Particulate invasion is one of the primary mechanisms of formation damage caused by drilling fluids. During the initial stages of filtercake growth particles are forced into the formation, building an internal filtercake which plugs the near surface pores. Where bridging is inadequate, the depth of invasion can be considerable. Removal of this internal cake can be difficult, leading to reduced productivity. Better understanding of its properties will lead to more effective cleanup and enhanced productivity.

As part of an EC-Joule funded study into formation damage mechanisms, we have investigated both the invasion of solids and their mobility during clean up. We have systematically investigated the effect of mud particle size on rock substrates of varying permeabilities. The results indicate that formation damage increases with increasing substrate permeability. Substrate permeability was also found to have a marked effect on the initiation of production; flow initiation pressures were greatest on low permeability cores.

Quantitative profiles of solids invasion into sandstone cores for a range of water-based drilling fluids were obtained using scanning electron microscopy/ X-ray mapping (SEM-EDS) and synchrotron energy-dispersive X-ray diffraction tomography (EDD-T). This latter technique, using a synchrotron source, samples the intact core rather than a fractured surface, and gives analytical powder diffraction data, whereas a conventional CAT scanner provides attenuation data only. The effect of back flooding on the invasion profile was then examined, and permeability damage associated with the invasion was also determined. The invasion was compared to damage profiles obtained by core sectioning. Invasion profiles were found to drop quite steeply but fines could be observed deep with in the core. The presence of external cake was found to influence the ease of clean up of the invaded zone.

A deep bed filtration model has been developed for the effect of solids invasion on permeability reduction. This model is compared to the profiles obtained in the invasion experiments.

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