Particle movement and scale formation in porous media.

The flow of particulate suspensions and brines in porous media is encountered in a wide range of industrial situations such as oil production, soil erosion, and ground water pollution and in the operation of filter beds. Three major classes of such problems are addressed in this thesis, namely particle movement, scale formation and filtration. Particle movement and scale formation are known to be serious problems in oil reservoirs where incompatible brine is brought into contact with the formation. Dramatic reductions in permeability are observed in such cases due to the release of fine particles that subsequently plug flow channels downstream. This study deals with an experimental and theoretical study of formation damage resulting from particle movement and scale formation in porous media. An experimental investigation was undertaken to look into the possible causes of the injectivity loss in a typical Iranian oilfield. Sets of experimental investigations were undertaken with different objectives in mind. Glass and sand bead packs were used to test the experimental set up and to observe the general behavior of fine particle movement and scale formation in porous media. The experiments were conducted with injection rates from 25 cm3/min to 100 cm3/min. Particles of alumina oxide were suspended in the injected water to simulate fine particle migration in porous media. The particles were injected at concentrations ranging from 500 ppm to 2000 ppm. It was observed that the build-up of flow resistance was mostly due to frontal face plugging. Alumina particles were added to the glass beads to study the effect of particles initially present in the glass bead pack. A mathematical model is presented that simulates the porosity impairment by particle movement. This model, which is based on the mass balance of particles flowing through the porous media, at first, proposes an overall equation to determine the porosity and by a special mechanism it can predict the overall change in porosity. The equations are quite general and can be used to model the permeability reduction for any given pore or particle size distribution. Also this study aims to investigate the calcium sulphate and calcium carbonate scaling in a porous medium to discover its possible effect upon the permeability of the porous medium. The information on the effect of scale deposition is obtained experimentally by flooding porous medium with incompatible brines. The experimental data are used to develop and validate models for predicting the permeability decline caused by scale. The model is intended to provide a link between what is known or can be assumed about brines in an oil reservoir. A computer program was developed which can predict the scaling tendency of the sulphate minerals and calcium carbonate, in water disposal wells, water injection systems, surface equipments and facilities that commonly form scale. This program is based on the latest correlations of experimental data relating to North Sea and Iranian oilfields. It is used to determine the degree to which brine becomes supersaturated with a scale forming mineral under specified conditions. Processes capable of creating supersatuartion in oilfields are described and it is shown that the distribution of scale depends on the process involved