A laboratory study of tracer tomography

A tracer tomographic laboratory study was performed with consolidated fractured rock in three-dimensional space. The investigated fractured sandstone sample was characterized by significant matrix permeability. The laboratory transport experiments were conducted using gas-flow and gas-tracer transport techniques that enable the generation of various flow-field patterns via adjustable boundary conditions within a short experimental time period. In total, 72 gas-tracer (helium) tests were performed by systematically changing the injection and monitoring configuration after each test. For the inversion of the tracer breakthrough curves an inversion scheme was applied, based on the transformation of the governing transport equation into a form of the eikonal equation. The reliability of the inversion results was assessed with singular value decomposition of the trajectory density matrix. The applied inversion technique allowed for the three-dimensional reconstruction of the interstitial velocity with a high resolution. The three-dimensional interstitial velocity distribution shows clearly that the transport is dominated by the matrix while the fractures show no apparent influence on the transport responses.