Abstract
The creation of fractures in boreholes by hydraulic stimulation is of importance for enhancing fluid transport in the context of geothermal reservoirs. Detecting whether a fracture is created and evaluating its capacity to transport fluid are usually performed by locating the acoustic emissions generated during the rock failure and by comparing the hydraulic properties before and after this type of hydraulic stimulation using pumping tests, respectively. However, free oscillations, exerted by rapid changes in pumping parameters, can be used as well to detect the existence of a fracture and to evaluate its permeability. The diagnostic properties of free pressure oscillations are their spectral components, i.e., frequency and damping coefficient. The hydraulic system, which includes technical equipment such as tubes and hoses, and the rock formation, which can be tight or leaky. In a tight system, the free pressure oscillations are attenuated by the viscous interaction of the fluid and the borehole wall and the local coupling of the fluid compression and deformation of the borehole due to the pressure variation. In a leaky system, the attenuation of free pressure oscillations includes fluid exchange between the borehole and the hydraulic conduits of the rock. We developed an analytical solution starting from the dispersion relation of fluid-flow waves in a tight borehole by accounting for the fluid exchange as a modified boundary condition. Deviation of spectral components of observed oscillation from the analytical solution for a tight borehole is an indication that the free pressure oscillations contain information of the hydraulic properties of the penetrated formation. The oscillations typically last for tens of seconds, which allows assessing the success of the stimulation operation on a near-real-time basis. We analyzed the characteristics of free operations recorded in boreholes during several hydraulic stimulation campaigns. We investigated the evolution of the spectral components in the course of the stimulation and with changes in mean injection pressure and obtained transmissivity values that favorably compare to the results of conventional analyses.
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