It is commonly understood that a reservoir drilling fluid must be compatible with the reservoir rock, both chemically and physically. One important aspect in optimizing drilling fluid is distribution of bridging particle sizes. Appropriate particle sizes can limit fluid loss to the formation and minimize solid plugging into reservoir pores. Reservoir rocks have complicated fluid-flow systems and it is important to understand pore characteristics of the rocks including pore type, size distribution and connectivity; and, secondary changes such as dissolution and fracturing. If inter-particle pores are the dominant fluid-flow channels for the sandstone and pore distribution is relatively homogeneous, a full range of particle-size distribution of bridging material may effectively bridge all pore spaces. But for many types of sandstone, natural dissolution of feldspar, rock fragments, or carbonate material has created oversized pores. Appropriately sized particles should be used to bridge them, and smaller particles should be retained in fluids to bridge smaller pores and to improve filtercake quality. For carbonate rocks, fracturing and/or dissolution may have created large pores or fluid-flow channels. They are usually inhomogeneous in space. Their sizes vary greatly and could be up to centimeter scale. The bridging particle size distribution should be comparable to the inhomogeneous pores or channels, if practical.

Differential pressure between wellbore and formation pore fluid is also a factor which influences bridging size selection. At high differential pressure, relatively large particle sizes may help limit the solids plugging into pore spaces and decrease the effect of filtercake compaction. However, a combination of pore size, fluid property, and filtercake quality considerations are needed to achieve the most effective particle size distribution.

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