超低渗储层压裂液返排特性及组成变化研究

Study on Flowback Characteristics and Composition of Flowback Fluid in Ultra-low Permeability

返排液组成动态分析是评估压裂作业方法和表征储层特性的一种有效手段,本文以长庆油田多个致密砂岩储层的天然气井压裂作业为研究对象,采用X射线荧光光谱法分析了多个井位探压后不同返排时间返排液的离子组成及变化规律,同时通过pH值、化学需氧量、流变学、表面张力及接触角等,分析压裂液渗透进入岩层后,岩层中矿物溶解、迁移过程及相互作用机理。研究发现,返排液中离子含量趋于稳定的时间为10~20天,大多数离子含量在18天左右趋于稳定;返排液每日排出液量稳定的时间为19天,每日产气量达到稳定的时间为20天,三者趋于稳定的时间基本相当,因此压后返排液的离子含量变化特征能够反映压裂液与岩心的相互作用,为优化生产制度提供理论依据。

Dynamic analysis of flowback fluid composition is considered to be an effective means to evaluate reservoir characteristics and guide fracturing operation.Taking the natural gas well fracturing operation in the tight sandstone reservoir in the eastern Ordos Basin of Changqing Oilfield as the research object,this work analyzes by X-ray fluorescence spectrometry the ionic composition and volume change of the flowback fluid against different flowback times after the pressure detection in gas wells at various reservoir layers.At the same time,after the fracturing fluid penetrates into the rock and through the tests of pH values,chemical oxygen demand(COD),rheology,surface tension and contact angle,etc.,the process of mineral dissolution and migration in reservoir and the interaction mechanism between fracturing fluid and reservoir rock and the ion source of post fracturing flowback fluid was revealed.It is found that the ion contents in the flowback fluid after fracturing tend to be stable after 10∼20 days and most of the ion contents tend to be stable after about 18 days.It is around 19 days for the volume of the flowback fluid to be stable and around 20 days when the daily gas production becomes steady.The time duration for the three parameters is basically similar.Therefore,the variation characteristics of the ion contents of the flowback fluid after fracturing can reflect the interaction between the fracturing fluid,shale and proppant.This work is expected to provide a theoretical basis for further understanding of the interaction mechanism between tight sandstone and fracturing fluid,and provide a theoretical basis for optimizing production system.