Abstract

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.

1. Introduction

Re-injection is in many cases the cheapest and most environmentally-friendly solution for produced water disposal. Water forms 75% of all fluids produced worldwide from oil reservoirs and the volume of water will grow in future. Re-injection can also be applied as a water flooding mechanism. It has thus gained significant importance in the last few decades. The main problem associated with Produced Water Re-injection is the unpredictable injectivity decline of the injection wells.

Produced water contains solid particles and oil droplets at a certain varying concentration. The produced water is often cleaned to a degree. However, it is impossible to get rid of all the contaminants before injecting them into the reservoir. During the injection process these contaminants are deposited in the formation pore throats, blocking the pores and consequently causing rapid decline in the permeability of reservoir rock. Even in injection under fracturing conditions, these factors are important since the rate of leak-off from the fracture into the formation helps to determine the fracture growth rate.

To predict permeability decline and reduce the impairment caused, an understanding is needed of the relationship between the concentration of suspended particles, grain size, injection velocity and the surface charge of suspended solids. This paper introduces a new experimental technique, which can give new insight into the processes which are affecting permeability decline. It reports on tests in which coloured latex particles have been injected into sandstone cores. The aim is to track the path of the coloured particles to show the trapping processes at the entrance to the core and inside the core. The reason for using colours is to determine how the process of particle deposition changes with time.

One of the aims of this study was to examine the concept of Transition Time introduced by Pang & Sharma (1997). They conjectured that when a suspension of particles is injected into a porous medium the external filtercake does not start to form immediately. Initially deposition takes place inside the porous medium, forming the internal filtercake. When this filtercake reaches a critical density, at the Transition Time, then external filtercake starts to form.

This content is only available via PDF.
You can access this article if you purchase or spend a download.