Abstract
Casing pipes in oil well constructions may suddenly buckle inward as their inside and outside hydrostatic pressure difference increases. For the safety of construction workers and the steady development of oil industries, it is critically important to measure the stress state of a casing pipe. This study develops a rugged, real-time monitoring, and warning system that combines the distributed Brillouin Scattering Time Domain Reflectometry (BOTDR) and the discrete fiber Bragg grating (FBG) measurement. The BOTDR optical fiber sensors were embedded with no optical fiber splice joints in a fiber-reinforced polymer (FRP) rebar and the FBG sensors were wrapped in epoxy resins and glass clothes, both installed during the segmental construction of casing pipes. In situ tests indicate that the proposed sensing system and installation technique can survive the downhole driving process of casing pipes, withstand a harsh service environment, and remain intact with the casing pipes for compatible strain measurements. The relative error of the measured strains between the distributed and discrete sensors is less than 12%. The FBG sensors successfully measured the maximum horizontal principal stress with a relative error of 6.7% in comparison with a cross multipole array acoustic instrument.
Highlights
Many oil and gas wells in China are drilled with highpressure water injections and shored with casing pipes bolted or welded together from many sections
This study develops a rugged, real-time monitoring, and warning system that combines the distributed Brillouin Scattering Time Domain Reflectometry (BOTDR) and the discrete fiber Bragg grating (FBG) measurement
The BOTDR optical fiber sensors were embedded with no optical fiber splice joints in a fiber-reinforced polymer (FRP) rebar and the FBG sensors were wrapped in epoxy resins and glass clothes, both installed during the segmental construction of casing pipes
Summary
Many oil and gas wells in China are drilled with highpressure water injections and shored with casing pipes bolted or welded together from many sections. Casing damage often occurs as a result of pipe corrosion, joint dislocation, and long-term water injection development It represents the most important factor for unproductive oil and gas wells. Since the 1990s, optical fiber sensing techniques, both locally high-precision FBG sensors and distributed Brillouin Scattering Time Domain Reflectometry (BOTDR) sensors, have been considered to be most promising for longterm structural health monitoring. They show significant advantages over conventional sensing technologies in terms of accuracy, durability, and sensing distance [7,8,9,10,11,12]. The effectiveness of the technique was validated by the in situ testing in Daqing oilfield
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