Over the past decade formate brines have proved to be viable fluids for high temperature and high pressure drilling applications. These high density, low solid, and corrosion free drilling fluids improve many aspects of drilling performance and enhance well productivity. However, formate fluid systems can cause significant effects on nuclear logging tool responses, and these effects have not been previously characterized. Recently, a decision was made to use formate mud fluid in a well that was to be drilled in tile North Sea. During the planning phase for this well, we were able to perform numerical modeling and make laboratory measurements to determine the effects of the formate drilling fluid on nuclear log responses. Correction charts and algorithms were developed for tile specific mud formulation planned for the well. The resulting corrected log porosity was comparable with measured core porosity in this well, and was used in tile reservoir evaluation model. The effects of all of these factors on nuclear log responses were modeled using Monte Carlo simulation in which complete geometrical and elemental description of the problem and tile most recent nuclear data libraries were used. The accuracy of the models was verified using laboratory measurements made in Aberdeen. Modeling of fully invaded and non-invaded formations demonstrates that knowledge of the level of formate mud invasion is essential for use of the correction algorithms.

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