大規模水圧破砕で造成された亀裂の進展範囲と形状の推定

Abstract

A hydraulic fracturing test was conducted in 1992 at the Hijiori hot dry rock test site. In this test, water was injected into injection well HDR-1. The depth of the injection well was about 2, 200m, and the maximum ternperature of the well was about 270 degC. The main fracture, which was extended along natural joints, was initiated from the bottom hole and a radius of the fracture was estimated to be about 500m from the acoustic emission observation. In this paper, the radius of the fracture, and the pressure, permeability and aperture distribution within the fracture was estimated by numerical simulation. The FEHM (Finite Element Heat and Mass transfer) code was used to estimate the radius of the growing fracture and a pressure distribution within the fracture. The Gangi bed-of-nails model was used to relate the fracture aperture to the fluid pressure. The permeability of the fracture was then calculated from the fracture aperture using Lomize's friction factor. A pressure history match between measured and calculated value was obtained. The simulation result shows that the radius of the fracture at the end of the experiment was about 500m, which agrees well with observations. The estimated apertures of the fracture at the vicinity of the well were 1.4mm and 2.6mm when the injection water rates were kept constant at 6kg/s and 20kg/s, respectively. In this simulation, three fitting parameters were used to match the pressure history. The sensitivities of three parameters on the simulation results were also examined.