Countercurrent imbibition is an important recovery mechanism during waterflooding in fractured reservoirs. This may be a rapid and efficient recovery process in strongly water-wet systems, but if the reservoir is mixed-wet, while it is possible for some water to imbibe spontaneously, the ultimate recovery is lower and the imbibition rate may be several orders of magnitude slower than for strongly water-wet rock. We use quasistatic pore-scale network modeling as a tool to study the behavior of mixed-wet rocks and to predict relative permeability and capillary pressure. The model uses a topologically disordered network that represents the pore space of Berea sandstone. We adjust the distribution of contact angles at the pore scale to match previously published experimental cocurrent waterflood recoveries and wettability indices on Berea. We then input the relative permeabilities and capillary pressures into a conventional grid-based code and simulate countercurrent imbibition in 1D. We make predictions, with no matching parameters, of the recovery as a function of time and compare the results with the experimental measurements. We are able to reproduce the observed dramatic increase in imbibition time as the system changes from being water-wet to mixed-wet. In a mixed-wet system, spontaneous imbibition, where the capillary pressure is positive, is limited to a narrow saturation range where the water saturation is small. At these low saturations, the water is poorly connected through the network in wetting layers and the water relative permeability is extremely low, leading to recovery rates tens to thousands of times slower than for water-wet media. We present a semiempirical equation to correlate imbibition recovery in mixed-wet rocks of different wettability and viscosity ratio. The recovery rate is proportional to the water mobility at the end of imbibition.
Counter-current imbibition is an important recovery mechanism during waterflooding in fractured reservoirs. While this may be a rapid and efficient recovery process in strongly water-wet systems, the vast majority of reservoirs show some mixed-wet or oil-wet characteristics. If the reservoir is mixed-wet, it is possible for some water to imbibe spontaneously, but the ultimate recovery is lower and the imbibition rate may be several orders of magnitude slower than for strongly water-wet rock. We use quasi-static pore-scale network modeling as a tool to study the behavior of mixed-wet rocks and to predict relative permeability and capillary pressure. The model uses a topologically disordered network that represents the pore space of Berea sandstone. We adjust the distribution of contact angles at the pore scale to match previously publised experimental co-current waterflood recoveries and wettability indices on Berea. We then input the relative permeabilities and capillary pressures into a conventional grid-based code and simulate counter-current imbibition in one dimension. We make predictions, with no matching parameters, of the recovery as a function of time and compare the results with the experimental measurements. We are able to reproduce the observed dramatic increase in imbibition time as the system changes from being water-wet to mixed-wet. In a mixed-wet system spontaneous imbibition, where the capillary pressure is positive, is limited to a narrow saturation range where the water saturation is small. At these low saturations the water is poorly connected through the network in wetting layers and the water relative permeability is extremely low, leading to recovery rates tens to thousands of times slower than for water-wet media. We present a semi-empirical equation to correlate imbibition recovery in mixed-wet rocks with different wettability states and for a wide range of viscosity ratios. We show that the recovery rate is proportional to the water mobility at the end of imbibition. Introduction Fractured reservoirs are important oil and gas resources. The fracture network contains a small amount of oil in place compared to the lower permeability matrix that it is connected to. High well productivity and relatively low ultimate recovery are typical characteristics of these reservoirs. Waterflooding is one of the most important mechanisms of oil production from fractured reservoirs. Imbibition is the displacement of non-wetting phase by wetting phase. In a strongly water-wet rock water rapidly imbibes into the rock and displaces the non-wetting phase, oil. However, the majority of reservoir rocks are not strongly water-wet.1 Salathiel2 introduced the term mixed wettability for cases where the rock contains both water-wet and oil-wet fractions. After primary oil flooding those larger pores occupied by oil may change their wettability, while smaller water-filled regions of the pore space remain water-wet. The adsorption of surface-active agents in the oil, such as asphaltenes, to the pore surface in direct contact with the oil causes wettability alteration.3–6 One of the important characteristics of mixed-wet rock is its ability to imbibe both water and oil.6–9 Zhou et al.10 performed 23 spontaneous imbibition and 27 waterflood experiments on Berea cores with different wettabilities and initial water saturations, corresponding to different wettabilities. To establish different initial water saturations after primary drainage, brine was displaced by Prudhoe Bay crude oil at different injection pressures. Then the samples were aged at a temperature of 88°C for between 0 and 240 hours to alter the wettability of the samples from water-wet towards mixed wettability. The oil that was in the core during aging was displaced with fresh crude oil prior to imbibition and waterflooding tests. Counter-current imbibition recovery was measured by the change in weight of the aged samples that were hung in a degassed brine solution. In flooding tests, the samples at initial water saturation were flooded at slow rates using a constant injection pressure. The waterfloods were stopped after 4 to 15 pore volumes of brine injection when the water-oil ratio, rwo, was greater than 99. The recovery at this point was used as an operational definition for the final waterflood recovery Rwf.
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