Considerable emphasis is placed on the selection of carefully sized bridging agents (including calcium carbonate, sized salt and cellulosic materials) for low damage reservoir drilling fluids. The rationale behind this selection is to optimise the filter cake-building properties of the fluids, and hence minimise formation damage due to invasion of mud particles into the near wellbore formation. This feature is particularly desirable in non-cemented completions where there are no perforations to bypass such near wellbore impairment. In addition the filter cake needs to be compatible with sand control techniques and equipment so that the productivity is not adversely affected.

Laboratory experiments on clean reservoir drilling fluids can, indeed, show the benefits of correct particle sizing, particularly in situations where the formations have high matrix permeability. There is some doubt, however, that these laboratory observations can always be repeated in the field, where the introduction of drilled solids and exposure of the fluid to high shear can result in a decrease of particle size.

The paper presents data from tests carried out on laboratory and field reservoir drilling fluids which show the extent and speed of the changes that occur in bridging solids composition and size distributions during drilling. The effects these changes have in terms of filter cake properties have been demonstrated.

Practical recommendations are made, which have been applied to wells drilled in West Africa. One well was a +600 m long horizontal section, where the filter cake was determined to have a neutral effect on the skin factor, without treatments for filter cake removal. A standard fluid would normally require a treatment, even if it is claimed to be non damaging.

Correct fluid composition, bridging material selection, and good maintenance of the drilling fluid system are the keys to success for better productivity. A filter cake that allows a better flow profile along the entire wellbore reduces the risks of water coning or erosion associated with non uniform flow and pressure drop. Very often, the cost of a specialised reservoir drilling fluid system is less than that of any treatment required to remove a standard filter cake and restore the original permeability along the entire borehole.

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