FRACTURE RESERVOIR CHARACTERIZATION BY FIBER-OPTIC DISTRIBUTED TEMPERATURE LOG
One of the important features of the fiber-optic distributed temperature log is its ability to acquire continuous, instantaneous and simultaneous temperature profile along the entire wellbore. Series of successive temperature profiles sampled every one minute or so enable to analyze transient temperature phenomena, which may pertain to important reservoir properties. For example, when change of rate occurred to the fluid flow such as due to water injection, the temperature profile over the section exhibits transient phenomena associated with the fluid flow. The characteristics of the phenomena depend on several factors including the flow profile which itself exhibits transient nature depending on fluid compressibility, permeability and reservoir extent, the original temperature profile, the temperature of injected fluid and that of the reservoir fluid flowing into the wellbore, the degree of rate alteration, well geometry, flow regime and etc.
Evaluation of a Distributed Fiber-Optic Temperature Sensor for Logging Wellbore Temperature at the Beowawe and Dixie Valley Geothermal Fields
A distributed temperature sensor (DTS) system, utilizing Raman backscattering to measure temperatures of optical fiber, has recently been installed in production wells at the Beowawe and Dixie Valley, NV, geothermal fields. The system has the potential to reduce the cost and complexity of acquiring temperature logs. However, the optical transmission of the initial fibers installed at Beawawe degraded over several months, resulting in temperature errors. Optical transmission spectra of the failed fibers indicate hydroxide contamination via hydrogen diffusion as a possible failure mechanism. Additional fibers with coatings designed to resist hydrogen diffusion were installed and have maintained their optical transmission over several months in the 340-360 F Beowawe wells. The same fibers installed in a 470 F Dixie Valley well rapidly failed. Possible methods to prevent fiber degradation include encasing the fiber in metallic buffer layer that resists hydrogen diffusion. Additional methods to correct temperature errors include using additional optical sources to measure fiber losses at the operating wavelengths. Although the DTS system is expected to have one degree F accuracy, we have observed an average accuracy of five degrees. The fiber connections appear to be the uncertainty source. Using connectors with greater stability should restore accuracy.
OH reduction in preforms by isotope exchange
To reduce OH-ion content in MCVD preforms a method, based on a thermal-activated deuterium-hydrogen isotope exchange, has been developed. Reaction has been applied on the internal walls of silica substrate tubes to produce a buffering layer against hydrogen diffusion. Experimental results are presented and process potentialities are discussed.