岩性差异对裂缝穿层扩展的率相关断裂特征影响

Influence of rock property difference on rate-dependent fracture characteristics during fracture passing through bedding

薄互层致密砂岩储层由于层间岩性差异、纵向非均质性强等原因,导致主裂缝难以垂向扩展,提高裂缝扩展速率可有效促进裂缝穿层扩展。为研究裂缝穿层断裂过程中的率效应机制,采用人工预制水泥-砂岩试件进行了三点弯断裂试验,通过数字图像法监测了裂缝扩展至层间接触界面处时的断裂过程区(fracture process zone,FPZ)发育特征;基于断裂动力学理论,提出了考虑率效应的裂缝扩展路径预测模型。研究结果表明:低速扩展时裂缝形态曲折,FPZ呈现短、宽的现象,高速扩展时裂缝平直,FPZ呈现长、窄的特点;FPZ存在离散性,且存在相互吸引的特点,裂缝从低弹性模量岩石向高弹性模量岩石低速扩展时层间接触界面处会提前产生高应变区,导致裂缝扩展至层间接触界面处时沿层扩展,高速扩展无此现象;岩石抗拉强度与裂缝穿越单元体的平均抗拉强度呈正相关关系,低扩展速率裂缝优先沿微缺陷扩展,导致岩石抗拉强度降低,高扩展速率裂缝优先沿自相似方向扩展,穿过大量高强度单元导致岩石抗拉强度增加;裂缝与层间接触界面夹角越大,裂缝越容易穿层扩展,裂缝与层间接触界面夹角为30°时,夹角影响效果最大,夹角超过30°时影响效果逐渐下降。研究结果对水力压裂参数优化、增加水力裂缝高度、提高薄互层致密砂岩油气产量具有重要意义。

Due to the lithology difference between layers and pronounced vertical heterogeneity,the main fractures are difficult to extend vertically in thin interbedding tight sandstone reservoirs.Increasing the fracture extension rate can effectively promote the fracture extension crossing the layer.A three-point bending fracture experiment was carried out using prefabricated cement-sandstone specimens to study the effect of extension rate on fracture path.The digital image correlation method monitored the fracture process zone(FPZ)development characteristics when the fracture extends to the bedding.A prediction model of fracture extension path considering rate effect is proposed based on fracture dynamics theory.The results indicate that at low fracture extension rates,the fracture geometry exhibits a tortuous pattern,accompanied by a short and wide FPZ.Conversely,at high extension rates,the fracture geometry becomes smooth,with a long and narrow FPZ.FPZ is discrete and has the characteristics of mutual attraction.The fracture expands from low-strength rock to high-strength rock.At low propagation speeds,a high-strain zone(FPZ)forms in advance at the bedding plane,facilitating the extension of fractures along the bedding,causing the cracks to propagate along the layers upon reaching these interfaces.In contrast,this phenomenon is not observed during high-speed crack propagation.A positive correlation exists between the tensile strength of the rock and the average tensile strength of fractures traversing the unit.Fractures with low extension rates preferentially propagate along micro-defects,leading to a reduction in the tensile strength of the rock.Conversely,fractures with high extension rates preferentially propagate along self-similar directions,causing an increase in the tensile strength of the rock due to the tearing of numerous high-strength components.As the angle between the fracture and bedding increases,the fracture's ability to traverse the layer improves.The effect of the angle is maximized when it reaches 30°between the fracture and bedding,gradually diminishing beyond this threshold.These research findings hold significant implications for optimizing hydraulic fracturing parameters,enhancing fracture height,and boosting oil and gas production in thin interbedded tight sandstone reservoirs.