THE CONCEPTUALIZATION OF CHANNEL NETWORKS THROUGH MACROSCOPIC ANALYSIS OF PUMPING AND TRACER TESTS IN FRACTURED CHALK

A methodology for improving conceptual models of flow and transport, through macroscopic interpretation of interference and tracer tests was developed and applied in a fractured chalk aquitard. The generalized radial flow (GRF) model suggested by Barker (1988) is used to interpret an interference test conducted between a pair of boreholes. In this model, the characterization of the flow-contributing zone includes the determination of the flow dimension. Based on this flow dimension, we suggest a homogeneous dilution factor, which is to be expected in the pumping borehole if, a forced-gradient tracer test is carried out between the same pair of boreholes. A steady-state convergent tracer test is then preformed and the actual dilution factor is inferred using a 1D advection-based interpretation of the breakthrough curve (Javandel et al., 1984). The aforementioned homogeneous expected dilution factor (HEDF) is then compared to the actual dilution factor (DF) derived from the tracer test. Differences between these two dilution factors reflect the deviation of the actual flow regime from that expected from a homogenous flow regime. This methodology was applied to interference and tracer tests performed in inclined boreholes intersecting fractured chalk in the Negev desert, Israel. HEDFs were calculated for conceptual models assuming homogenous parallel-plate flow in large horizontal and vertical fractures. The large differences between these HEDFs and the actual DF implied that either the flow is channeled to narrow sections of these fractures, or an invalidity of these conceptual models. Additional observations such as, fractures’ length and orientation distributions derived from surveys in nearby outcrops, and breakthrough of tracers injected to other boreholes at the same tracer test, lead us to the conclusion that the dominant flow at this site is in horizontal networks of channels. Many dissolution channels at the intersections of vertical fractures and bedding plains were observed in outcrops and support this channel-networks conceptual model.