A double-porosity slug test model for a sloping fracture zone

Abstract

Field investigation indicates the dip angle of a fracture zone in a Cenozoic folded sandstone formation is as large as 47°. When the dip angle is large, the flow field generated by slug test becomes radially asymmetrical with respect to test well. Taking into account the dip angle effects, a new double-porosity slug test model and its Laplace-domain solution are necessarily developed for both the oscillatory test response (as for a highly permeable condition) and non-oscillatory test response (as for a low permeable condition). For the oscillatory response, it is found that a larger dip angle causes larger amplitude while introduces little impact on period. The dip angle effect on the test response, oscillatory or non-oscillatory, is found to be more pronounced for a larger hydraulic conductivity of the fracture zone, a smaller specific storativity of the matrix, or a smaller λ* ; and the application range of the current model is determined for the limiting dip angle. That is, the dip angle effect should be considered in data analysis when dip angle is larger than 30°. A set of test responses from a slug test data in the fracture zone of a 47° dip angle is analyzed by using the current model. It is found that neglecting the dip angle can result in a 63.4 % overestimate of the hydraulic conductivity of the fracture zone, a 95.5 % underestimate of the specific storativity of the matrix, and a 63.3 % underestimate of λ*.