The prediction of hydraulically induced fractures and the characterization of these fractures for fluid flow and heat transfer is a limiting factor in the development of Enhanced Geothermal Systems (EGS) technology. To study hydraulically induced fractures in crystalline rock reservoirs, a new true-triaxial apparatus was developed and tested. The apparatus is capable of applying three independently controlled principal stresses up to 12 MPa and temperatures as high as 180 °C to a 30x30x30 cm cubical rock sample. Hydraulic fracturing is performed from hydraulic tubing installed in 10 mm oriented boreholes which are drilled into a pressurized sample. Fracturing fluid pressures up to 70 MPa can be applied with volumetric flow rates as low as 0.00001 mL/min allowing for high strength rocks to be fractured and fracture extents to be contained within the 30 cm sample. With a second borehole drilled to intercept the induced fracture, fluid circulation experiments are performed to characterize the thermal properties of the reservoir. Sensor systems measure temperatures, pressures, flow rates, and strains such that the fluid state and rock loading parameters can be monitored for use in the validation of a computer model currently being developed. Initial testing has been performed on an artificial granite sample to calibrate the system. The artificial granitite sample is created using a specialized high performance concrete designed to replicate Colorado Red Granite as available from local quarries. Using fine aggregate silica fume concrete allows for the creation of cast homogenous samples with a greater uniformity of axial stress distributions and controllable internal discontinuities. Tested parameters for characterizing the rock include unconfined compressive strength, indirect tensile strength, mode I fracture toughness, elastic modulus, thermal conductivity, specific heat capacity, coefficient of thermal expansion, porosity and permeability. The triaxial system is designed to allow for future application of acoustic emission (AE) fracture monitoring for relating the laboratory data to field tests and a proppant injection system will also be applied to analyze the effect of proppant and high viscosity fluid injections on the fracture thermal characteristics.

Press the Back button in your browser, or search again.

Copyright 2012, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.