From Stochastic toward Deterministic Characterization of Discrete Fracture Network via Thermal Tracer Tests

Abstract:

The presence of fractures play an essential role in different disciplines, including hydrogeology, geothermal and hydrocarbon industries, as fractures introduce new pathways for flow and transport in the host rocks. Understanding the physical properties of these planar features would reduce the uncertainty of the numerical models and enhance the reliability of their results. Among the fracture properties, orientation and spacing are relatively easily estimated via borehole logs, core images, and outcrops, whereas the fracture geometry (i.e. length, width, and height) is more difficult to investigate. As the fracture geometry controls the hydraulic and thermal behavior of the fracture network through the strong dependency of the fracture conductivity with fracture aperture, it is possible to estimate these geometrical properties indirectly through hydraulic and thermal tomography investigations.

To reach this goal, an innovative approach is introduced for discrete fracture network (DFN) characterization of heterogeneous fractured media via active thermal tracer testing. A synthetic DFN model is constructed based on the geological properties of an arbitrary fracture medium such as fracture orientation, length, spacing and persistency. Different realization are then constructed by considering all the above mentioned fracture properties except the length of fracture segments. Pressure and temperature fields are estimated inside the fracture network by means of an implicit upwind finite difference method, which is used to compute heat tracer travel times between injection and observation points and record the full temperature breakthrough curves at the monitoring points. A trans-dimensional inversion is then adopted to update the lengths fracture segment (add or remove) of the DFN model by comparison between proposed and observed travel times (Figure 1). The resulting assemble of the models can be used as an input geometry for deterministic simulations of fracture medium in the above mentioned disciplines. Moreover, the obtained ensemble improves stochastic flow and transport simulations by constraining the uncertainties in DFN characterization.

Figure 1. Schematic representation of the proposed stochastic approach for DFN characterization of heterogeneous aquifers