(149d) Assessment of Effective Heat Transfer Area with Sorbing Tracers

Assessing the surface area from which the working fluid draws heat in geothermal energy extraction systems could help with the management of existing resources and with the design and construction of engineered geothermal systems in hot dry rocks. In otherwise identical aquifers hosting coupled injection & production wells, a difference in the amount of surface area available to heat the injected fluid will influence the amount of energy that can be extracted without a loss in fluid enthalpy at the production well occurring. Thermal drawdown can occur when the heat extraction rate through fluid convection exceeds the conduction rate from the host rocks to the fracture surfaces where most heat transfer occurs. Predicting the host rock thermal response to the induced fluid convection requires an estimation of, among other things, the effective surface area available to transfer heat to the fluid. Operating geothermal reservoirs often have flow paths plugged by mineral precipitation near the injection wells. This decreases the available heat transfer area and could lead to premature thermal drawdown if not identified and addressed. Creation of new fractures within hot dry rocks will open up unconventional geothermal reservoirs. A tool to assess the exposed surface area within these new reservoirs would help to support this advancing technology. Since adsorption of solutes is a function (in part) of how much surface area/fluid interaction occurs in a system, sequential tracer breakthrough curves through the same system with only a change in its surface area is a potential surface area measurement tool.

We are working on developing thermally stable tracers that will help geothermal energy producers to assess the surface area available between injection and production wells. Research results are presented that show how the use of tracers that interact (i.e. sorb) with the surface of the geomedia can help to refine the estimation of the available effective heat transfer area. A series of high temperature miscible displacement experiments through media with different size fractions of Ottawa sand demonstrates the relationship between the retardation of a reactive tracer relative to a conservative tracer and the amount of surface area that the fluid encounters. Thermal characteristics of the tracer and temperature effects on its retardation through the media are presented, as well as a discussion on analysis, detectability, and availability of potential reactive tracer candidates for field applications.