Dielectric properties of nonaqueous drilling fluids from 1 MHz up to 20 GHz

Measuring the water volume and the rock textural information downhole in wells with a logging tool requires an accurate knowledge of the dielectric properties of the borehole fluid. A small error on these properties can generate significant misinterpretation of the data, especially in the presence of resistive oil-based mud (OBM) or synthetic-based mud (SBM), both of which are also called nonaqueous drilling fluid (NAF). A model predicting the complex permittivity of NAF as a function of its density and the oil/water (O/W) ratio was developed in the 1970s, but since then, the NAF compositions have changed, which modified their dielectric properties. To revisit this model, a measurements campaign was launched in July 2012 to characterize the dielectric properties of NAF. NAF that results from an emulsion of water in oil consists of many components such as emulsifiers, salt, lime, weighting material, and surfactants. Different NAF formulations were prepared and the interactions between key parameters such as the O/W ratio (90/10 to 60/40) and density (9 to 15 lbm/gal) were evaluated using a central composite design (CCD).The complex permittivities of the selected NAF compositions were measured using two set-ups covering a wide frequency range, from 1 MHz up to 20 GHz. A statistical analysis of the measurements shows that the complex permittivity of NAF depends on two main influences: brine volumetric fraction and weighting material volumetric fraction. These two parameters can respectively be expressed as a function of the O/W ratio and density. The surface quadratic model responses developed during this study are validated by measuring the complex permittivities of different NAF samples from the field.