Microwave properties of drilling fluids

Abstract

The application of electromagnetic (EM) techniques to well logging is initiated in an environment dominated by the properties of the drilling fluids. An impulse technique using nanosecond pulses is applied to a coaxial waveguide containing drilling fluids to measure the velocity (dielectric constant ε) and absorption (attenuation coefficient α) of EM impulses. It is the large difference in dielectric constants of water and oil which makes EM propagation techniques attractive for logging. Dielectric properties of some nondispersed drilling fluids (bentonite and attapulgite clays) are found to be largely dependent upon the volume of water present. Both bentonite and attapulgite clays exhibit the same range of dielectric constants (ε = 81 → 75) when the weight percent of clay is increased to 10 percent. In contrast, the microwave attenuations of these two clays are quite different, with that of the bentonite increasing at about 4 times the rate of the attapulgite suspensions. Microwave attenuation measured for a variety of commercial drilling fluids varies over a wide range, with the lignosulfonates the largest (91 dB/m) and oil inverts the smallest (3 dB/m). The oil inverts also have a small dielectric constant (ε = 3 → 6). Temperature dependence of the attenuation for these same drilling fluids is determined in the range from 23 °C to 45 °C to indicate their behavior under in situ conditions.