Methods to Determination of Brittle-Ductile Transition Zone of Shale

Abstract

Brittle - ductile of shale is closely related to shale gas conservation, hydraulic fracturing and natural fracture formation and evolution. It is also a key parameter for both geological and engineering “sweet spots”. Therefore, it is of great significance in shale gas exploration and development. The Paleozoic shale in southern China has experienced early deeply buried and late significant uplifting. What the brittleness and ductility of the shale are today? How to predict the brittle - ductile of the shale is currently one of the key issues faced in shale gas exploration. The magnitude of shale brittleness can be determined by mineral content, but the precondition is that the shale has been trusted as a brittle one. Aiming at this issue, we set up a set of method to determination of shale brittle - ductile: first of all the pre-consolidation pressure is determined by uniaxial strain test, and then the over consolidation ratio (OCR) and the OCR brittle threshold are determined by combination of uniaxial strain with triaxial compression test. Finally, the bottom depth of the shale brittle zone is determined by OCR brittle threshold combined with maximum vertical effective stress the shale experienced. The critical confining pressure can also be determined by triaxial compression test, based on the critical confining pressure the top depth of the shale ductile zone can be determined. Accordingly, the depth interval of the brittle - ductile transition zone can be restricted with both the bottom depth of the brittle zone and the top depth of the ductile zone. Based on the above technical methods, the brittle -ductile transition zone of the Lower Silurian shale is determined in eastern Sichuan, Shizhu and western Hubei areas. The results show that the bottom depth of brittle zone of the Lower Silurian shale in Jiaoshi 1 well (eastern Sichuan), Jiangsheng 1 well (Shizhu) and Heye 1 well (western Hubei) are 2195 m and 2763 m and 2485 m respectively and the top depth of ductile zone is about 4200 m. Generally, the brittle-ductile transition zone is the best depth interval of shale gas exploration.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015