Title

Seismic Waveform Based Criteria for Phase Transition of CO2 in Sample Rocks

Author

Andi Jakupi

Date of Award

5-1-2007

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Electrical Engineering

Abstract

Storing carbon dioxide (CO2) in depleted oil and gas reservoirs, un-mineable coal beds or deep saline aquifer reservoirs is being considered as the most effective CO2 sequestration option. A crucial element in CO2 storage is a dependable monitoring method and assessment criteria of the mechanical integrity of the C02-rock system. One method to assess this integrity is designing a system to measure elastic properties via nondestructive seismic waves on various sample rocks under numerous conditions. Due to the nature of rock formation, their material content is neither isotropic nor homogeneous. Their elastic properties can be characterized indirectly by measuring the seismic velocities of compressional P-waves and shear S-waves.

In this study, a system to measure elastic properties of rock samples under different confining pressures and fluid/gas injected flow rate using nondestructive seismic waves was designed and validated. Seismic waves are generated and transmitted through the core by one of three piezoelectric ultrasonic transducers excited by a voltage pulse. One of the transducers is used to generate a compressional wave and the other two transducers independently generate shear waves (one vertical and one horizontal). A similar set of transducers is used on the receiver end to measure the waveforms propagated through the core sample. The received waveform signals are acquired via a digital oscilloscope which sends the digitized data to a local computer for further processing. Various algorithms are implemented in this computer in order to filter the noise from the signal and measure the time delay for the waveform to travel from one end of the sample to the other. Knowing the length of the sample and time delay s, the velocities of the seismic waves can be determined. Once the compressional and shear velocities have been measured, the elastic properties of the rock sample can be calculated. The system also measures the sample’s confining pressures, fluid or gas injected flow rate and pressure, and various temperatures and stores the information to a | database installed in the local computer. This allows real-time analysis of the rock sample while samples are saturated with CO2 under conditions similar to those found in geological formation.

After the system was built and calibrated with known elastic properties of a test material, various experiments were conducted to monitor the overall performance and functionalities of the system. Temperature effect and confining pressure effect on the elastic properties were observed over different rock samples. Experiments with porous materials saturated with water and CO2 were also carried out to prove the main purpose of this laboratory instrument.

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