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. This transient state eventually converges to pseudosteady state and so does the temperature profile. But especially during the early time of its progress, the movement of fluid mass can be deduced by the movement of the temperature profile which has specific characteristics pertaining to the fluid mass. Especially when the adjacent wellbore fluid masses exhibit large temperature contrast such as for cold injected water and hot thermal brine, the fluid flow in wellbore is clearly seen by the movement of the corresponding temperature boundary. In other words the sequential temperature profiles measured by fiber-optic distributed temperature log could render fluid flow profile along the wellbore. Because the temperature profile along the entire wellbore is simultaneously acquired, the derived flow profile has no time delay over different depths. Effect of flow mixtures from fractures at several different depths can be clearly identified not only by the flow speed but also by the shift on temperature profile. Since the measurement principle for the fluid flow detection employed by the fiber-optic distributed temperature log and by the spinner is very different, these two methods can complement each other to improve interpretation quality which is not so obvious all the time. The application of the fiber-optic distributed temperature log is particularly interesting for geothermal wells where fresh water flows in and out of the permeable beds, as one can additionally deduce pressure profile from the temperature profile and can clearly differentiate characteristics of several fractures which belong to different hydrological regimes. The fiber-optic distributed temperature log if coupled with appropriately designed measurement procedure, may render enough information to characterize each layer of permeable beds. In this paper, one of such studies for a geothermal producer located in the northern Kyushu, Japan is presented. The outcome is interesting as it clearly demonstrates presence of annulus flow behind casing, presence of pressure difference in the different layers or a derivation of coherent reservoir permeability and more.