基于三维数字岩心的油页岩裂解孔隙重构及表征研究

Characterization of oil shale pore and fractures during pyrolysis using digital rock reconstruction

油页岩孔隙结构作为油气渗流的主要通道,高温热解过程中将产生复杂变化,然而其变化规律不明严重制约储集层分类评价和高效开发。因此,开展不同温度及不同有机质含量条件下油页岩热解对孔隙结构影响的研究对于油页岩的有效开发具有重要意义。针对热解前后油页岩孔缝结构变化精细表征难的问题,本文通过编程方法提出了一种基于三维数字岩心重构的裂缝及孔隙结构表征新方法,定量分析了热解过程中样品裂缝宽度、裂缝方向、分形维数及孔隙度等参数的变化规律。研究结果表明:(1)本文所提出的裂缝性数字岩心精细化处理方法结合等位精细化中面提取算法,在准确计算岩石物性参数的基础上可实现对缝宽及裂缝方向分布的精确计算,可达到对裂缝孔隙精细表征的目的;(2)随着热解温度的升高,油页岩样品孔隙度由18.3%升至20.9%,且裂缝宽度不断增大,油页岩样品物性得到持续改善,但当热解进行到一定程度后油页岩孔隙结构变化趋势放缓;(3)相同温度条件下随着有机质含量的升高,热解后油页岩样品孔隙度由12.1%升至29.3%,但4种样品分形维数未随着有机质含量升高呈现出单调递增趋势,表明热解裂缝复杂性除受有机质含量影响外,也受到样品中有机质初始分布的影响。本文可为油页岩热解后孔隙结构精细表征提供新思路。

As the primary conduit for oil and gas seepage,the pore and fracture structure of oil shale undergoes intricate changes during pyrolysis.However,the lack of understanding regarding its variations hinders effective reservoir classification and devel-opment.Therefore,it is important to investigate the influence of oil shale pyrolysis on pore structure at different temperatures and organic matter content.This paper proposes a new method based on 3D digital core reconstruction to quantify fractures and pore structures.The methodology employs parameters such as fracture width,fracture orientation,fractal dimension,and porosity to assess changes in pore and fracture structures throughout the pyrolysis process.The findings are as follows:(1)The proposed digital rock processing method and midplane extraction algorithm for fractured rock can accurately calculate fracture width and fracture orientation distribution.(2)As pyrolysis temperature increased,the porosity of oil shale samples rose from 18.3%to 20.9%,with a corresponding increase in fracture width,indicating improvements in the physical properties of the samples.However,the rate of change in pore and fracture structures decreased after pyrolysis(3)For fractured oil shale samples,porosity increased from 12.1%to 29.3%with higher organic matter content at consistent temperatures.However,Fractal analysis shows that pyrolysis fracture complexity is governed by both the content and,importantly,the initial spatial distribution of organic matter.This provides a new perspective for the precise characterization of pore structure after fracturing oil shale.