Burial depth interval of the shale brittle–ductile transition zone and its implications in shale gas exploration and production

Brittleness and ductility of shale are closely related to shale gas exploration and production. How to predict brittleness and ductility of shale is one of the key issues in the study of shale gas preservation and hydraulic fracturing treatments. The magnitude of shale brittleness was often determined by brittle mineral content(for example, quartz and feldspars) in shale gas exploration.However, the shale brittleness is also controlled by burial depth. Shale brittle/ductile properties such as brittle, semibrittle and ductile can mutually transform with burial depth variation. We established a work flow of determining the burial depth interval of brittle–ductile transition zone for a given shale. Two boundaries were employed to divide the burial depth interval of shale brittle/ductile properties. One is the bottom boundary of the brittle zone(BZ), and the other is the top boundary of the ductile zone(DZ). The brittle–ductile transition zone(BDTZ) is between them.The bottom boundary of BZ was determined by the overconsolidation ratio(OCR) threshold value combined with pre-consolidation stress which the shale experienced over geological time. The top boundary of DZ was determined based on the critical confining pressure of brittle–ductile transition. The OCR threshold value and the critical confining pressure were obtained from uniaxial strain andtriaxial compression tests. The BZ, DZ and BDTZ of the Lower Silurian Longmaxi shale in some representative shale gas exploration wells in eastern Sichuan and western Hubei areas were determined according to the above work flow. The results show that the BZ varies with the maximum burial depth and the DZ varies with the density of the overlying rocks except for the critical confining pressure.Moreover, the BDTZ determined by the above work flow is probably the best burial depth interval for marine shale gas exploration and production in Southern China. Shale located in the BDTZ is semi-brittle and is not prone to be severely naturally fractured but likely to respond well to hydraulic fracturing. The depth interval of BDTZ determined by our work flow could be a valuable parameter of shale gas estimation in geology and engineering.

TaN纳米颗粒对电泳沉积PEEK涂层显微组织、力学性能和摩擦学性能的影响

为解决钛的摩擦、磨损和润滑问题,通过电泳沉积制备一系列TaN/PEEK(聚醚−醚−酮)涂层,并探讨TaN纳米颗粒对涂层显微组织、力学性能和摩擦学性能的影响。结果表明,在PEEK涂层中引入TaN纳米颗粒可有效提高沉积效率,增强材料抗变形能力,同时还可增加涂层的硬度、弹性模量以及与基底之间的结合强度。与纯PEEK涂层相比,P-TN-3样品的摩擦因数降低31.25%,耐磨性大幅度提高,其比磨损率由9.42×10^(−5)降低到1.62×10^(−5 )mm^(3)·N^(−1)·m^(−1)。电泳沉积制备的复合TaN/PEEK涂层均匀分布于钛合金基体表面,且与钛合金基体结合良好,TaN颗粒可有效提高PEEK涂层的强度,为钛合金提供耐磨保护作用。