We have studied oil-in-water emulsions stabilized by monodisperse, fluorescent silica colloids presenting either a smooth or a rough surface. The presence of the fluorescent core allows for direct visualization of the colloids on the surface of the emulsion droplets. Droplet interfacial tension, measured by micropipet tensiometry, is not modified by particle adsorption at the interface, suggesting a purely steric stabilization mechanism. Surface roughness is shown to considerably lessen the ability of particles to stabilize droplets. At variance with what is commonly assumed, no straightforward relation exists between the extent of particle interfacial adsorption and emulsion macroscopic stability; stable emulsions can be obtained even with very low droplet surface coverage. Finally, we directly monitor the Brownian motion of the adsorbed particles, showing that their surface diffusion coefficient is very close to the bulk value. Evidence of a possible role of particle surface dynamics on the stabilization of poorly covered droplets is presented.

A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.

The high-molecular-weight paraffinic (‘wax’) fraction separates from crude oils at low temperatures, a process that can lead to a sol–gel transition when the mass of wax solids exceeds 1–2%. Attractive interactions between the micron-size wax solids suspended in the non-polar medium have been suggested to be responsible for gel formation. The present study reports an optically transparent model oil system, based on a mixture of linear and branched paraffins. Rheological measurements and optical microscopy show that the model system reproduces essential features of crude oil gels. Small-angle light scattering studies conducted at temperatures intermediate between the cloud point (58 °C) and sol–gel transition (39 °C) show that phase separation and wax solid aggregation are rapid processes, leading to the formation of dynamically arrested structures well above the sol–gel transition determined rheologically. Analysis of gravity settling effects has provided a rough estimate for the yield stress of the wax particle network formed (greater than 0.7 Pa at 45 °C and 0.07 Pa at 55 °C). Clusters formed by the aggregated wax solids possess a fractal dimension of about 1.8, consistent with diffusion-limited cluster–cluster aggregation.

Solubilization of membrane proteins requires surfactants, whose structural properties play a crucial role in determining the protein phase behavior. We show that ionization of a pH-sensitive surfactant, lauryldymethylamino-N-oxide, bound to the bacterial photosynthetic Reaction Center, induces protein phase segregation in micrometric "droplets." Liquid-liquid phase separation takes place in a narrow pH range, is promoted by increasing temperature, and vanishes by adding salt. After a fast initial droplet growth, the nearly arrested kinetics at a later stage leaves the system in a finely divided, long-lasting emulsified state.

Streptococcus agalactiae is an etiological agent of several infective diseases in humans. We previously demonstrated that FbsA, a fibrinogen-binding protein expressed by this bacterium, elicits a fibrinogen-dependent aggregation of platelets. In the present communication, we show that the binding of FbsA to fibrinogen is specific and saturable, and that the FbsA-binding site resides in the D region of fibrinogen. In accordance with the repetitive nature of the protein, we found that FbsA contains multiple binding sites for fibrinogen. By using several biophysical methods, we provide evidence that the addition of FbsA induces extensive fibrinogen aggregation and has noticeable effects on thrombin-catalyzed fibrin clot formation. Fibrinogen aggregation was also found to depend on FbsA concentration and on the number of FbsA repeat units. Scanning electron microscopy evidentiated that, while fibrin clot is made of a fine fibrillar network, FbsA-induced Fbg aggregates consist of thicker fibers organized in a cage-like structure. The structural difference of the two structures was further indicated by the diverse immunological reactivity and capability to bind tissue-type plasminogen activator or plasminogen. The mechanisms of FbsA-induced fibrinogen aggregation and fibrin polymerization followed distinct pathways since Fbg assembly was not inhibited by GPRP, a specific inhibitor of fibrin polymerization. This finding was supported by the different sensitivity of the aggregates to the disruptive effects of urea and guanidine hydrochloride. We suggest that FbsA and fibrinogen play complementary roles in contributing to thrombogenesis associated with S. agalactiae infection.

We show that ionization of a pH-sensitive detergent, DDAO, bound to a bacterial photosynthetic reaction center (RC), induces reversible emulsification of the protein over a narrow acidic pH range, resulting in stable micrometric RC-surfactant droplets. Electrostatic interactions play a key role in the phase separation process, as shown by a systematic analysis of ionic strength effects and by the use of a cationic detergent (DTAB) that mimics, also at basic pH, the ionized form of DDAO. Under all the conditions we tested, phase segregation seems to be coupled to a 15 nm blue-shift of the low energy absorption band of the primary electron donor P of the RC. This spectral change strongly suggests that surfactant-protein interactions leading to phase separation also induce a conformational transition of the RC. Time-resolved visible-NIR spectra recorded during the emulsification process reveal that the conformational change probed by P spectral shift is always faster than droplets formation. In line with these observations, phase segregation affects charge recombination kinetics following RC photoexcitation, as well as electron transfer from soluble cytochrome c(2) to the photoxidized primary donor P+

Nowadays oilfield development has become more technically and economically challenging and a high degree of interdisciplinary interaction is needed to have an effective and efficient management of the field. The achievement of this goal is made possible only if all the different resources of an organisation work together sharing the same reservoir model. Indeed, the main breakthrough behind Integrated Asset Modeling (IAM) is to combine reservoir, production and surface engineering modeling into an asset management tool that allows the simulation of the whole oilfield system. Though the need and the benefits of IAM were already recognised during the seventies, it is only in the last decade that software tools required to perform integrated production simulation have become available. Coupling dynamic reservoir and surface facility models into a single integrated model may address the following issues: pressure interaction between the surface and the subsurface; mixing of different fluids and flow assurance; accounting for facilities constraints; and identification of system bottlenecks and backpressures. In this way, unnecessary drilling can be avoided, new opportunities can be discovered, optimal artificial lift programs can be implemented to meet production targets. In this paper the state of the art of IAM tools is reviewed, with emphasis on the solution implemented in our company. Benefits and criticalities are then discussed on the basis of three cases, including integrated models for regional gas production systems, deep water mixed oil-gas assets and gas lifted reservoir. Indeed, it has been noted that integrated surface-subsurface modeling will have a critical impact on field management by offering increased accuracy in forecasting reservoir behaviour and maximising the recovery factor at minimum cost. Introduction During the last decade, oil companies attempted to reorganise their structures into interdisciplinary asset teams and include IAM in industry workflows. This was mainly due to an acknowledgement of the need for a better understanding and description of the interaction between subsurface-surface systems. It was facilitated by more powerful IT resources and commercial tools for IAM. Despite the availability of several commercial platforms, many major petroleum companies have implemented their own proprietary softwares over time. One of the pioneer solutions to IAM dates back to the 1960s when Amoco (Tingas et al., 1998) developed on a mainframe computer RAISEGAS (Mohamed et al., 1979), a single phase - 2D reservoir and surface network simulator, to manage the production of coupled Southern North Sea UK gas fields. Extension to two phase gas-water systems was presented by Dempsey in 1971 for gas field deliverability. It is with the works of Chevron (Startzman et al., 1977; Emanuel et al., 1981; Breaux et al., 1985) that IAM began to be applied to three dimensional black oil reservoirs. Startzman coupled a black oil simulator to an in-house surface model. To reduce execution time Emanuel moved the interface between the models to the wellhead and used look-up tables for wellbore pressure losses, whilst Breaux presented two applications of the methodology that combined cost-effective drilling and facility scheduling with balanced reservoir development. More recently Chevron tightly coupled its own 3D reservoir simulator CHEARS with the commercial network simulator PIPESOFT2 and presented an application for the development plan of the Gorgon field offshore Northwest Australia (Zapata et al., 2001). Arco (Stoisits et al., 1992) developed its own integrated simulator and presented a case study of gas lift optimisation for the Kuparuk river field on the Alaskan North Slope.

Compositional reservoir simulations are widely used to simulate reservoir processes with strong compositional effects, such as gas injection. The equations of state (EoS) based phase equilibrium calculation is a time consuming part in this type of simulations. The phase equilibrium problem can be either decoupled from or coupled with the transport problem. In the former case, flash calculation is required, which consists of stability analysis and subsequent phase split calculation; in the latter case, no explicit phase split calculation is required but efficient stability analysis and optimized coding of the basic thermodynamic subroutines are still crucial to the overall speed. This work tries to provide a comprehensive strategy to increase the speed for compositional simulation. This strategy begins with the coding of the basic thermodynamic properties, including the derivatives of fugacities with respect to molar numbers. Then, in the algorithms for stability analysis and phase split calculation, successive substitution with acceleration and minimization-based second-order methods are combined to gain both robustness and efficiency. For compositional simulations, the results from previous simulation steps provide the possibility to skip stability analysis by the shadow region method in the single phase regions. The approach was implemented in the general purpose research simulator (GPRS) developed by Stanford University. GPRS is a modular, state of the art reservoir simulation and its architecture makes the implementation and evaluation of new ideas and concepts easy. Tests on several 2-D and 3-D gas injection examples indicate that with an efficient implementation of the thermodynamic package and the conventional stability analysis algorithm, the speed can be increased by several folds. Application of the shadow region method to skip stability analysis can further cut the phase equilibrium calculation time.