低渗透油藏注水诱导裂缝及其开发意义

Waterflood-induced fractures and its significance for development of lowpermeability sandstone oil reservoirs

本文以鄂尔多斯盆地安塞油田王窑区长6低渗透砂岩油藏为例,研究了在油藏注水开发过程中形成的注水诱导裂缝特征及其形成机理,探讨了注水诱导裂缝对低渗透油藏注水开发调整的影响。注水诱导裂缝是指低渗透油藏在长期注水开发过程中由于注水压力过高形成的以水井为中心的高渗透性开启大裂缝。注水诱导裂缝主要表现为张裂缝,其规模大,延伸长,一般沿着主渗流裂缝方向或现今地应力方向延伸和扩展,并随着低渗透油藏注水开发,注水诱导裂缝不断地扩展和生长。注水诱导裂缝的渗透率高,沿诱导裂缝方向容易造成油井的快速水窜和水淹。注水诱导裂缝通常有3种形成机制,其中由于注水压力超过天然裂缝开启压力,导致天然裂缝张开、扩展和延伸,形成开启的大裂缝是研究区裂缝的主要成因机理。注水诱导裂缝形成以后,彻底改变了低渗透油藏的渗流场和压力场,影响低渗透油藏的注水开发效果,同时给低渗透油藏的中高含水期注水开发及其调整带来一系列更深层次问题,需要引起重视。

The characteristics and formation mechanisms of waterflood-induced fractures formed during the process of water injection development and their influence on development adjustment of low-permeability reservoir are studied.This paper uses the low-permeability sandstone oil reservoir of the sixth member of the Triassic Yanchang Formation in the Wangyao area of the Ordos Basin as an example.A waterflood-induced fracture is a water well-centric open fracture which is formed due to high water injection pressure during the long period of water injection development.Waterflood-induced fractures are tensional fractures with large scale and length.They extend and develop along the main flow fracture direction or in-situ stress direction.With the development of low permeability sandstone oil reservoirs,waterflood-induced fractures have grown continuously and extended from water well to oil well.Because of the high permeability in the fractures,waterflood-induced fractures can cause rapid waterflood and water breakthrough of oil wells,which is the most serious anisotropy of low-permeability sandstone oil reservoirs during the middle and later stages of water injection development.Waterflood-induced fractures have three formation mechanisms.Among them,the main formation mechanism is because water injection pressure is higher and makes natural fractures open and extend.After formation of a waterflood-induced fracture,the flow field and pressure field in low-permeability sandstone reservoir are completely changed,which impacts on the water injection-development effect of the low permeability sandstone reservoir.At the same time,we need to pay attention to a series of deeper problems to development and adjustment of low-permeability sandstone oil reservoir during the mid to high water content stage.