致密砂岩裂缝网络复杂性及其影响因素研究

Fracture network complexity of tight sandstone and its influencing factors

裂缝网络分析在油气藏勘探开发过程中发挥着着重要作用,致密砂岩裂缝网络复杂性分析对水力压裂优化、裂缝网络预测、裂缝建模等具有重要意义。文章结合致密砂岩复杂的裂缝网络动态演化的实验研究,明确了裂缝网络的分形和多重分形谱特征,深入分析了裂缝网络的复杂性及其主控因素。通过岩石力学和X射线CT扫描实验确定了岩石力学和裂缝网络特征;通过扫描电镜实验、裂缝网络的分形分析定量化表征了致密砂岩微观组构和裂缝网络的分形特征。研究结果表明:致密砂岩的石英含量为28.08~52.88%,黏土含量为11.54~25.45%,粒度为61.18~184.55μm,孔隙度为8.125%~10.296%;单轴抗压强度介于69.09~188.33 MPa,弹性模量介于31.69~92.76 GPa;分形维数(DB)为1.28~2.35,谱宽(Δα)平均值为1.0851~1.3638。裂缝的萌生、扩展贯穿于应力–应变的全过程,裂缝网络的复杂性主要受控于致密砂岩的微观组构特征,并且具有明显的围压和尺度效应。三维裂缝网络的分形维数、多重分形谱的谱宽平均值可分别表征裂缝空间分布的复杂性和非均质性,两者之间具有相对的独立性。砂岩中石英、长石等脆性矿物含量越高、储层孔隙度越大、砂岩组成粒度越小裂缝网络分形维数越大,谱宽平均值越小;无围压情况下,样品裂缝网络的复杂性主要受控于微观组构特征,且随着轴向压力的增加而增加;存在围压的情况下,围压起主导作用,围压越大分形维数越小,谱宽平均值越大。而黏土矿物不利于复杂裂缝的形成;小尺度样品的分形维数和谱宽平均值大于尺度大样品的分形维数和谱宽平均值。砂岩的弹性模量和抗压强度与分形维数和谱宽平均值具有一定的正相关性。

[Objective]Fracture network analysis plays an important role in oil and gas exploration and development.However,complexity analysis of tight sandstone fracture networks and their control factors is relatively lagging.Based on an experimental study of the dynamic evolution of the complex fracture network in tight sandstone,the fractal and multifractal spectral characteristics of the fracture network were defined,and the complexity and main controlling factors of the fracture network were analyzed.Fracture network complexity analysis of tight sandstone plays an important role in hydraulic fracturing optimization,fracture network prediction,and fracture modeling.[Methods]Rock mechanics and Xray computed tomography scan experiments determined the characteristics of rock mechanics and fracture networks.The microstructure and fracture network fractal characteristics of tight sandstone were quantitatively characterized by SEM and fracture network fractal analysis.[Results]The results showed that the quartz content of tight sandstone ranges from 28.08 to 52.88%,clay content ranges from 11.54 to 25.45%,particle size ranges from 61.18 to 184.55μm,and porosity ranges from 8.125 to 10.296%.Uniaxial compressive strength ranges from 69.09 to 188.33 MPa,and the elastic modulus ranges from 31.69 to 92.76 GPa.The fractal dimension(DB)ranges from 1.28 to 2.35 and average spectral width(Δα)ranges from 1.0851 to 1.3638.[Conclusion]The initiation and propagation of fractures extend through the entire stress–strain process.The complexity of the fracture network of tight sandstone is mainly controlled by microscopic fabric characteristics,and has obvious confining pressure as well as scale effects.The DB of the three-dimensional fracture network and averageΔαof the multifractal spectrum represents the complexity and heterogeneity of the fracture spatial distribution,respectively,and are relatively independent.As the content of quartz,feldspar,and other brittle minerals in sandstone increases,the porosity of the reservoir increases,particle size of the sandstone decreases,DB of the fracture network increases,and averageΔαdecreases.In the absence of confining pressure,the complexity of the sample fracture network is mainly controlled by the microscopic fabric characteristics,and the complexity increases with increase of axial pressure.When present confining pressure plays a leading role;the higher it is,the lower the DB value,and the higher the meanΔαvalue.Clay minerals are unconducive to complex fractures formation.The mean values of DB andΔαof small-scale samples are greater than those of large-scale samples.The elastic modulus and compressive strength of sandstone are positively correlated with DB and meanΔα.