Three-dimensional physical simulation experiments on large-scale hydraulic fracturing in multi-thin interbedded tight sandstone reservoirs
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摘要: 鄂尔多斯盆地东北缘临兴气田以多薄互层型致密砂岩储层为主,储层岩性复杂、渗透率低,受多种因素影响且作用机理不明确,导致压裂施工难度大、效果差异大。为此,针对临兴气田致密砂岩储层不同岩石组分、黏土含量、粒径、沉积旋回、平面及纵向非均质性等特性,设计并开展了不同地质条件下的水力压裂室内大型三维物理模拟实验。根据相似准则,参照二叠系石盒子组三向地应力、岩石强度、井身结构参数、现场压裂施工参数,确定了实验基本参数。根据临兴气田典型井主力储层特征,制作了考虑不同岩石组分、黏土含量、粒径、沉积旋回组合、平面非均质性组合、纵向非均质性组合的15块立方体岩心,开展了15组压裂模拟实验,并从分析注入压力曲线和观察岩样破裂面两个方面总结了影响水力压裂缝扩展规律的储层主控因素。研究表明,岩石矿物和粒径、沉积旋回、平面及纵向非均质性对致密储层裂缝扩展形态有显著影响。砂岩粒度较大、胶结较弱、黏土含量较高、平面非均质性较强,使得裂缝面更易于屈曲,扩展压力增大,导致加砂困难;沉积旋回会使得水力压裂缝易于沿旋回层面扩展,从而形成水平缝,其中反旋回界面突破难度比正旋回大;由于砂—泥层间界面、砂—煤层间界面、天然砂岩弱面易被激活,从而产生“工”或“T”字形裂缝,对于砂泥多薄互层还会产生“工”+“T”+“十”字形的组合裂缝。实验研究揭示了不同地质条件下的水力压裂缝扩展形态,也为类似区块的研究提供了借鉴。Abstract: The Linxing gas field on the northeastern edge of Ordos Basin is mainly composed of multi-thin interbedded tight sandstone reservoirs. These reservoirs feature complex lithologies and low permeability, and are affected by multiple factors with unclear mechanisms, leading to difficulties in hydraulic fracturing operations and significant variability in operation outcomes. Therefore, this study designed and conducted a series of large-scale three-dimensional (3D) physical simulation experiments of hydraulic fracturing under different geological conditions, focusing on different rock components, clay contents, particle sizes, sedimentary cycles, and planar and longitudinal heterogeneities of the tight sandstone reservoirs in the Linxing gas field. According to the similarity criteria, the basic parameters of the experiments were determined by referencing the triaxial geostress, rock strength, wellbore structural parameters, and on-site fracturing operational parameters of the Permian Shihezi Formation. Based on the characteristics of the main reservoirs in typical wells of the Linxing gas field, 15 cubic rock cores were produced to account for different rock components, clay contents, particle sizes, sedimentary cycle combinations, and planar and longitudinal heterogeneity combinations. Fifteen sets of hydraulic fracturing simulation experiments were conducted, and the main controlling factors affecting the propagation of hydraulic fractures were summarized by analyzing the injection pressure curves and observing the fracture surfaces of rock samples. The results indicate that rock minerals, particle sizes, sedimentary cycles, and planar and longitudinal heterogeneities have a significant impact on fracture propagation patterns in tight reservoirs. The fracture surfaces are more prone to buckling with larger sandstone particle sizes, weaker cementation, higher clay content, and stronger planar heterogeneity, increasing the expansion pressure and difficulty in sand addition. Sedimentary cycles facilitate hydraulic fractures to propagate along the cycle planes, resulting in horizontal fractures. The difficulty of breaking through in retrograde cycle interfaces is greater than in prograde cycles. Interfaces between sand and mud layers, sand and coal layers, and natural weak sandstone surfaces are easily activated, leading to "工" or "T" shaped fractures. A combination of "工", "T", and "十" shaped fractures may occur in sand-mud multi-thin interlayers. This experimental study reveals the propagation patterns of hydraulic fractures under different geological conditions, providing insights for research in similar blocks.
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表 1 水力压裂三维物理模拟实验参数与现场参数对比
Table 1. Comparison between parameters in three-dimensional physical simulation experiments for hydraulic fracturing and on-site parameters
几何参数 现场实验参数范围 现场实验参考值 室内实验参数 井筒直径/mm 215.9 215.9 18 射孔直径/mm 20~30 20 2 X方向地应力/MPa 25~36 28 14 Y方向地应力/MPa 35~39 36 18 Z方向地应力/MPa 40~44 42 21 破裂压力/MPa 30~34 31 30 压裂排量/(m3/min) 3~5 3 1×10-5 表 2 考虑不同岩石组分和沉积旋回实验分组
Table 2. Experimental grouping considering different rock components and sedimentary cycles
组号 岩石组分 主要粒度设置 备注 1-1 中砂60%、细粉砂10%、水泥30% 恒定中值粒度为375 μm 中砂标准组 1-2 粗砂60%、细粉砂10%、水泥30% 恒定中值粒度为710 μm 粗砂标准组 1-3 细砂30%、细粉砂10%、水泥30%、黏土30% 恒定中值粒度为160 μm 泥岩组 1-4 中砂60%、细粉砂10%、水泥20%、黏土10% 恒定中值粒度为375 μm 中砂加黏土组 1-5 底层:粗砂60%、黏土0%,从下到上渐变至细砂30%,黏土30%(每层均含细粉砂10%、水泥30%) 从下到上由710 μm渐变至160 μm 正旋回组 1-6 底层:细砂30%,黏土30%,从下到上渐变至粗砂60%、黏土0%(每层均含细粉砂10%、水泥30%) 从下到上由160 μm渐变至710 μm 反旋回组 表 3 考虑不同非均质性平面组合实验分组
Table 3. Experimental grouping considering different planar heterogeneous combinations
组号 岩性变化 渗透性变化 备注 2-1 泥—砂—泥 低—高—低 盒2段某水平井 2-2 砂—泥—砂 高—低—高 盒4段某水平井 2-3 标准组 标准组 均质对照 表 4 考虑不同纵向非均质性组合实验分组
Table 4. Experimental grouping considering different longitudinal heterogeneity combinations
组号 从上到下地层岩性 各层厚比例设置 备注 3-1 泥—砂—泥 单砂体 下石盒子组盒6段 3-2 泥—砂—泥 单砂体 上石盒子组盒4段 3-3 煤—砂—煤 单砂体上下含煤板 太原组太2段 3-4 砂—泥—砂—泥—砂 砂泥交互多层 太原组太1段 3-5 砂—砂—砂 弱胶结面 均质对照 3-6 天然露头砂岩 含薄弱面、粒间弱胶结 均质对照 -
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