Benchtop Porosity Measurement Technique Using Electrical Capacitance Tomography (ECT) Sensor

The literature review explained about the research done on topics related to the project, such as the design of ECT sensor and porosity measurement. ECT sensor is used to visualize and measure the permittivity distribution of a cross-sectional area of a vessel. The ECT sensor may not be effective in measuring porosity, but with further improvement, the possibility of measuring and visualizing the porosity distribution of core sample can be done.

Areeba Shafquet and Muhammad Faidhi Bin Mat Sobri who offered their help and time in designing and manufacturing the ECT sensor.

Background of Study

ECT Sensor
Porosity

Problem Statement
Objectives
Scope of Study

Relevancy of the Project
Feasibility of the Project within the Time Scope and Frame

The design of the sensor involves many considerations in order for it to be usable. For this part, the sensor will be used to measure the porosity distribution of the core sample. The scope of investigation for this project will be divided into four main categories.

The second part will be the examination of the Electrical Capacitance Tomography (ECT) overview.

Porosity

Absolute Porosity
Effective Porosity

Porosity Measurement

Porosity Measurement Technique using Helium Porosimeter
Porosity Measurement Using Saturation Method

Core Sample
Electrical Capacitance Tomography

Measurement Principle

Sensor Design

Capacitance Measurement Concept
Electrodes Number
Electrodes Length
External and Internal Electrodes
Earthed Screen
Normalization of Measurement

Porosity Distribution Measurement Method
Data Acquisition System

In industry, there are several techniques that can be used to measure the porosity of the core sample of a rock in the laboratory. For this method bulk volume and pore volume are measured to get the porosity of the core sample. Although bulk volume can be calculated from measurement of the uniform core sample dimension, the usual procedure is to observe the volume of liquid displaced by the core sample.

In any case, it is necessary to prevent the liquid from penetrating into the pore space of the core. The electrodes surround the cross-section of the content that will be imaged by ECT. The information is then used to create an image of the content by measuring the capacitance between pairs of electrodes.

Yang suggested that the length of the electrode should be made larger than the diameter of the ECT sensor[12]. For internal electrodes, they must be placed outside the insulating frame of the ECT sensor. The measurement of porosity distribution using ECT is based on the concept of detecting the dielectric constant of the material.

Since the optimal number of electrodes used in the design of the sensor is 12, the number of independent electrode capacitances that can be measured is 66, and it will be projected onto a (32 X 32) square pixel grid to generate the permittivity distribution image. An image can only be created on the pixels that lie on the cross-sectional image of the vessel. On a (32 X 32) square pixel grid containing 1024 pixels, only 812 pixels are required to construct the cross-sectional image of the core sample.

The normalized permittivity distribution refers to the fractional concentration distribution of the material with the highest permittivity.

Project Activities
Procedure Identification

Process Flow
Key Milestones
Gantt Chart

Tools and Equipment
Experimental Procedure

The data obtained from the measurement of the ECT sensor is then compared to the standard method of obtaining porosity from the laboratory, using the helium porosimeter method and the water saturation method. The result of the porosity percentage of both measuring techniques is then compared with the porosity measured using the ECT sensor. The core sample is measured using an ECT sensor and the data is collected by the data acquisition system and uploaded to the computer.

The porosity of the core sample is measured using the conventional technique of helium porosity and water saturation. The title of the project was selected in week 1. Literature review covered week 3 through week 6. The comprehensive proposal was submitted in week 6, while week 9 was the defense of the proposal. Because the ECT sensor is expected to be ready during the FYP 1 timeline, the FYP 2 will begin measuring porosity activity using ECT and the conventional technique in the lab.

The saturated core sample with distilled water will then be removed from the beaker full of water. The saturated core sample will then be placed within the area surrounded by electrode within the sensor. The core sample will be exactly the same as the core sample used by ECT sensor measurement technique.

The core sample would be exactly the same as the core sample used for ECT sensor and helium porosimeter measurement. The effectiveness of porosity measurement using ECT would be determined by the difference between both techniques.

Sensor Design
Sensor Fabrication
Calibration Measurement
Core Sample Preparation
Online Sensor Measurement
Measurement Results
Discussions

To identify the porosity of the core sample, the distilled water is used to fill the pores in the core samples. The results of the saturation of distilled water with the core sample are shown in Table 8. An increase in weight for all three core samples indicated that they were saturated, where distilled water entered the pore space of the core samples.

For eight-electrode ECT, the distribution is concentrated only at the bottom of the tomogram images for all three core samples. Based on the distribution pattern, it is assumed that the distilled water is concentrated at the bottom of the core sample due to the gravitational effect as during the measurement stage; the sensor is installed horizontally. Therefore, ECT detected the region with the highest porosity distribution at the bottom of the core sample.

At the beginning of the project, a 12-electrode ECT was envisaged as a tool to measure the porosity distribution in the core sample. Another problem with saturation is that the core pattern is wet even on the outside of the core pattern. Looking at the porosity distribution of all core samples imaged by ECT, the porosity distribution is centered at the bottom of the core samples.

This may be due to the effect of gravity where the water moved to the bottom of the core sample when it was horizontal in the sensor. It could be due to a mistake, since the sensor is most sensitive on the wall instead of in the middle. Since the core sample is saturated with water, the outside of the core sample is also wet.

Therefore, the sensor is most likely to detect the wet surface of the core samples and define it as the porous region that has been occupied by water.

Table 5 and table 6 shows the specifications lists of the ECT sensor, which stated the electrodes number, electrodes length and width, the distance between two electrodes, ground electrode width and also the inner and outer diame

Conclusion

Based on the result using both eight and twelve electrode sensor, both of them are unable to provide the accurate percentage of porosity distribution in the wet rock core sample. These are due to the design and fabrication of the sensor, where fewer electrodes are used in this project, which leads to a lower quality image of porosity distribution, while the sensor is not up to industry standard, and thus the possibility of having errors during measurement is high. During the calibration stage, when the sensor is placed horizontally, there is a gap between the core sample and distilled water to the sensor wall, which is occupied by air, the difference in properties between air with distilled water and core sample will result in errors in calibration.

The first is the design and fabrication of a workable sensor suitable for use in porosity measurement. The 8-electrode ECT was developed later, and the porosity measurement result is much more satisfactory and has not suffered from the performance problem, thus proving that the design and fabrication of the sensor is working. ECT was able to obtain the porosity measurement and the results are being compared with the standard technique.

Based on the percentage error, using ECT with 12 electrodes, the errors are more than 100%, making it ineffective to be used. The errors are more than 10%, so it may not be accurate to use ECT as a tool to measure the porosity distribution in the core sample. In conclusion, this project achieved its main objectives, which is to test the suitability of using ECT as a tool to provide image distribution of porosity.

Based on the results of image distribution of porosity as well as the measurement value, the ECT is considered not. If all the errors are corrected and further improvement is made in the future, but the result is not satisfactory again, the ECT may not be really suitable to be used as a tool to measure porosity.

Recommendations

The ECT sensor developed by the student may suffer from performance issues as the ECT sensor is not carefully manufactured, resulting in errors in measurement. Additionally, the ECT size must be exactly the same size as the core sample to prevent air from occupying the space between the core sample and the ECT sensor wall. Fourth, in order to improve the result of measurement using ECT, the core sample size must be consistent with the standard size used in the industry.

Currently, the size of the core sample, 7.5 inches long and 1.5 inches in diameter, is too small, which may prevent the ECT from detecting the correct representation of the core sample's porosity distribution. The standard core sample size used in industry can be obtained from the geology or mineralogy laboratory. This project will continue using sixteen electrodes mounted around the external body of the sensor, to get a better picture of the porosity distribution of the core sample.

In addition, the method in the project can also be improved, where the result of the image of the porosity distribution of the core sample can be compared with the result of X-ray CT scanning. If ECT is proven to be capable of measuring porosity distribution in the core sample, ECT can be implemented as a tool that can be used in the borehole to provide a real-time image of the reservoir. Yang, W.Q., Chondronasios, A., Natrass, S., Nguyen, V.T., Betting, M., Ismail, I., McCann, H., Adaptive calibration of capacitance tomography system for imaging water droplet distribution.

Gamio, J., C., Ortiz-Aleman, C., Martin, R., Electrical capacitance tomography Two-phase oil-gas pipe flow imaging by the linear back-projection algorithm. Rautenbach, C., Mudde, R.F., Yang, X., Melaaen, M.C., Halvorsen, B.M., A Comparative Study between Electrical Capacitance Tomography and Time-Resolved X-Ray Tomography, at the Department of Process, Energy and Environment Technology2012, Telemark University College, Porsgrunn, Norway.