Measurement of Water Content of Oil Downhole Using Fiber Optic Sensors and Nanotechonology
Authors: Jiang Liping , Ruan Hang , Chen Yaosheng , Liu Yanjing , Claus Richard O. 
Published: October 2008

Since water occurs naturally in petroleum formation, and is used in the secondary recovery of oil from wells, it is particularly desirable to be able to measure the water content of oil (watercut) during the exploration, drilling, production and transmission of oil and gas. In general, oil/water/gas cut measurements have been traditionally carried out by taking samples from wells or pipes, separating the oil, water and gas, and measuring the volumes of the separated components. As the water content of the oil flow is nonhomogeneous and changes over time in a pipe, there is no guarantee that a sample is representative. A new method for measuring the borehole watercut is given in this paper. It uses fiber-optic evanescent wave sensing (FOEWS) technology and nanotechnology based on the electrostatic self-assembly monolayer (ESAM) method. The watercut measured by this sensor ranges from 0% to 100% and its precision is lower than 2%. The characteristics of this sensor are analyzed.

Introduction

As more oil fields move into secondary and tertiary recovery, the need for production logging is increasing1. In a well producing both oil and water, an accurate flow profile indicates the production rates of the two fluids from each interval of interest. This profile is essential for raising the oil production, reducing the water production, characterizing and managing the reservoir. Watercut is one of the most important parameters among the production logging information.

The major source of error found with traditional capacity based sensors is due to salinity. The capacitive coupling current consists of displacement current and conduction current affected by salinity. When watercut is greater than 50%, the oil and water mixture becomes a water-continuous emulsion, and the conduction current is significantly larger than the displacement current. Consequently, the watercut measured by the capacitive based method ranges only from 0% to 50%. The use of high frequency methods can partially solve this problem2. However by implementing fiber-optic sensing technology the range is increased to 100%.

Fiber-optic sensing (FOS) technology has developed dramatically since the late 1970's. It has some advantages over the conventional sensing technology such as high sensivity, high flexibility and immunity from electromagnetic fields. The optical fiber sensor we describe in this paper has been modified using electrostatic self-assembled nanostructured materials to enhance its sensitivity, environmental stability and robustness. A novel method for determining water cut downhole based on FOEWS and the ESAM material synthesis method is presented.

Design Concept

FOEWS Technique.

For a straight fiber, the electric field of the guided beam decays exponentially into the cladding to form an evanescent wave. When the fiber is bent, the phase velocity of the wavefront around the outside of the bend must be greater than it is in the medium, as shown in Figure 1. The velocity of propagation increases as we move father away from the core, until it becomes equal to the local velocity of light. At this point the wave is no longer guided and is instead radiated.