Abstract: The flow field and pressure field of low-permeability reservoirs are changed by dynamic fractures, significantly affecting the swept volume of water flooding. Comprehensively using the data of imaging logging, core analysis, rock mechanics experiments, production performance, and dynamic monitoring, the paper analyzes the characteristics and controlling factors of dynamic fractures in four typical ultra-low permeability reservoirs in the Ordos Basin, including Jing’an, Ansai, Xifeng, and Huaqing. Based on their geneses, dynamic fractures can be classified into two types, i.e., being formed by exceeding the rock formation’s fracture pressure and activated by natural fractures. According to impacts on the water cut of oil wells, they can be divided into unidirectional and multidirectional dynamic fractures. The evolution of dynamic fractures undergoes four stages:rapid growth to a certain scale, a fixed length, and a maximum length, and contraction stage; the extension direction of fractures is primarily controlled by the current maximum horizontal principal stress direction. The development intensity of dynamic fractures is influenced by factors such as matrix permeability, natural fractures, rock brittleness index, as well as development technology policies. The lower matrix permeability, more developed natural fractures, higher brittleness index, and stronger water injection development technology policies are easier for the formation of dynamic fractures, and the transition from unidirectional to multidirectional fractures. Dynamic fractures significantly affect the water cut increase regularity of oil wells and the distribution of remaining oil.

Key words: Ordos Basin, ultra-low permeability reservoirs, water injection development, dynamic fractures, controlling factors

摘要： 动态裂缝改变了低渗透油藏的渗流场和压力场,严重影响水驱波及体积。综合利用成像测井、岩心、岩石力学实验、开发生产数据和动态监测等资料,对鄂尔多斯盆地靖安、安塞、西峰、华庆4个典型特/超低渗透油藏动态裂缝特征和控制因素进行了分析。基于成因,动态裂缝可分为超岩层破裂压力型和天然裂缝激活型;根据对油井含水率的影响,可分为单方向和多方向动态裂缝。动态裂缝的演化经历裂缝快速增长一定规模、裂缝固定长度、裂缝增长至最大长度、裂缝收缩4个阶段,其延伸方位主要受现今最大水平主应力方向控制。动态裂缝发育强度受基质渗透率、天然裂缝、岩层脆性指数、开发技术政策等影响。基质渗透率越低,天然裂缝越发育,脆性指数越大,注水开发技术政策较强,越易形成动态裂缝,并且由单方向裂缝向多方向裂缝转化。动态裂缝对油井含水上升规律和剩余油分布影响较大。

关键词: 鄂尔多斯盆地, 特/超低渗透油藏, 注水开发, 动态裂缝, 控制因素