HOW GOOD ARE ESTIMATES OF TRANSMISSIVITY FROM SLUG TESTS IN FRACTUREDROCK

Slug tests in fractured rock usually are interpreted with models that assume homogeneous formation properties, even though hydraulic propert ies of fractures can vary by many orders of magnitude over the length of boreholes, To investigate the impact of heterogeneity on the interp retation of slug tests in fractured rock, slug tests were conducted ov er large intervals of boreholes in crystalline rock in central New Ham pshire, and interpreted using a homogeneous model, The results of the slug tests were then compared with estimates of transmissivity from fl uid-injection tests conducted over shorter intervals in the same boreh oles, The fluid-injection tests showed transmissivity to vary more tha n six orders of magnitude over the length of the boreholes; however, t he sum of the transmissivities from the fluid-injection tests were wit hin an order of magnitude of the transmissivity estimated from the slu g tests, Although the two estimates of transmissivity were within an o rder of magnitude of each other, the water level responses during the slug tests did not exactly match the responses predicted by the homoge neous model, To investigate the effect of heterogeneity on water level responses during slug tests, a Laplace-transform solution was develop ed for slug tests conducted in boreholes containing multiple fractures with hydraulic properties that vary over the length of the borehole, A comparison of this solution with the homogeneous model shows no diff erence between the shape of water level responses in a homogeneous for mation and a (layered) heterogeneous formation, Furthermore, the trans missivity estimated using a homogeneous model is within an order of ma gnitude of the prescribed transmissivity in the heterogeneous model, T hus, differences between responses predicted from a homogeneous model and measured water levels during slug tests can be attributed to pheno mena such as nonradial now in the vicinity of the borehole, and not he terogeneous hydraulic properties over the length of the borehole, The experimental results of this investigation show that even when conditi ons such as nonradial flow are present in the vicinity of the borehole , interpretations of slug tests using a homogeneous model provided ord er-of-magnitude estimates of transmissivity in the crystalline rock te rrane under consideration.