基于岩土介质三维孔隙结构的两相流模型
Two-phase flow model based on 3D pore structure of geomaterials
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摘要: 地下储层中岩土介质一般具有较低的孔隙连通性,宏观流动模拟一般忽略微观尺度的孔隙连通性,通过渗透率、弯曲度等参数反映储层的整体特性。但岩土介质的多孔性及孔隙间复杂的连通性,使得宏观描述流体在岩土介质中流动不能反映其内在流动特征。孔隙结构模型的建立可以反映岩土介质中孔隙的几何形态及空间连通性,为解释流体在复杂多孔介质中的流动特性提供有效手段。通过考虑岩土介质孔隙尺寸分布、孔隙孔喉空间相关性、孔隙连通性等特征参数,建立了反映不同岩土介质连通性、各向异性特征的等效孔隙网络模型。等效孔隙网络模型通过水力特征参数等效的方式反映岩土介质三维微观孔隙结构,通过渗透率计算验证了模型的有效性。此外,基于建立的孔隙结构模型,开发了孔隙尺度动态两相流计算模型,模型可以反映孔隙内弯液面的动态运动过程,直观反映多孔介质中的优势渗流,可以为不同孔隙尺度岩土介质提供表观渗透率、击穿曲线、相对渗透率曲线等宏观计算参数。将孔隙尺度两相流模型应用于页岩气开采中水力阻滞特性研究,结果表明:页岩基质的残余饱和度约为30%,随着平均配位数的增加,残余饱和度显著降低。Abstract: Geomaterials normally have low pore-connectivity in underground reservoir, and the macro-scale flow simulation normally ignores the micro pore connectivity and uses macro parameters such as permeability and tortuosity to reflect the conductivity of underground reservoir. However, due to the complex pore structure and pore connectivity of geomaterials, the macro-scale method cannot reflect the micro flow mechanisms. The pore-structure model provides an effective way to reflect the micro-flow mechanisms for complex porous media since the pore geometry and pore connectivity can be included in the model itself. In this work, an equivalent pore-network model (EPNM) is established considering pore-size distribution, spatial correlation and pore-connectivity. EPNM aims at reflecting 3D pore structure of geomaterials by the equivalent hydraulic parameters, and the effectiveness is verified by permeability tests. Furthermore, a dynamic two-phase flow model is developed based on EPNM, and simulate the dynamic invasion of each phase, reflect the preferential flow in porous media, and it can provide apparent permeability, relative permeability curve, breakthrough curve for macro-scale simulation. Finally, the dynamic two-phase flow model is applied to the wetting phase trap during shale gas exploitation. The results show that the residual saturation in shale matrix is around 30%, and this residual saturation decreases significantly with the increase of the average pore coordination number.