| Abstract |
In fluid-based geothermal reservoirs, thermal breakthrough depends on mainly two parameters: fluid residence time, and heat exchange area density. However, “heat exchange area” is often used in a sense similar to residence times or their distribution; we examine the implications of that. Further, we discuss two non-equivalent meanings of “HEA density”, depending upon reservoir structure and differing in respect to their determinability from SWIW or from inter-well tracer tests. In systems with negligible natural flow, SWIW tracer tests are a good method for measuring transport parameters other than advective-dispersive, out of which parameters associated with matrix diffusion are especially relevant to heat transport. In single-fracture systems, HEA per fracture volume is equivalent to fracture aperture, w. r. to which SWIW tracer tests indeed exhibit good sensitivity (better than from inter-well tests). In multiple-fracture systems, HEA per bulk reservoir volume is equivalent to fracture spacing, w. r. to which SWIW tracer tests also exhibit good sensitivity, yet heat transfer in such systems is controlled by both parameters, fracture spacing and individual-fracture aperture, with different weightings during early, late and very-late time stages. Thus, in order to reliably determine both parameters, multiple SWIW tests are recommendable, using tracers with contrasting diffusivities (like heat vs. solute tracers) and conducted at dual scales. To be noted, HEA (fracture length  height  number of fracture walls) is completely independent upon HEA density (reciprocal of either aperture, or spacing). The mobile-fluid volume in the reservoir results from the product of fracture number, aperture, height, length, and porosity within fractures; while the latter three can be regarded as purely advective parameters, MRT and RTD will result from both AD and non-AD parameters, which makes the identification between “heat exchange area” and RTD features of a geothermal reservoir feel natural, but act highly confusing w. r. to what can or cannot be measured from SWIW tests. |