled with good carrying capability, but its use remains hazardous due to the incomplete knowledge of its bottomhole properties, and especially of its flowing properties. Pressure drop estimation is crucial for underbalanced drilling (UBD) operations in order to be able to keep the bottomhole pressure in the adequate range in real time. In this study, we analyze the pressure drop variation with the flow rate in a circular pipe for different foam qualities and formulations. Experimental investigations are realized in a pressure and temperature circular conduct flow. We show that lubrication at the wall plays a crucial role. Indeed, the intrinsic viscosity of the foam can be very high, leading to the development of a water layer at the wall responsible for the lubrication of the flow. A two-phase description of the system allows for the analytical estimation of the pressure drop. The size of the lubricated layer is then deduced and its range of existence is discussed. Main parameters of its formation are also discussed. We show that this lubrication effect can be significant in pressure drop estimation for underbalanced operations.

We study a suspension of water droplets in oil which, due to organoclay particles links, exhibits a yielding and thixotropic behavior. The local rheological behavior in time is determined with the help of MRI velocimetry in a Couette flow. Under constant rotation velocity, in a first stage we observe a progressive displacement of the fluid/solid interface towards the inner cylinder, which is associated with the increase of a critical shear rate below which there is apparently no flow. During this stage our local measurements show that the constitutive equation of the liquid region does not vary in time: only the thickness of the sheared region and the critical shear rate vary. Then we focus on the behavior in the solid and liquid regimes as a function of the droplet concentration phi (from 20 to 70%). The solid regime is studied by measuring the evolution of the elastic modulus (G) in time. The initial level of G increases with phi but G also increases in time, witch is the hallmark of the thixotropy (aging) at rest. We find that the effect of time on the restructuring is much larger than the effect of concentration increase. In order to study the liquid regime, we build the local flow curves from MRI velocity profiles as a function of time for all the formulations. The effect of a significant increase in droplet concentration on the material behavior in the liquid regime is found to be minor, in contrast with its effect on the behavior in the solid regime. This is explained by the fact that the behavior in the solid regime is mainly controlled by the droplet aggregation, a process which plays a much smaller role as soon as the liquid regime is reached.

From magnetic resonance imaging rheometry we show that a pure emulsion can be turned from a simple yield stress fluid to a thixotropic material by adding a small fraction of colloidal particles. The two fluids have the same behavior in the liquid regime but the loaded emulsion exhibits a critical shear rate below which no steady flows can be observed. For a stress below the yield stress, the pure emulsion abruptly stops flowing, whereas the viscosity of the loaded emulsion continuously increases in time, which leads to an apparent flow stoppage. This phenomenon can be very well represented by a model assuming a progressive increase of the number of droplet links via colloidal particles.