An on-line linear X-ray apparatus has been used to examine deep bed filtration during water injection in Bentheimer sandstone core samples. The aim of this project was to investigate the effect of residual oil saturation on the deposition profile of suspended solids in the injected water, and to compare that with a previous study at fully brine saturated cores. Hematite particles were used as suspended solids in the brine. The X-ray apparatus was used to estimate the amount of deposited material at any time and distance along the core sample. Permeability decline as determined by the pressure change and chemical analysis of hematite concentration were used to confirm the X-ray analysis.

The effect of residual oil saturation was seen in most of the samples. The presence of oil caused greater apparent damage (reduction of permeability).It was also observed that there is rather deeper invasion at residual oil saturation than at fully brine saturation, and it was inferred from the X-ray measurements that the external filter cake is thicker.

1. Introduction

Re-injection of produced water is of increasing importance as water cuts continue to increase worldwide. It provides an environmentally acceptable way of the disposing of water, and contributes to pressure maintenance if injection takes place into the reservoir itself.

When designing a water flooding project, it is very important that the injecting water is sufficiently clean. Solid particles and oily-water can block the reservoir pore throats and cause rapid and severe permeability decline depending on water quality and reservoir characteristics. Total removal of solids and oil may not be economically feasible and therefore a relationship must be found between the concentration of suspensions, particle size, oil concentration and other parameters that help to minimize or slow the rate of impairment (1–7).

The performance of the injection well and the distribution of the injected water is strongly influenced by the build up of formation impairment around the wellbore or the fracture face. Solid particles and small oil droplets in the injection water, which are deposited in the formation by a process of filtration, will cause this impairment.

As part of a study on filtration mechanisms, an online single detector x-ray system was used to examine deep-bed filtration during water injection in sandstone cores saturated with residual oil.Hematite particles were suspended in the brine at different concentrations. Several researchers tried to study deep-bed filtration on cores that are either 100% saturated with brine, oil, or residual oil while injecting suspended particles or oily-water, or a combination of both, but very few (8,9) have tried injecting suspended particles into cores at residual oil saturation. Such cores were used in the tests reported here to more closely approximate wettability effects.

In the previous analysis (7), the X-ray system provided a means of quantitatively measuring the deposition profile of hematite as a function of time during the test. The measurement offered a direct way of testing deep-bed filtration and filter-cake build-up theories.

Several models are available in the literature for deep bed filtration. Most of these models are based mainly on measurement of effluent concentration and only the final deposition profile in the core, so the tests reported here provide a more rigorous check of the models. The results of the tests will be discussed, and the implications for the models will be examined. Future work will be carried out to model the filtration phenomena.

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