Yongan Gu

A model for a liquid drop spreading on a solid surface

Abstract This paper presents an overall energy balance (OEB) method for modelling the spreading process of a liquid drop on a solid surface. This method combines surface physics and fluid mechanics and contains only one adjustable parameter. Using the de Gennes model for the viscous force of a moving contact line and the OEB method, this novel theoretical spreading model accounts for the inertial, viscous and gravitational forces, the interfacial tensions and the contact angle of the solid–liquid–fluid system. A computer program for implementing the spreading model is developed. A set of measured spreading data of silicone liquid drops on a soda-lime glass plate is used to verify the model predictions for the spontaneous spreading case. The comparison shows that the simple spreading model presented in this paper can predict well the spontaneous spreading of a liquid drop on a solid surface.

Drop size dependence of contact angles of oil drops on a solid surface in water

Abstract Contact angle is a fundamental quantity in colloid and surface science. Of all the methods employed to measure the contact angles, direct measurement from sessile drops is probably the most popular approach. However, it has long been found that the measured contact angle is not unique for a given solid–liquid–fluid system. There are two types of contact angle multiple-value phenomena — the drop size dependence of contact angles and the contact angle hysteresis. In the literature, the contact angle dependence on the liquid drop size for the solid–liquid–vapour systems has been studied extensively. In this paper, the contact angle dependence on the oil drop size for a solid–oil–water system is measured by applying the axisymmetric drop shape analysis (ADSA) technique. More specifically, a natural sedimentation method is employed to deposit drops of a silicone oil with a density slightly higher than that of water onto the hydrophobic FC725 coated glass slide in the deionized ultra filtered (DIUF) water. It has been observed that the measured equilibrium contact angle decreases by approximately 7.3° as the equilibrium base radius of the silicone oil drop increases from 0.0205 to 0.4684 cm. According to the modified Young equation, the measured contact angle changes are then interpreted in terms of the so-called line tension effect. The line tension of the solid–oil–water system is found to be 0.82 μJ m−1, which is very close to those for similar solid–oil–air systems. Unlike other conventional drop formation methods, the natural sedimentation method used in this study can eliminate all the appreciable mechanical disturbance and vibrations. Furthermore, this deposition can generate liquid drops in a much larger range of drop size such that the line tension effect becomes much more pronounced.

A novel contact angle measurement technique by analysis of capillary rise profile around a cylinder (ACRPAC)

A new experimental technique and its computational scheme to determine the contact angles of capillary rise profiles around a cylinder are presented. In the experiment, a carefully coated conic glass cylinder was inserted vertically and slowly into a tested liquid. Then the precise image of the partial capillary rise profile of the liquid around the conic cylinder was acquired and digitized by applying computer image processing and analysis techniques. From the digitized profiles of the liquid-vapour interface and the conic cylinder, the local inclination angle β and the local radius Rc, of the conic cylinder at the three-phase contact circle were calculated directly. Furthermore, an objective function was constructed, which expresses the discrepancy between the physically observed capillary rise profile and the theoretically predicted curve, i.e. the curve representing a solution of the Laplace equation of capillarity. The contact angle of the capillary rise profile on the conic cylinder was used as an adjustable parameter in optimizing the objective function and determined once the minimum objective function had been achieved. The accuracy of the measured contact angles is approximately 0.1°. In addition to the local gravity, the densities of the liquid and vapour phases and the liquid-vapour surface tension, the input requirement is the digital information of the partial capillary rise profile which is provided by a specially designed computer image analysis program. This method was tested by measuring contact angles of four n-alkane liquids around cylindrical glass fibres coated with FC725. The measured contact angles are in very good agreement with those determined by the Wilhelmy plate technique. Finally, the present technique was also applied to study the dependence of contact angles on the geometry of the conic cylinder, i.e. on cos βRc. Contact angles of the four n-alkane liquids on a conic glass cylinder coated with FC725 were measured at different positions along the cylinder. The results were interpreted in terms of the line tension effect. The calculated line tensions were positive and of the order of 1 μJ m−1, which is consistent with the published data for similar solid-liquid systems obtained by using the sessile drop method. In particular, the contact angle without line tension effect θ∞ for a given solid-liquid system can be measured directly by this method. The validity of the derived contact angle θ∞ and line tension σ was also confirmed by means of the axisymmetric drop shape analysis (ADSA) technique. This novel technique is particularly suitable to the study of the wetting and spreading phenomena of a liquid on fibres, as most fibre surfaces are rough and their shapes may deviate considerably from those of right circular cylinders. A general user-oriented computer program to implement the technique was developed.

In Situ Upgrading of Heavy Oil in a Solvent-Based Heavy Oil Recovery Process

In this paper, a series of laboratory experiments are conducted under reservoir conditions to quantify the in situ upgrading of heavy oil due to the solvent dissolution and asphaltene precipitation by using a pure solvent (propane) and a solvent mixture (70 mol% methane + 25 mol% propane + 3.5 mol% n-butane + 1.5 mol% iso-butane). It is found that after a solvent is placed in contact with heavy oil at a relatively high pressure for a sufficiently long time, the heavy oil-solvent system at equilibrium state can be roughly divided into three different layers. The top layer is a solvent-enriched oil phase, the middle layer comprises heavy oil with the dissolved solvent and the bottom layer mainly consists of heavy components. The solvent-saturated heavy oils in these three layers have rather different physicochemical properties, such as the solvent concentration, carbon number distribution and viscosity. The top layer has the highest concentrations of solvent and light components and the lowest viscosity of heavy oil even after its dissolved solvent is flashed off. The heavy oil in the middle layer has similar carbon number distribution to the original heavy oil. The bottom layer has the lowest solvent concentration and the highest concentration of heavy components. The heavy oil in the bottom layer, after its dissolved solvent is flashed off, has much higher viscosity than the original heavy oil. These experimental results indicate that in a solvent-based heavy oil recovery process, the solvent-saturated heavy oil in the top and middle layers can be recovered because of its lower viscosity, whereas the heavy oil in the bottom layer may be left behind in the heavy oil reservoir because of its higher viscosity. In this way, the produced heavy oil is in situ upgraded during the solvent-based heavy oil recovery process.

Liquid drop spreading on solid surfaces at low impact speeds

In this paper, a previously developed drop spreading model is applied to studying the spreading kinetics of water drops with low impact speeds on an anodized aluminium surface and a glass plate. A set of measured spreading data is used to verify the model predictions for low-speed impact spreading cases, in which the impact speed was adjusted by varying the drop release height. It is found that, for relatively lower impact speeds up to U=250 cm s−1, the spreading model can simulate well, the spreading of the water drops on the flat metal surface and the glass plate. At higher impact speeds, the simple spreading model overestimates the spreading rates. Some possible reasons for the overestimation are discussed.

Integrated optimization and control of the production-injection operation systems for hydrocarbon reservoirs

This paper presents novel integrated models to analyze the production-injection operation systems (PIOS) for the reservoirs based on the concepts of systems engineering. More specifically, gas and/or water injection, reservoir and production performance, and the economic assessment are incorporated into these models. Four production operation methods are considered in the production models. The reservoir geological model is improved with time by the on-going monitoring results for better performance evaluation and prediction. The non-numerical parallel algorithms, including genetic algorithms and simulated annealing algorithms, are employed to optimize and control the nonlinear PIOS under different practical constraints throughout the life of a reservoir. A direct field application shows that the bottomhole flowing pressure, reservoir pressure, injection pressure, oil rate, gas-oil ratio and water-cut can be adjusted and optimized. Such optimized injection and production parameters maintain the reservoir pressure and ensure the optimum reservoir performance so as to slow down the flooding front in a stable way. Meanwhile, the optimized results are desirable in the field production and injection operations. Therefore, the economic benefits can be maximized and the reservoir life can be extended. A PC-based software for optimizing the overall PIOS is also developed.

A novel experimental technique for studying solvent mass transfer and oil-swelling effect in the vapour extraction (VAPEX) process

In this paper, a new experimental technique is presented to study the solvent mass transfer in heavy oil and the resultant oil-swelling effect by applying dynamic pendant drop volume analysis (DPDVA). In the experiment, a pendant drop of heavy oil is formed inside a visual high-pressure cell, which is initially filled with a solvent (e.g. propane). As the solvent gradually dissolves into heavy oil, the volume of the pendant oil drop keeps increasing due to the oil-swelling effect. The sequential digital images of the dynamic pendant oil drop are acquired and analyzed to determine its volumes at different times. Theoretically, a previously formulated mathematical model is applied to describe the solvent mass transfer in heavy oil and the oil-swelling effect. The diffusion coefficient of the solvent in heavy oil and the oil-swelling factor are determined by finding the best fit of the theoretically predicted volumes of the dynamic pendant oil drop to the experimentally measured data. Experimental tests are conducted for the heavy oil-propane system at constant pressures of P = 0.4 - 0.9 MPa and a constant temperature of T= 23.9°C. It is found that both the diffusion coefficient and the oil-swelling factor of the heavy oil-propane system increase with pressure. The major advantage of this new technique is that simultaneous measurements of the solvent diffusivity in heavy oil and the oil-swelling factor can be completed within two hours at the prespecified constant pressure and temperature.

Pressure Pulsing Cyclic Solvent Injection (PP-CSI): A New Way to Enhance the Recovery of Heavy Oil Through Solvent-Based Enhanced Oil Recovery Techniques

Hydrocarbon solvent-based enhanced oil recovery (EOR) techniques, such as vapor extraction (VAPEX) and cyclic solvent injection (CSI), have shown a great potential to recover heavy oil reserves. Solvent dissolution into heavy oil and diluted oil flow are the two main processes of these techniques. VAPEX suffers a low production rate because of the slow mass transfer and inefficient gravity drainage. Although benefited from solution gas drive and foamy oil flow, CSI is bothered with the solvent gas release and viscosity re-increase during the production period, which might seriously slow down the oil production. In this study, it is found that foamy oil flow would generate a foamy oil zone which re-saturated part of the solvent chamber. On the basis of this a phenomenon, this paper proposes a new way to enhance the CSI performance through a three-step pressure control scheme during the production period: first, reduce the model pressure to induce foamy oil flow and foamy oil zone; second, re-increase the model pressure to a preset value; third, maintain a certain pressure difference between injector and producer for gas flooding. This three-step process is called one pressure pulse and it can be consecutively repeated several times during the production period. A series of laboratory experiments have been conducted with a visual rectangular sand-packed physical model. The sandpack permeability is ~5 Darcy. Propane is used to recover a heavy oil sample with a viscosity of 5,875 cP. Results show that (1) foamy oil could effectively re-saturate the solvent chamber; (2) more pressure pulses would lead to more oil production, while the oil production decreases with pulse number in one cycle; (3) the oil production rate of PP-CSI is 4.37 times of that of conventional CSI, and the final recovery factor of PP-CSI is 17.9% higher than that of CSI. (4) flooding-induced viscous fingering can accelerate the mass-transfer rate; meanwhile, the viscous fingering will not cause an early breakthrough of the solvent chamber.

The effects of capillary force and gravity on the interfacial profile in a reservoir fracture or pore

Abstract This paper presents a mathematical model for determining the interfacial profile between two immiscible fluids in a reservoir fracture or pore with a random orientation. This model is derived from the Laplace equation of capillarity by considering the effects of the capillary force, the gravity, and the wettability of fluids on reservoir rocks. The fourth-order Runge–Kutta method is employed to obtain the numerical solution. The mathematical model together with its numerical scheme is verified by the measured interfacial profiles between a heavy oil sample and a water phase in a circular tube. The numerical results show that the equilibrium shape of the interfacial profile depends on the dimensionless Bond number, the orientation of the reservoir fracture or pore and the contact angle. In particular, it is found that the Bond number, which is defined as the ratio of the gravity to the capillary force, has a strong effect on the shape of the interfacial profile. For a sufficiently small Bond number, the interfacial profile is governed by the capillary force and the contact angle formed with reservoir rocks. In this case, the orientation of the reservoir fracture or pore has no influence on the interfacial profile. Based on the numerical results, a universal critical Bond number is obtained and further used in determining the so-called critical permeability of hydrocarbon reservoirs. The interfacial profile in a reservoir fracture or pore is dominated by the capillary force as long as the reservoir permeability is smaller than this critical permeability.

Experimental determination of the Hamaker constants for solid–water–oil systems

The van der Waals interaction (vdW) is a fundamental interaction in colloid and interface science. Regardless of the methods used in deriving the vdW interaction between two bodies as a function of their separation distance, the Hamaker constant is always an essential parameter involved. In this paper, a simple experimental method is presented to determine the Hamaker constant. As an example, the Hamaker constant of a solid-water-oil system is related to its surface and interfacial energies, which can be measured accurately. Based on the proposed method, the effects of two typical solid surfaces and three kinds of aqueous solutions on the Hamaker constant and wettability of the solid-water-oil system are studied. It is found that hydrophilic and hydrophobic solid surfaces will lead to rather different Hamaker constants and wettability behaviour. The detailed experimental results also show that the ionic surfactant solutions have a strong influence, whereas the pH value of the aqueous phase has a limited effect on the Hamaker constant. In addition, the electrolyte solutions do not strongly affect the Hamaker constant for the oil phase interacting with the solid surface across an electrolyte solution. Such determined Hamaker constants are in reasonable agreement with the reported Hamaker constants for oils (dodecane and hexadecane), mica, and metals (Ag, Au, and Cu) interacting across a pure water phase.