Single-Well Tracer Push-Pull Test Sensitivity to Fracture Aperture and Spacing

Abstract

Dealing with a parallel-fracture system of infinite lateral and radial extension, four characteristic regimes of tracer signal sensitivity towards fracture aperture b and fracture spacing s (whose reciprocal 1/s defines fracture density) can be identified during the pull stage of a single-well tracer push-pull test, also depending upon the ratio between push-stage duration Tpush and a characteristic time scale Ts (defined by s^2 / D = Ts , with D denoting the tracer’s effective diffusion coefficient): (i) early-time regime: tracer signals are sensitive to b, but insensitive to s; sensitivity to b first increases, then decreases with Tpush / Ts  (thus there will be an optimum in terms of Tpush / Ts , at early pull times); (ii) mid-time regime: tracer signals are sensitive to s, but insensitive to b; sensitivity to s increases with Tpush / Ts ; (iii) late-time regime: with increasing pull duration, tracer signals become increasingly insensitive to s, while regaining sensitivity to b; (iv) ‘very late’-time regime: sensitivity towards b becomes independent upon Tpush / Ts . From these different regimes, some recommendations can be derived regarding the design and dimensioning of dual-tracer single-well push-pull tests for the specific purposes of geothermal reservoir characterization, using conservative solutes and heat as tracers.