Microwave Sensing of Water-Cut in Production Fluids

J. Oliverio Alvarez
Aramco Services Company: Aramco Research Center -- Houston,
United States

Keywords: microwave measurements, permittivity, water-cut sensing, water-cut imaging


A microwave water-cut meter for geological applications was designed and tested. The meter uses a vector network analyzer to measure the reflection (S11) and transmission (S21) coefficients of the material under test, such as production fluids, oil spills, rock cores or soil. The initial design of meter consisted of a pair of waveguides whose ends face each other and are placed on the inner surface of the pipe/core holder. The waveguides have a diameter similar to that of the main pipe and are filled with specific low loss materials whose dielectric constant is close to that of the fluid in the pipe. The feasibility of the microwave meter was studied initially by numerical simulations. Three dimensional simulations were conducted in the frequency domain and the values of the S parameters at the transmitting port and at the receiving port were obtained. The numerical experiments were conducted using two different filling materials, quartz and sapphire. These materials satisfy the conditions to be low-loss (tanδ < 0.0001) and to have low varying dielectric properties for a wide range of temperatures. Different oil and water mixtures at 40ºC were considered by varying the water salinity. Permittivity of the mixture was modeled by the refractive mixing model. Results showed a clear sensitivity to the water-cut. Moreover, when water-cut increases the initial filling (quartz) material can be changed for a higher dielectric constant material. A new and optimized meter was recently designed and built. In order to have mode propagation over a larger bandwidth, the new design changed from a regular waveguide to a ridge waveguide. Feasibility simulations were performed with both quartz and glass-filled Noryl as the waveguide filling materials. Results showed that the ridges in the waveguide provide a significantly better transmission than just the waveguide alone. In addition, between one and three frequencies between 2 GHz and 4.5 GHz provide both the transmission and the reflection coefficients even in the case where just low salinity water is present. Furthermore, for high salinity water, high power amplifiers can be used to reach the -80 dB threshold for transmission detection in a vector network analyzer. Thus, these results give a possible alternative to switching waveguides for higher water-cuts. A second meter prototype, made from aluminum and with glass-filled Noryl filling the waveguides, that incorporates the ridge waveguide design, was built and tested. The waveguides are feed through an N-type connector. As in the previous prototype, the diameter of the waveguides is slightly smaller than that of pipe. Initial testing was conducted with the pipe filled with only gas (air), mineral oil or water filling. Initial results show the new design is capable to differentiate between water and hydrocarbons. Multiple measurements will be performed with different water and oil contents. These measurements will provide a physics-based measurement database that will be used as an initial training set for a machine learning algorithm.