Impacts of interactions with low-mineralized water on permeability and pore behavior of carbonate reservoirs

Laboratory filtration experiments are employed to investigate effective well killing while minimizing its impacts on surrounding rocks.The novelty of this experimental study lies in the prolonged exposure of rock samples to the killing fluid for seven days,corresponding to the average duration of well workovers in the oilfields in Perm Krai,Russia.Our findings indicate that critical factors influencing the interactions between rocks and the killing fluid include the chemical composition of the killing fluid,the mineralogical composition of the carbonate rocks,reservoir pressure and temperature,and the contact time.Petrophysical analyses using multi-scale X-ray computed tomography,field emission scanning electron microscopy,and X-ray diffraction were conducted on samples both before and after the well killing simulation.The experiments were performed using real samples of cores,crude oil,and the killing fluid.The results from this study indicate that low-mineralized water(practically fresh water)is a carbonate rock solvent.Such water causes the dissolution of rock components,the formation of new calcite crystals and amoeba-like secretions,and the migration of small particles(clay,quartz,and carbonates).The formation of deep channels was also recorded.The assessment reveals that the change in the pH of the killing fluid indicates that the observed mineral reactions were caused by carbonate dissolution.These combined phenomena led to a decrease in the total number of voids in the core samples,which was 25%on average,predominantly among voids measuring between 45 and 70μm in size.The change in the pore distribution in the bulk of the samples resulted in decreases in porosity of 1.8%and permeability of 67.0%in the studied core samples.The results from this study indicate the unsuitability of low-mineralized water as a well killing fluid in carbonate reservoirs.The composition of the killing fluid should be optimized,for example,in terms of the ionic composition of water,which we intend to investigate in future research.

Viscous fingering and its effect on areal sweep efficiency during waterflooding: an experimental study

Viscous fingering is one of the main challenges that could reduce areal sweep efficiency during waterflooding in oil reservoirs. A series of waterflooding experiments were carried out in a Hele-Shaw cell at ambient temperature during which areal sweep efficiency was estimated and techniques to ease the fingering problem were examined. The onset and propagation of viscous fingers were monitored as a function of both injection rate and injection/production positions. Image processing techniques were utilized to quantitatively investigate the propagation of fingers. The experimental results show that, under specific conditions, increasing the number of finger branches could improve the areal sweep efficiency, whereas growth of a single narrow finger has a negative impact on oil displacement efficiency. According to the obtained results,increasing the injection rate improves the areal sweep efficiency up to a critical rate at which viscous fingers start to grow.The impact of heterogeneity of the medium on distributing the viscous fingers was also investigated by introducing two different arrangements of fractures in the model. The results show that fractures perpendicular to the direction of flow would distribute the displacing water more uniformly, while fractures in the direction of flow would amplify the unfavorable sweep efficiency.