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Oil recovery by an imbibition process at elevated temperatures depends not only on fluid composition but also on the history of the process, that is, the composition of the initial fluid in contact with the carbonate rock before the drainage process. In this work, extreme cases have been investigated with ion-free water and water containing Mg2+ or SO4 2-. Modified carbonate rocks with stearic acid that was initially saturated with ion-free water followed by an imbibition process with fluids containing Mg2+ or SO4 2-, at the same concentration as the injected seawater, shows the highest oil recovery when Mg2+ is present in the imbibing fluids. Whereas the initially saturated fluid and imbibing fluids contain SO4 2-, an inconsiderable difference between the oil recovery factor with SO4 2- and that for the ion-free water was observed. This is in contrast to Mg2+-containing imbibing fluid, which showed the lowest recovery. Computation of the disjoining pressure for the three systems (ion-free water, SO4 2-, and Mg2+) indicates that the presence of magnesium ions gives a more stable water film and requires increasing capillary pressure to rupture the water film as the temperature increases, as shown by the maximum in the repulsive energy. In addition, the calculation by Derjaguin, Landau, Verwey, and Overbeek theory and experimental observations may suggest that fine detachment is one of the mechanisms that alters the wettability of the rock by increasing the temperature.

This paper describes the chemistry and treatment design of the first field application of a recently developed, organically crosslinked polymer (OCP) system for water shutoff. The candidate for which this treatment was designed is a production well in the North Sea that was completed in naturally fractured chalk. Each of the completed intervals were acid fractured to further open the existing natural fractures and increase production. The recommended treatment volume calculations took into account the leak-off considerations and the acid fracture volume. The method by which the leak-off requirements were calculated is described. The placement and shut-in temperatures of the recommended treatment volume were then calculated as a function of time. These simulations used a production - operation wellbore simulator in combination with an advanced processes reservoir simulator. The fractures in the target zone were defined in the grid-block system that was used in these temperature simulations. The results of these temperature calculations were used to formulate the chemical compositions of the stages of the OCP treatment and to calculate gel times and shut-in times. The calculations of these gel times and how they correspond to the gel times during the actual treatment are also described.

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The effects of SO4 2- and Mg2+ on the adsorption/displacement of stearic acid (SA), N,N-dimethyldodecylamine (NN-DMDA) and asphaltene, as oil soluble additives, onto or from calcite, quartz, and clay minerals are addressed in this paper. Thermal gravimetric analysis, isothermal water vapor adsorption, and contact angle methods are used to determine the extent of surface modification and evaluate the hydrophilicity/hydrophobicity of the modified powders and minerals, respectively. The experimental results of the modified mineral surfaces are analyzed using a suggested wettability index based on water vapor adsorption isotherm and contact angle. It is interesting to observe that SO4 2- and Mg2+ ions enhance hydrophilicity characteristic of the modified calcite surface while SO4 2- ions have insignificant effect on adsorption of the tested polar components on the silicate minerals. Mg2+ ions enhance the hydrophilicity of quartz and kaolinite surfaces modified by N,N-dimethyldodecylamine. On the other hand Mg2+ ions increase the hydrophobicity of silicate minerals when asphaltene is the surface modifying component. This may be due to bridging of the hydrated Mg2+ ions with asphaltene. The suggested bridging mechanism is also consistent in the case of alteration of calcite surface with asphaltene, however to lesser extent due to the more affinity toward calcite surfaces.

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