A Wireline-Deployed Tool for Monitoring Fluid Flow within an EGS Borehole

Critical to determining the success of an EGS well is the measurement of the flow distribution along the open-hole length of that well. Knowledge of not only the depth but the spacing of intersected fractures is required in order to determine the sweep efficiency of fluids flowing through the reservoir. Currently, the only device available for measuring fluid flow within a geothermal wellbore is the pressure- temperature-spinner (PTS) tool. Whereas this device can be quite accurate in its measurement of temperature and pressure, it often fails as a flow meter. This is because spinner tools require minimum velocities in order to turn the impeller, which means that flow rates in the low-laminar range are not measurable. At very high velocities, spinner tools are not reliable; bearings fail and impellers break. Even within the operable range of spinner tools, the pitch of impellers must be changed frequently, requiring time-consuming trips into and out of the wellbore. Another serious drawback of propeller-based tools is their inability to accurately measure flow in washed-out sections. If such a tool is deployed within in a wellbore section of unknown diameter, the rate at which the impeller turns cannot be related to the true volumetric flow rate. Therefore, in washed-out regions, the impeller rotation leads to misleadingly low flow measurements. The objective of this project is to design, fabricate, demonstrate and market a borehole flowmeter for measuring flow within geothermal and EGS wellbores at temperatures as high as 300oC. This tool is based upon the well-established tracer-dilution method that has become the standard for accurately measuring single- and two-phase flow in surface pipes. In applying the tracer-dilution method to our application, tracer is injected at one end of the tool at a constant known flow rate and concentration. The tracer then mixes evenly within the wellbore as the brine flows up and around the tool to its opposite end where its diluted concentration is measured. From these tracer-flow and concentration parameters, a simple calculation (performed using software within the tool) then determines the flow of brine around the tool and sends these data to the surface via an electrically conductive wireline. As the tool is made to ascend or descend along the wellbore, the flow is measured as a function of depth and a flow log is created. With no moving parts other than a syringe pump, this tool will provide for the reliable measurement of flow rate as a function of depth within a geothermal wellbore and thus provide for improved reservoir management. Since it will measure volumetric flow rate, it will be able to accurately measure rates even in washed-out regions where spinner tools fail. Likewise, it will be able to measure all flow regimes (including both laminar and turbulent) without time-consuming trips to the surface for tool adjustment. By improving wellbore flow measurements, predictions of well performance will improve and EGS energy production will become more reliable and more cost effective.