海陆过渡相储层不同岩性断裂特征及其对压裂的影响

Fracture characteristics of reservoirs with different lithologies in marine-continental transitional facies and its influence on fracturing

岩石断裂力学性质是决定储层压裂改造效果的内在地质力学基础,明确海陆过渡相储层不同岩性断裂力学性质对于优化压裂改造措施、改善造缝效果具有现实意义。以鄂尔多斯盆地东缘海陆过渡相储层为研究对象,开展储层多种岩性断裂韧性测试,明确储层不同岩性断裂力学特征及影响因素,并在有限元平台建立了具有测井岩石力学参数分布特征的压裂数值模型,着重研究了岩性组合对裂缝高度扩展规律的影响。结果表明:海陆过渡相储层岩性复杂,富含石英和黏土矿物,力学变形破坏呈较强的脆性特征;与海相页岩相比,海陆过渡相储层岩石具有低断裂韧性的特点,断裂韧性值大小呈现煤岩<碎屑沉积岩<灰岩的特征,其中植物碎屑、纹层等弱结构面通过改变断裂路径影响相应岩性的断裂韧性,有较强的劣化作用;储层岩石静态弹性模量是控制不同岩性断裂韧性的重要力学因素,断裂韧性与静态弹性模量具有线性正相关性;储层岩石具有较强的层间力学非均质性,水力裂缝高度倾向钝化于由泥岩层进入砂岩层过程中,储层夹煤层将明显降低裂缝高度,增大支撑剂在储层非均匀铺置的概率。储层地应力对裂缝高度形态的影响与岩性组合有关,因此在前期水平井穿行轨迹选择需要重视水平井上、下岩性组合特征对后续水平井压裂效果的影响。

Rock fracture mechanics properties are the intrinsic geo-mechanical basis for determining the effectiveness of reservoir fracturing stimulation.It is of practical significance to clarify the fracture mechanics properties of reservoirs with different lithologies in marine-continental transitional facies to optimize the fracturing stimulation measures and improve the effect of fracture networks.Taking the reservoir in marine-continental transitional facies at the eastern edge of Ordos Basin as the research object,this study carried out the fracture toughness test of various lithologies in the reservoir,clarified the fracture mechanics characteristics of the reservoir with different lithologies and the influencing factors,and finally established a numerical model of fracturing with the logging lithology depositional sequence on the finite element platform,and emphasized the influence of the combination of lithologies on the fracture height propagation.The results show that reservoir lithologies in the marine-continental transitional facies are complex and rich in quartz and clay minerals,and the mechanical deformation shows strong brittleness characteristics.Compared with marine shale,the reservoir rocks in the marine-continental transitional facies are characterized by low fracture toughness.The fracture toughness value shows the characteristics of coal rock<clastic sedimentary rock<gray rock.Among them,weak structural surfaces such as plant debris and grain layers have a strong deterioration effect by changing the fracture paths and thus affecting the fracture toughness of the corresponding lithologies.The static elastic modulus of reservoir rocks is an important mechanical factor controlling the fracture toughness of different lithologies,and the fracture toughness has a linear correlation with the static elastic modulus.The reservoir rock is characterized by strong interlayer mechanical heterogeneity.The hydraulic fracture height tendency is blunted in the process of moving from the mudstone layer into the sandstone layer,and the interbedded coal layer in the reservoir will significantly reduce the fracture height and increase the probability of non-uniform proppant placement in the reservoir.The influence of reservoir in-situ stress on fracture height morphology is related to the lithology combination,so the selection of horizontal well traversing trajectory in the early stage needs to pay attention to the influence of the lithology combination characteristics above and below the horizontal wells on the fracturing effect of the subsequent horizontal wells.