基于脆性、断裂韧性和抗压强度的可压裂性评价模型及应用

    A fractureability evaluation model based on brittleness, fracture toughness, and compressive strength and its application

    • 摘要: 可压裂性是衡量非常规油气储层是否易于通过压裂实现增产的关键性能指标,也是页岩油井评价的重要参数。目前可压裂性评价主要采用基于岩性或岩石弹性特性的脆性指数,但由于岩性与孔隙组合的多样性,相同岩性条件下储层的弹性和可压裂性可能存在显著差异。为了解决这一问题,本研究针对渤海湾盆地沧东凹陷孔二段页岩油储层岩性复杂性和孔隙多样性的特点,提出了一种综合弹性脆性指数、抗压强度和断裂韧性的储层可压裂性评价模型。首先,利用阵列声波测井资料,用速度与密度计算弹性参数、泊松比、杨氏模量等弹性力学参数;其次,结合实验室测试结果,构建静态与动态弹性模量的关系,获得弹性参数、岩石力学参数和弹性脆性指数;再次,对于缺乏剪切波信息的常规测井资料的井区,基于Xu-White、孔隙度和页岩中不同矿物含量,建立估算页岩与砂岩横波速度的方法,实现储层工程甜点参数的计算;最后,选取研究区的典型井,计算其可压裂性指数 FI并进行分级评价。研究结果表明:该区域 FI<0.45为可压裂性差,0.45≤FI<0.55为可压裂性适中,FI≥0.55为可压裂性好;根据该标准划分的区域与实际水力压裂效果一致。研究结果为页岩油勘探的工程甜点评价提供了新思路和技术手段。

       

      Abstract: Fracability serves as a critical indicator for assessing whether the production of unconventional oil and gas reservoirs can be increased by fracturing. Besides, it also represents a key parameter for evaluating shale oil wells. Currently, fracability evaluation is primarily conducted using the brittleness index based on the lithology or rock elasticity. However, due to the diversity of lithology and pore combinations, the elasticity and fracability of reservoirs may vary significantly even under identical lithological conditions. To address this issue, targeting the complex lithologies and diverse pores of shale oil reservoirs in the second member of the Kongdian Formation, Cangdong Sag, Bohai Bay Basin, this study proposes a reservoir fracability evaluation model that integrates the elastic-based brittleness index, compressive strength, and fault toughness. First, using the velocity and density data in array acoustic log data, elastic mechanical parameters, including the elastic modulus, Poisson's ratio, and Young's modulus, are calculated. Second, in combination with laboratory test results, the relationship between static and dynamic elastic moduli is established. Accordingly, elastic parameters, rock mechanics parameters, and the elastic-based brittleness index are determined. Third, for well blocks lacking shear wave information in conventional log data, a method for estimating the shear wave velocities of shales and sandstones is developed based on the Xu-White model, porosity, and contents of various minerals in shales. This model enables the calculation of parameters for the engineering sweet spots of reservoirs. Finally, a typical well in the study area is selected to calculate its fracability index (FI), followed by a grading evaluation. The results of this study indicate that FI values of < 0.45, 0.45~0.55, and ≥ 0.55 represent a low, moderate, and high fracability, respectively. The zones with low, moderate, and high fracability delineated in the study area agree well with the actual hydraulic fracturing results. This study provides a novel thought and technical approach for evaluating engineering sweet spots in shale oil exploration.

       

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