Experimental study of the heat transfer characteristics of single geothermal fracture

Abstract

Mineral dissolution and mechanical deformation of granite are two main mechanisms that affect permeability evolution of rock fracture. In this study, two water flow-through experiments with large granite fractures were conducted at 200 °C with a constant flow rate for 24 h, under confining pressures of 5 and 10 MPa, respectively. Water pressure and temperature were measured, fracture aperture and permeability were calculated, and chemical element concentrations in effluent water were tested for mechanism analysis. The permeability fluctuates up and down between 2.62×10-12 and 3.16×10-12 m2 at confining pressure of 5 MPa; while it decreased monotonously by 24% from 1.92×10-12 to 1.45×10-12 m2 at a confining pressure of 10 MPa. The heat transfer rates at both experiments stay stable at about 0.25 J/s. The mass concentration of Ca, Na, K, and Si in effluent water are between 5 to 23 mg/L, indicating slight dissolution of Ca-plagioclase, Na-plagioclase, and K-feldspar, as well as possible precipitation of minor amount of Kaolinite or Qartz. The total amount of free-face dissolution and pressure dissolution are similar at 5 and 10 MPa. The geochemical reaction counts only small part of aperture change, and the mechanical deformation counts the major part of aperture change.