EPFL, ENAC, LEMR, Station 18, CH-1015, Lausanne, Switzerland.
Géophysique Expérimentale, Institut de Physique du Globe de Strasbourg, Univerisité de Strasbourg/EOST, CNRS UMR7516, Strasbourg, France.
Sci Rep. 2017 Aug 9;7(1):7705. doi: 10.1038/s41598-017-08108-5.
Recently, projects have been proposed to engineer deep geothermal reservoirs in the ductile crust. To examine their feasibility, we performed high-temperature (up to 1000 °C), high-pressure (130 MPa) triaxial experiments on granite (initially-intact and shock-cooled samples) in which we measured the evolution of porosity during deformation. Mechanical data and post-mortem microstuctural characterisation (X-ray computed tomography and scanning electron microscopy) indicate that (1) the failure mode was brittle up to 900 °C (shear fracture formation) but ductile at 1000 °C (no strain localisation); (2) only deformation up to 800 °C was dilatant; (3) deformation at 900 °C was brittle but associated with net compaction due to an increase in the efficiency of crystal plastic processes; (4) ductile deformation at 1000 °C was compactant; (5) thermally-shocking the granite did not influence strength or failure mode. Our data show that, while brittle behaviour increases porosity, porosity loss is associated with both ductile behaviour and transitional behaviour as the failure mode evolves from brittle to ductile. Extrapolating our data to geological strain rates suggests that the brittle-ductile transition occurs at a temperature of 400 ± 100 °C, and is associated with the limit of fluid circulation in the deep continental crust.
最近，有人提议在韧性地壳中对深层地热储层进行工程改造。为了检验其可行性，我们对花岗岩（初始完整和冲击冷却样品）进行了高温（高达 1000°C）、高压（130MPa）三轴实验，在实验中测量了变形过程中孔隙率的演化。机械数据和死后微观结构特征（X 射线计算机断层扫描和扫描电子显微镜）表明：（1）在 900°C 以下，破坏模式为脆性（剪切断裂形成），但在 1000°C 时为韧性（无应变局部化）；（2）只有在 800°C 以下的变形为扩容；（3）在 900°C 下的变形是脆性的，但由于晶体塑性过程效率的提高而导致净压实；（4）在 1000°C 下的韧性变形是压实的；（5）对花岗岩进行热冲击不会影响强度或破坏模式。我们的数据表明，虽然脆性行为会增加孔隙率，但随着破坏模式从脆性向韧性转变，孔隙率的损失与韧性行为和过渡行为都有关。将我们的数据外推到地质应变率表明，脆性-韧性转变发生在 400±100°C 的温度下，与深部大陆地壳中流体循环的极限有关。
