Implementation of a LED-Photodiode and CMOS Camera System for Water Detection and pH Measurement in a Multiphase System, 18-R8245
Leonardo J. Caseres
Inclusive Dates: 07/25/11 – 11/25/11
Background — Determination of free-water content in a multiphase fluid is of great practical interest to the oil and gas industry from the point of view of corrosion monitoring and mitigation. The presence of free water in crude oil during transportation in offshore and onshore pipelines has been associated with accelerated corrosion rates in oil and gas production operations. Today, portable devices for rapid analysis of free-water content in crude oil in deep-sea environments are lacking. As a result, there is a need for a tool to measure the presence of free water in a multiphase system for both offshore and onshore applications. In addition to the detection of free water in crude oil, it is of critical importance to determine the properties of the free water in a multiphase system, such as its pH. Accurate pH measurement allows for more accurate selection of appropriate completion materials and effective planning for scale formation treatment and inhibition. Numerous corrosion predictive models are being developed to accurately predict corrosivity as a function of pH and other parameters in multiphase and define accurate limits of use of carbon steel pipes. However, many of these models are overly conservative or focus only on a narrow range of parametric effects, thereby limiting their scope of applicability. Thus, there is a need to measure and validate the pH in the field by using a tool capable of measuring in-situ pH in multiphase pipelines.
Approach — The objective of this project was to test and validate a proof of concept consisting of a combination of an LED/photodiode and a CMOS color camera/pH-sensitive coating for measuring the amount of free water and pH in a multiphase system. The sensor prototype (Figure 1) consisted of two sealed prisms, 4-inches long, 2-inches high, and 3-inches wide made of transparent acrylic. One prism contained three off-the-shelf LEDs (RGB type LED, white, and an infrared LED) and the other prism contained a CMOS color camera as well as two photodiodes. For simplicity, the CMOS color camera, LEDs, and photodiodes were hardwired to a data acquisition unit. Then, the surfaces containing the electronic components in both prisms were coated with a commercially available oleophobic coating. A mixture of the oleophobic coating with a bromocresol green pH indicator was applied in front of CMOS camera. The sensor prototype was exposed to varying mixtures of crude oil and 3.5%wt NaCl solution to attain water fractions from 0 to 100 percent at flow rates of 70 and 100 mL/min. The distances between the sensing elements were set to 1.6, 3.175, and 5 mm. Sensor results were correlated with the mixture flow rates, water fraction, and distance between the LEDs and photodiodes. In addition, an ac impedance technique was used to compare the results obtained by the proposed optical measurement.
Accomplishments — SwRI has successfully tested and validated a cost-effective optical measuring device for water fraction determination and pH in multiphase systems. The results of the pH measurements showed that the CMOS camera in conjunction with the pH-sensitive coating was effective in the determination of solution pH. The sensor response to water fraction determination demonstrated that the IR LED/photodiode proved to be the best combination for measuring water fraction in crude oil in the range from 1 to 100 percent (Figure 2). An increase in water detection sensitivity, especially for small water fractions, can be increased by reducing the separation between the LED source and photodiode. The results also showed that flow rates lower than 100 mL/min did not have an influence on the water detection. However, it is expected that for higher flow rates water detection might be affected. If this is the case, increasing the sampling time could resolve this potential issue. In addition, the optical measurement technique proposed here was far more sensitive to water detection in crude oil than the ac impedance technique, especially at water fractions lower than 20 percent.