Formation damage induced by the invasion of low ionic strength brines is well understood and frequently documented. However, very little has been recorded regarding damage induced by very high ionic strength brines. A formation damage reaction between specific pore lining clay species and saturated salt drilling mud has been identified with the potential for significant permeability impairment in the near-wellbore region. This reaction was found during a reservoir condition core flood investigation into well performance in a field in South America operated by BP Exploration. Damage levels around 70% were recorded in reservoir core samples. Characterization of the formation damage mechanism was carried out by scanning electron microscopy. This revealed that flocculation of the pore lining clays had occurred, due to the invasion of the very high salinity brine, producing significant pore throat bridging and plugging. The type and concentration of the clays was determined in order to predict if this damage mechanism was an issue in future investigations. Further testing with an alternative drilling system showed that a non-damaging mud could be formulated for this field. A subsequent formation damage investigation into the likely impact of saturated salt drilling mud on the productivity of a North Sea field operated by BP Exploration revealed a sandstone with an almost identical clay fraction. It was predicted that the same damaging mechanism would occur. Return permeability testing confirmed this and validated the mechanism. However, in this specific case the extremely high permeability of the formation significantly reduced the impact of the pore bridging and plugging. It is recommended that when high salinity water based drilling muds are being considered for inclusion in reservoir drilling programmes the clay fractions of the formation are carefully considered to identify potentially sensitive sandstones. Specific formation damage testing should then be carried out to assess the likely impact on well productivity and whether this is acceptable.

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