All water injection schemes suffer injectivity decline to some degree. It is important to understand the factors that influence the formation of an external filtercake and improve injectivity. This paper introduces a new technique for examining time-dependent particle filtration processes in a sandstone core.

Experimental work and analysis is presented of the filtration phenomena that occur during injection of water containing suspended particles into a sandstone core. The technique involves injecting a sequence of suspensions of different-coloured latex particles It gives valuable qualitative information about the processes taking place, such as the location and timing of the trapping of particles and gives quantitative predictions, by using Image Analysis and the RGB-colour system

Tests has been performed in which coloured latex particles are injected into sandstone cores. The particles form an external filtercake on the entry face of the core, and are deposited inside the core (deep-bed filtration). By switching the colour of the injected latex particles, and analysing where the different colours have been deposited, qualitative insight is gained into the process of external and internal cake forming. Using Image Analysis based on the RGB-colour system, the particle analysis can be quantified to determine effluent concentration and particle colour. Conclusions can be made about the filtration processes taking place. The conclusions are that current theories of how and when an external filtercake forms are too simple, and that external filtercake permeability for individual particles is larger than anticipated.

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