Wideband Microwave Sensor for Downhole Water-Cut Monitoring

Abstract

In the petroleum industry, downhole water-cut loggings are essential for reservoir management. Previously, most of them are based on permittivity measurements by low-frequency capacitance methods. However, they fail to work in aging oil fields with high water cuts and highly mineralized subterranean water. In this article, a nonresonant self-calibrated wideband microwave sensor is proposed to solve this problem. Taking advantage of the Cerenkov-like enhanced radiation of coaxial leaky-wave antennas, sufficient signals can be received even when the fluid under test (FUT) is highly conductive. An accurate wideband forward model has been established with no empirical or undetermined quantities. Complex permittivities and water fractions of the FUT can be retrieved without calibrations, making the sensor especially suitable for the downhole working conditions with unknown temperature drifts. NaCl solutions with different concentrations (0–130 g/L), oil–water emulsions with different water fractions (70%–100%), and ethanol–water solutions have been tested in the frequency band ranging from 1 to 8 GHz. Dielectric constants and conductivities of the NaCl solutions range from 40 to 80 and from 0 to 15 S/m, respectively, covering the main measurement ranges of high-water-cut monitoring.