A photoelastic experiment for fracture extending in fractured rocks: An example from the Shahejie Formation in the southern North China basin

Abstract

Five types of fracture assemblage were summarized from the siltstone of the Tertiary Shahejie Formation in the southern North China basin, i. e., (1) conjugated shear fracture belts arranged in en échelon N-S-striking tension fracture; 2 conjugated shear fracture belts arranged in en échelon E-W-striking tension fracture; 3 tangly shear fractures with variable striking; 4 shear and tension fractures cutting each other; and 5 sporatic fractures. Furthermore, a photoelastic physical model for experiment was established with well-proportioned and gradually increased loading along the direction of 96° (ESE-WNW). The results suggest that 3 types of isochrome map, which marks the local stress along the fractures, could be identified: type I has a symmetrical shape in respect to the fracture with the main axis perpenticular to the fracture, and type II has a symmetrical shape in respect to the fracture with the main axis parallel to the fracture, whereas the compound type (type (III) has an asymmetrical shape in respect to the fracture with the oblique main axis. Fracture assemblage of type 1 has type I isochrome map with tensional or tensional-shear stress, and fracture assemblage of type 2 has compound type (type III) isochrome map with compressional stress, and fracture assemblage of type 3, 4 and 5 have very complicated stresses. Micro-fracture starts to form when loading is increased to 1000 kg, and macro-fracture starts to extend when loading is increased to 1100-1200 kg. The order of the extending of broken fracture from strong to weak is 1, 3, 5, 2 and 4. The fracture assemblage of type 1 is the earliest one to start tension with the greatest tensional velocity; Therefore, the fact that the best tendency to connect the fractures occurs to type 1 suggests that a perpendicular loading to the en échelon fracture could easily result in the occurrence of connectedness.

以南華北盆地戶部寨第三系沙河階組粉砂巖低滲透儲層為例,總結了其中發育的5種類型構造裂縫組合,即:①南北向雁形張節理組成的共軛剪節理帶;②東西向雁形張節理組成的共軛剪節理帶;③纏結狀多組剪切構造裂縫;④組合狀交切的多組剪切構造裂縫及張裂縫;⑤散狀無序排列的構造裂縫。在此基礎上,建立光彈性試驗構造物理模型,沿現應力場96°方向,均勻地對該模型逐步增加外載力。結果表明,沿各組合裂縫尖端分別形成對稱于裂縫走向、且主軸垂直于裂縫的Ⅰ型等色線條紋;對稱于裂縫走向、且主軸平行于裂縫走向的Ⅱ型條紋和斜交于裂縫走向的復合型(Ⅲ型)條紋。第①類組合裂縫尖端以Ⅰ型等色線條紋的拉張應力和張剪應力為主;第②類組合裂縫尖端以復合型(Ⅲ型)等色線條紋的擠壓應力為主;第③、④和⑤類組合裂縫尖端的應力分布較為復雜。當模型外載荷接近1000kg時,裂縫尖端起裂,并開始發育顯微裂縫,而當外載荷達1100-1200 kg時,沿裂縫尖端部位形成宏觀裂縫,并開始擴展。5種類型的裂縫組合擴展發育程度由強至弱依次為:①,③,⑤,②,④。其中第①類共軛裂縫組合起裂最早,擴展最快,裂縫間的連通趨勢最好,說明規則分布的構造裂縫組合,在垂直于外施加載衙時更易達到連通的目的。