低渗透油藏注水诱导裂缝特征及形成机理——以鄂尔多斯盆地安塞油田长6油藏为例

Characteristics and formation mechanisms of waterflood induced fractures in low-permeability reservoirs: A case study from Chang 6 reservoir in Ansai oilfield,Ordos Basin

以鄂尔多斯盆地安塞油田长6油藏为例,在分析低渗透油藏注水开发动态特征的基础上,阐明了注水诱导裂缝的基本特征并研究其形成机理,最后利用数值模拟技术对该区注水诱导裂缝的主要形成机理进行了模拟。注水诱导裂缝指低渗透油藏在长期的注水开发过程中,当注水压力超过各类裂缝开启压力或地层破裂压力而形成的以水井为中心的高渗透性开启大裂缝或快速水流通道。它是低渗透油藏长期注水开发过程中所表现出的新的开发地质属性和最主要的非均质性,对于长期水驱的低渗透油藏来说具有普遍性和必然性。注水诱导裂缝有3种形成机理,当注水压力过高,超过天然裂缝的开启压力使天然裂缝张开、扩展和延伸,或超过地层破裂压力使地层中不断产生新的破裂,或使注水井周围因射孔、压裂等生产或增产措施所导致的不同类型的人工裂缝张开等均可形成注水诱导裂缝。安塞油田长6油藏普遍发育与现今最大水平主应力方向近一致的以雁列式排列的高角度构造剪切裂缝,当注水压力超过这类裂缝的开启压力并使裂缝张开、延伸扩展并相互连通,是该区注水诱导裂缝的主要形成机理。油藏数值模拟表明,随着注水诱导裂缝规模的不断扩大,注水井井底压力相应的表现出连续的不规则的周期性变化。

The study took Chang 6 reservoir in Ansai oilfield of Ordos Basin as an object to demostrate the essential ch-aracteristics and the formation mechanisms of waterflood induced fractures,based on the analysis of dynamic production features of the water-injecting lowpermeability reservoir.And the main formation mechanisms of waterflood induced fractures in the study area were numerically simulated in the end.Waterflood induced fracture is an injector-centered open fracture or a fast flow channel,which is formed when the water injection pressure exceeds the opening pressure of various fractures or the formation fracture pressure during the long-period waterflooding of low-permeability reservoirs.The fractures are an artificial geological attribute and the main cause of heterogeneity for waterflood development of low-permeability reservoirs,and they are widely occurring and inevitable in these reservoirs.Three formation mechanisms of waterflood induced fractures were summarized as follows:(1)when the water injection pressure exceeds the opening pressure of natural fractures,the natural fractures become open and are extended;(2)When the water injection pressure exceeds the formation fracture pressure,new fractures are developed throughout the injection life;(3)Other types of artificial fractures are generated by production or stimulation measures such as perforating and fracturing around the injector.In Chang 6 reservoir,high angle structural shearing fractures,in en echelon arrangement,are widespread approximately along the direction of major principal stress,and the dominate fractures are the first type of waterflood induced fractures.According to numerical simulation results,the bottom hole pressure of the injeciton well exhibits a continuous irregular periodic change with the continuous expansion of the waterflood induced fractures.