Characterizing Vertical Fracture Connections using Pressure Responses due to the Short-circuiting of Injection Water during Constant Head Tests

A discrete fracture approach is often required for modeling groundwater flow in complex bedrock aquifers at the local scale. This approach, however, depends on a number of parameters (i.e. fracture density, connectivity, and orientation) not easily obtained from traditional hydraulic testing techniques. Additionally, the hydraulic properties estimated from typical field methods generally reflect near borehole conditions, which may or may not be representative of dominant flow pathways in the larger aquifer system. A study is presented which illustrates how measuring head changes above and below the isolated interval during constant head tests can yield value-added information to address these limitations. A total of 25 tests were completed in two 152 mm diameter boreholes drilled to approximately 40 m depth in a granite gneiss aquifer. A constant-head testing system with packer spacing approximately two-m in length was used with separate pressure measurements recorded in the open hole interval above the system, in the test section, and in the shut-in section below. Significant pressure increases were observed in a number of shut-in intervals where the test section intersects low-moderate transmissivity zones. Responses with reduced magnitude also occurred in the upper open hole interval above the test section. It is hypothesized that: (1) Pressure increases occur when injection water short-circuits through adjacent vertical fractures into the observation intervals above and below the test zone; (2) Lower magnitude responses in the upper open hole section are due to wellbore storage; and (3) A lack of increased pressures outside the test zone may indicate no vertical connections exist. Numerical modeling of test results provide additional insights regarding the transmissivity and geometry of possible vertical connections. As the additional measurements collected during this study involved minimal extra cost and labour they represent a significant opportunity to augment information already collected with standard constant head testing techniques.