Determining transmissivity of fracture sets with statistical significance using single-borehole hydraulic tests: Methodology and implementation at Heshe well site in central Taiwan

Abstract

This work develops a novel method for determining the transmissivity of fracture sets with statistical significance and validates the method by performing in situ hydrogeological tests. An ideal mechanistic model is firstly considered, in which the injection discharge in a single-borehole hydraulic test is the sum of the flow of water through the intact rock and that through the fractures. The model assumes that fractures of similar attitudes have mutually consistent hydraulic properties and that the law of mass conservation applies in hydrogeological tests, facilitating statistical analysis. To validate the proposed method and demonstrate its practical implementation, an experimental well site is designed using a discrete fracture network concept and information about the spatial distribution of fractures at the site. Ten wells within 10m of each other are drilled to perform 59 groups of single-borehole hydraulic tests at specific locations where fractures of distinct attitudes are observed in the wells. A deviation of more than three orders of magnitude in the measured equivalent hydraulic conductivity among the wells reveals that the fractures dominate the hydraulic characteristics at the study site. Therefore, the transmissivity of each dominant fracture set is statistically determined using the proposed method, and fractures with transmissivity deviated from their mean are identified. Most test statistics are evaluated with a confidence level of greater than 95%, indicating that the transmissivity of dominant fracture sets can be statistically inferred with high confidence. The efficacy of the proposed method for determining the transmissivity of fracture sets in fractured rock masses is thus verified, so the method can be used to effectively describe the hydraulic conductivity of fractured rock masses using a DFN model.