Experimental measurements of creep deformation of Tournemire shale loaded at specified pressure (10 MPa) and room temperature (26°C)

Following the experimental protocol used in (Geng<em> et al.</em>, 2018), we performed the stepping creep experiments at a confining pressure of 10 MPa. We first loaded the samples under hydrostatic conditions up to 10 MPa at a pressure rate of 0.3 MPa/min. Hydrostatic conditions were maintained for ~18 h at 26 °C. Next, differential stress (axial stress minus confining pressure) was increased to a fixed initial stress (30 MPa) and maintained (creep status) for 24 h. The differential stress was repeatedly increased by 5 MPa and maintained for 24 h, until brittle failure. All the experiments were conducted using the triaxial apparatus installed at the Laboratoire de Géologie of ENS-Paris (France). There were few constraints on the natural saturation state of the samples because of their low permeability (10^-19 10^-21 m²). To avoid exposition redundancy, an additional description of the technical performance of the triaxial apparatus can be referred to (Brantut<em> et al.</em>, 2011, Sarout &amp; Guéguen, 2008). Compressive stresses and compactive strains are denoted as positive. Axial creep deformation was measured using three capacitive gap sensors that externally monitored the overall axial displacement of the piston during creep deformation. Volumetric strain during creep was estimated by adding the average of axial strains (axial displacement of the piston divided by the sample length) and two average radial strains measured by four radial strain gauges glued uniformly around the cylindrical rock surface. As the deformation rate generally stabilized during the last 8 h in most creep periods (Geng<em> et al.</em>, 2018), we estimated the average axial strain rate over the last 8 h of each step to characterize the creep strain rate under the corresponding axial loading stress. More technical details of the sample configuration and creep rates estimation can be found in (Geng<em> et al.</em>, 2018).本研究遵循Geng等人（2018）的实验方案，在围压10 MPa条件下开展阶梯式蠕变实验。首先以0.3 MPa/min的升压速率对样品进行静水压加载，直至围压达到10 MPa，随后在26 ℃下维持静水压环境约18小时。接下来，将差应力（轴向应力与围压的差值）提升至固定初始应力30 MPa，并维持该蠕变状态24小时；此后以5 MPa为步幅反复提升差应力并维持24小时，直至样品发生脆性破坏。所有实验均在法国巴黎高等师范学院地质实验室（Laboratoire de Géologie of ENS-Paris）配备的三轴试验机上完成。由于样品渗透率极低（10^-19 ~ 10^-21 m²），其天然饱和状态几乎不受约束。为避免表述冗余，三轴试验机的技术性能细节可参考Brantut等人（2011）以及Sarout与Guéguen（2008）的研究。本研究将压应力与压实应变均定义为正值。蠕变阶段的轴向变形通过3台电容式间隙传感器测量，该传感器可实时监测活塞在蠕变过程中的总轴向位移。蠕变过程中的体积应变通过如下方式估算：将轴向应变平均值（活塞轴向位移除以样品初始长度）与由均匀粘贴在圆柱岩样表面的4个径向应变计测得的两组平均径向应变相加。正如Geng等人（2018）的研究结论，多数蠕变阶段的变形速率通常在最后8小时趋于稳定，因此我们选取每个应力步最后8小时的平均轴向应变率，以表征对应轴向加载应力下的蠕变应变率。样品配置与蠕变应变率估算的更多技术细节可参考Geng等人（2018）的研究。