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RESUMO Os fluidos de perfuracao nao controlam apenas as condicoes operacionais de perfuracao. Tambem influenciam nas leituras dos perfis geofisicos dependentes da propagacao do campo eletrico. Nos pocos perfurados para explotacao de agua, a maioria dos fluidos e confeccionada a partir de materiais bentoniticos e polimericos. Experiencia de um fabricante deste ultimo produto mostrou diferenca significativa na resistividade aparente de camadas permeaveis em dois pocos vizinhos - um perfurado com bentonita e o outro com polimero. Atribuiram tal comportamento apenas ao material solido componente do fluido usado.O presente trabalho mostra que tal conclusao nao corresponde a realidade. Dados coletados por seus autores sinalizam que as curvas eletricas convencionais (normais e laterais) sao muito mais influenciadas pela maxima profundidade radial atingida pelo filtrado do fluido de perfuracao (invasao) do que pelo tipo de material solido nele usado. Fluidos polimericos formam finos filmes impermeabilizantes nas paredes dos furos e apresentam pequenos diâmetros de invasao. Por outro lado, os fluidos bentoniticos formam espessos rebocos impermeabilizantes e invadem profundamente, influenciando de tal sorte as curvas eletricas fazendo com que elas registrem a resistividade da zona lavada/invadida e nao a verdadeira, parâmetro necessario para a quantificacao dos sais dissolvidos totais. ABSTRACT Drilling fluids not only control the well and improve drilling operations, they also affect the interpretation of geophysical well logs which depend on the propagation of electrical field. In water wells, the drilling muds are mostly made up of argillaceous or polymeric materials. In an experiment, logging two wells ten feet apart, one drilled with a polymeric mud and the other with a bentonitic mud, a manufacturer of drilling fluids demonstrated that the same permeable beds showed a significant difference in resistivity, and reached the conclusion that electrical logs are affected by the type of materials composing the drilling fluid. This paper shows this conclusion to be erroneous. A large number of data gathered by HYDROLOG clearly shows that resistivity differences between electrical curves are mostly function of the diameter of invasion and not of the type of material used in the drilling fluid. Polymeric fluids form a thin filmcake associated with a shallow invasion. On the other hand, bentonitic fluids create a thick mudcake associated with a deep invasion, and the shallow and deep electrical penetration measurements usually record the same resistivity, that of the flushed/invaded zone.

OS FLUIDOS DE PERFURAÇÃO USADOS NA INDÚSTRIA DA ÁGUA SUBTERRÂNEA E SUA INFLUÊNCIA SOBRE OS PERFIS GEOFÍSICOS DE PRINCÍPIO ELÉTRICO