Interpretation of transient borehole flow metering data in a fissured granite formation

Abstract

Heat-pulse flowmeter (HPF) data obtained in a fractured granite formation under transient pumping conditions are interpreted using a borehole flow model. The phraeatic formation is modeled using cylindrically symmetric layers composed of thick low-permeability intervals between thin layers of much higher permeability corresponding to fracture zones. The inflow to the borehole at fracture zones generates sharp increases in the flow profiles obtained during pumping. Heat-pulse flowmeter measurements are given as a function of depth and time; drawdown and pumping rate are also recorded during the measurement period. The numerical flow model is used to calculate the drawdown and evolving profile of borehole flow as a function of time during the pumping period. The model allows for vertical flow across the horizontal interfaces between layers, and accounts for borehole storage effects. The model is also designed to allow for changes in fracture zone permeability with distance from the borehole (bi-zonal flow) by allowing for changes in permeability with radial distance. The model uses a finite difference technique to compute borehole flow and drawdown during pumping. The accuracy of the finite difference code was checked against available analytical and semi-analytical solutions derived by integral transform techniques. The model was calibrated using estimates of formation properties and distance to lateral boundaries derived from short, medium, and long-term packer tests. Results of two borehole flow experiments are evaluated for experiments in boreholes with 7 and 4 discrete fracture zones distributed over 59.50 and 121.16 m of open borehole interval.