Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Electrical and Computer Engineering (Holcomb Dept. of)

Committee Chair/Advisor

Dr. Hai Xiao

Committee Member

Dr. Fei Peng

Committee Member

Dr. Pingshan Wang

Committee Member

Dr. Lin Zhu


Technology advancements (e.g., hydraulic fracturing and horizontal drilling) to recover unconventional oil and gas (UOG) resources are critical in maintaining future U.S. oil and gas production levels. Permanently installed distributed downhole pressure sensors could monitor fracture propagation, assess the effectiveness of hydraulic fracturing, and optimize hydraulic fracturing placement so that overall UOG recovery efficiency can be increased. However, the harsh environment (high temperatures, high pressures, strong vibration, and presence of brine, mud, debris, hydrate, and various gases), the long data telemetry distance, and the requirements of reliability and service lifetime make the downhole monitoring a very challenging task. To combat these challenges, this thesis presents three sensing systems for downhole pressure monitoring.

First, A microwave-photonic low-coherence interferometry (MPLCI) system is proposed for optical fiber based distributed sensing. The system can be used to interrogate the intrinsic Fabry–Pérot interferometers (IFPIs) based distributed downhole pressure sensors. Assisted by an unbalanced Michelson interferometer (MI), a low-coherence laser source is used to interrogate IFPIs along with an optical fiber for a dark zone-free (or spatially continuous) distributed measurement. By combining the advantages of microwaves and photonics, the MPLCI system can synergistically achieve high sensitivity and high spatial resolution.

Second, to solve the packaging and drift problems in optical fiber sensors, an all-digital sensing method based on an electrical encoder is developed for downhole pressure monitoring. The key innovation of the all-digital sensor concept is the built-in nonelectric analog-to-digital converter (ADC), which eliminates the need for downhole electronics for signal conditioning and telemetry in conventional electrical downhole sensors. As such, the sensors are more robust, less expensive, and have less drift in comparison with the existing sensors. Because the sensor outputs are digital in nature, the developed sensors can be remotely logged over a long distance, and many sensors can be digitally multiplexed for distributed sensing using a single surface instrument. The all-digital pressure sensors and their surface instrument were designed, engineered, fabricated, and calibrated. The integrated sensing system was tested/validated at both laboratory and research wellbores.

Third, to solve the hysteresis problem induced by the electrical encoder, a non-contact optical encoder based all-digital pressure sensor for downhole applications is proposed. The proposed sensor combines the advantages of both optical fiber and all-digital sensing method. The noncontact-type encoder, which is composed of an encoding pad and an all-glass optical fiber sensing head. A glass additive and subtractive manufacturing (ASM) system was used to embed the multi-channel optical fibers into a bulk-fused silica glass substrate with high positioning accuracy and good thermal stability even at elevated temperatures. The optical fiber only serves as the telemetry channel to directly transmit the data in digital format, such that the system has long-distance telemetry capability as well as low drift. The proposed pressure sensor was manufactured and experimentally verified to have a high SNR, linear pressure response, and good long-term stability. In addition, a mathematical model to study the relationships between the sensor’s performances and design parameters was established.

Author ORCID Identifier




To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.