一种新的基于拓扑结构特征的微裂隙-孔隙空间描述方法

Extraction and identification methods of micro-fractured characteristics information in pore space of porous media based on morphological theory

微裂隙在多孔介质储层大量发育并与基质孔隙连通,是流体渗流特别是非常规储层油气渗流的重要通道.高精度无损成像技术以及数字岩心技术的快速发展,为孔裂隙空间描述、孔裂隙模型构建等研究提供了数据支撑.本文基于传统孔隙空间拓扑学,分析裂隙拓扑结构特征,充分考虑三维裂隙空间区别于普通孔隙的面状特征以及与基质孔隙之间的相互连通关系,提出"特征点化"的思路,将裂隙空间点进行分类,形成了适用于多种孔裂隙空间的描述以及结构特征分析方法.首先基于传统孔隙拓扑结构分析方法,提出"线面共存"的新骨架模型,提取裂隙中轴骨架面、中轴骨架线,精细刻画裂隙轮廓以及孔裂隙连通关系;其次,基于各类空间点的拓扑结构共性分别提取相应点集,分割孔隙、裂隙空间,构建微裂隙-孔隙骨架模型;第三,基于新的孔裂隙骨架模型提取孔裂隙空间的几何-拓扑结构特征参数,获取裂隙孔隙度、开度、倾角等重要裂隙几何参数,以及孔裂隙配位数、裂隙发育范围等拓扑参数;最后基于上述骨架提取描述方法,对理想与真实孔裂隙空间的数字岩心进行了骨架提取和分析.从微裂隙拓扑形态开展研究进而对裂隙-孔隙空间描述,可简化此类孔隙介质的建模过程,为后续气、液流动模拟提供准确数据基础,促进缝洞介质油气开发的进展.

Natural micro-fractures in ultra-tight formations, such as shales and coal seams, provide key information about pore structure and continuous channel in it. How to identify the geometrical and topological properties of micro-fractured structure and matrix pore space and the correlation between them are the major contributors which can lead to strong calculation results during the following seepage flow simulation. Through characterizing the pore space included micro-fractures （i.e., connectivity capability of pore space, extension of fracture, etc.）, we can therefore describe and develop pore-fracture structure and the suitable models to gain better understanding of the roles of micro-fractures on the drainage of hydrocarbons from matrix pores. In this work, we proposed a new skeleton model to distinguish fractures from pore space via extraction of surface points set of fracture. In the procedure of points set extraction, we improved the classic ＂medial axis based＂ shrink method to ＂medial surface-based＂ method for new fracture description through introducing a new set of skeleton points （i.e., surface points and edge points of the fracture）, one of which describes its aperture and the other is used for collecting connectivity information and determining the extension ranges of the fracture. The new skeleton model can show more comprehensible forms of the real connected junction instead of the former ideal model, voxel-thickness medial surface extracted can also satisfy demands of the classic skeleton extraction model and preserve the topology of the original pore space included micro-fractures in the meantime. New points set classification method mentioned above is determined by considering the difference of their topological properties. Through calculating and collecting their topological number one by one, we can obtain a new skeleton model formed with medial axis and surface. Among them, the simple points set was composed of values of topological number T6=T26=1 in the 3×3×3 direct neighborhood system as before and would be deleted in the process of shrinking pore space in the certain order of the distance values of the space. The surface points forming medial surface was composed of the points close to the center of the micro-fractures in all directions. An object point was defined as surface point if no background voxels continuously existed between any two neighbor voxel that shares a ＂face＂ in its 3×3×3 neighborhood system, which means its topological number T6〉1. Edge points represented as the junction between fractures and matrix if they are always the one of the 26 neighbors of surface points. Moreover, in the process of obtaining new skeleton model, characteristic parameters and connectivity of micro-fractures can then be easily got via statistics and calculation. Through combining Euclidean distance maps and geometric transformation, we can easily calculated the parameters of width, thickness, orientation, and inclination angle of micro-fractures. Connectivity location information from edge points would play the part of following simulation of flow interaction between fractures and the matrix. As a contrast, ideal and real fracture models were used to verify feasibility of our new methods, all results showed good effectiveness and accuracy. The study will lead to more realistic pore space models and help to extend the applicability to a wider range of porous media especially for the study of multi-scale pore space representation. This work was inspired by challenges in developing a fast and accurate method for micro-scale modeling in micro-fractured porous media, and potentially applicable for flow simulations in the tight porosity samples. Overall, our new methods improved the level of micro-fracture characterization representation of the pore space including fractures for the following flow simulation.