Richard A. Dawe

Influence of Mg2+ on the kinetics of calcite precipitation and calcite crystal morphology

Abstract The kinetics of calcite growth in the presence of Mg2 has been studied by both a pH free drift method and by visual observation. Our experiments show that the calcite growth rate is reduced by the presence of Mg2+. The higher the Mg/Ca ratio in the solution, the lower the growth rate in a CaCO3 supersaturated solution. The inhibition of calcite growth by the presence of Mg2+ is caused by Mg2+ being incorporated into the original calcite seed surfaces and developing new crystal surfaces. The Mg2+ distribution on the calcite surfaces is not uniform as there is likely to be different Mg2+ densities on the newly developed surfaces than on the original surfaces of the calcite seeds. This causes the calcite crystal morphology to change. The significance of creating error in using the initial rate data rather than the equilibrium rate data for the prediction of the calcite precipitation kinetics is discussed.

The kinetics of calcite precipitation from a high salinity water

Calcium carbonate is one of the most common and important scale-forming minerals in oilfield produced water, but the kinetics of CaCO3 precipitation has been ignored in most scale prediction models because of the lack of reliable precipitation rate model. There are none in the open literature for oilfield conditions (temperature > 100°C, pressure > 200 bar and salinity > 0.5 mol kg− 1). In this work the kinetics of calcite (CaCO3) precipitation from high salinity waters (up to 2 mol kg−1) have been studied by a pH-free-drift method in a closed water system. This method. is much easier to operate than the often used steady-state method. The experimental results indicate that the calcite precipitation rate is not only affected by the solution CaCO3 saturation level, but also by the solution pH, ionic strength and the concentration ratios of Ca to HCO3− ions (CCa2+/CHCO3−). When the concentration ratios of Ca to HCO3− ions are close to their chemical stoichiometric ratio of 0.5, the calcite growth from a supersaturated solution is believed to be surface reaction controlled. However, at higher CCa2+CHCO3− ratios, the transportation of the lattice ions to calcite crystal surface has to be considered.

Experimental investigation of capillary pressure effects on immiscible displacement in lensed and layered porous media

This paper reports the results of extensive experimental studies of the effects of well-defined heterogeneous porous media on immiscible flooding. The heterogeneities were layers and lenses, with some of the lenses being a wettability contrast. Drainage and imbibition displacements, with and without an initial residual fluid saturation, were carried out at a variety of flow rates on layered and lensed two-dimensional glass beads models of the size of a typical large core test (58×10×0.6 cm). These displacements were followed photographically and the effluent saturation profiles recorded. In most of the experiments the glass beads were water-wet, but in some the lens beads were coated with a water repellent chemical. In all experiments, the displacement fronts became highly irregular due to the different capillary pressures acting in the different areas of the models. In this paper, these displacements are fully reported and their implications for reservoir simulation and for interpretation of laboratory core tests, where the inner heterogeneities are not known, are discussed.

Kinetics of calcium carbonate scaling using observations from glass micromodels

Abstract Calcium carbonate, CaCO3, is one of the most common scale components found in oilfield production wells and surface facilities. Although many computer models have been developed to predict the thermodynamic tendency of scaling, the kinetics have usually been neglected due to a lack of a reliable model to predict how fast the scale will build-up. In this work, the kinetics of calcium carbonate (calcite seeds used) precipitation have been studied by using visual observations of crystal growth within a glass micromodel. The effects of the solution composition, temperature and the presence of a gas—liquid interface on the spontaneous nucleation and the calcite crystal growth rate have been examined. From the measurements of the crystal growth kinetic data, the calcite growth rate constant as a function of temperature (25–70°C) and ionic strength (up to 2 eq/kg) have been determined and an activation energy of 46.9 kJ/mol estimated. We believe that the presence of a gas—liquid interface can help spontaneous nucleation. The presence of Mg2+ at ratios of [Mg2+]/[Ca2+] between 0.1 to 0.5 causes a 50% reduction in calcite growth rate and must not be neglected in scale kinetics.

Mechanistic study of the selective action of oil and water penetrating into a gel emplaced in a porous medium

Abstract Liang and Seright observed that the reduction of the permeability of rock to oil and water by a gel was different. They have recently suggested reasons but have not found a fully convincing explanation. This work addresses the phenomena and gives visual evidence of what may be occurring inside the porous medium. Visual microscale studies have been carried out to observe the mechanisms of water and oil passing through a gel emplaced in a porous medium or in a fracture, and how wettability variations and residual oil affect the processes. The results show that oil and water can pass through the gel but by different mechanisms. Oil passes a gel by fingering from the centre of the pores and widens the pathway by taking away some water from inside the gel and syneresising the gel, whereas water passes a gel by diffusing into and swelling the gel.

Flow behaviour in the presence of wettability heterogeneities

The physical processes occurring during fluid flow and displacement within porous media having wettability heterogeneities have been investigated in specially designed heterogeneous visual models. The models were packed with glass beads, areas of which were treated with a water repellent to create wettability variations. Immiscible displacement experiments show visually the effect of wettability heterogeneities on the formation of residual oil and recovery due to capillary trapping. This work demonstrates by experiment the importance of incorporating reservoir heterogeneity into pore displacement analysis, essential for the correct interpretation of core data and for directing the route for scale-up of the processes to reservoir scale.

Photographic observations of unusual flow phenomena in porous media at interfacial tensions below 0.1 mN m−1

Abstract Two-phase flow within porous media at low interfacial tension and high capillary number (>10 −3 ) behaves very differently from flow at high interfacial tension and low capillary number. Qualitative features have been observed experimentally using 2,6-lutidine-water mixtures in a transparent model porous medium. Three photographic observations are presented; the first illustrates a “pore-to-pore hopping” mechanism of wetting phase movement, the second shows pore-to-pore transfer of wetting phase in a different pore geometry, and the third reveals a gravity induced instability.

Modelling fluid flow in cross-bedded sections

Beadpack experiments and numerical simulations have been carried out to study flow displacements, effluent profiles and streamline patterns for layered systems with flow not parallel to the layers. The effects of layer thickness, permeability contrast, angle of layer to flow direction, mobility ratio and flood rate have been examined. Each of these parameters influence the displacement profiles, and disperse the flood front. Such real effects must be considered when subsuming reservoir heterogeneities in average reservoir parameters in simulation studies, or interpreting core tests.

The effect of halides on the lower critical solution temperature of the 2,6-lutidine-water system

Abstract Seah, C.Y., Grattoni, C.A. and Dawe, R.A., 1993. The effect of halides on the lower critical solution temperature of the 2,6-lutidine-water system. Fluid Phase Equilibria 89: 345-350. The lower critical solution temperatures of 2,6-lutidine (2,6-dimethyl pyridine) in brine have been measured for a range of halide solutions. The effect of the anion is significant with iodide having a raising effect compared with lutidine in distilled water (34°C) and bromide, chloride and fluoride having a depressing effect. The effect of different cations is not so significant. The implications of such observations are wide ranging for both theoretical and industrial application.

Modeling Low Interfacial Tension Hydrocarbon Phenomena in Porous Media

This paper investigates pore-scale mechanisms of both wetting and nonwetting phase dropout and flow at low interfacial tensions (IFTs) (approximately 0.1 to {lt} 0.01 mN/m) by use of transparent 2D micromodels with a partially miscible binary fluid system. Quantification is possible with this liquid system because it has uniquely defined physical properties at any given temperature. Phase dropout and flow behavior at phase saturations of {lt}10% are emphasized. Wettability effects are dominant, but the fluid/fluid interactions change as the IFT drops. The ramifications for the relative permeability curves are discussed.