Abstract

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.

Introduction

The importance of minimizing formation damage induced by an incompatibility between the drilling and completion fluids with the reservoir in order to optimize well productivity is widely recognized. In modern high angle and horizontal wells the influence of formation damage effects can be of greater significance on subsequent well performance. The use of open hole rather than cased and perforated completions means that damage in the near wellbore region cannot be partially or fully penetrated, or bypassed, allowing it to have a major influence on well productivity. Therefore BP Exploration, in common with many operators, stresses great importance on determining the formation damage potential of drilling and completion fluids with laboratory based testing. Experience has shown that the potential for formation damage for a specific drilling or completion fluid is extremely dependent on the reservoir section to be drilled. Therefore testing tailored to a specific drilling and completion programme in a particular field is carried out.

Should an incompatibility between a drilling or completion fluid be detected it is extremely important to determine the formation damage mechanism. This allows a basis for proposing modifications to the fluid formulation or, if necessary, an acceptable alternative system. It is also important to be able to predict formation damage problems in other reservoirs. A sound basis of prediction can clearly save both time and costs when considering proposed drilling and completion fluids for future programmes.

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