Multi-scale fracture development characteristics and fracture network patterns of buried-hill in metamorphic rocks: A case study of the Bozhong Z metamorphic buried-hill

doi:10.13745/j.esf.sf.2024.6.19

Abstract

Abstract:

Natural fractures are important reservoir spaces and effective seepage channels for metamorphic buried hill reservoirs. For metamorphic rock reservoirs an interconnected network of multi-scale fractures is key to forming high quality contiguous reservoirs and achieving high, stable reservoir productivity. In this study, the power law distribution of fractures is established based on detailed characterization of multi-scale fractures using image log, core, thin section and SEM data. The contribution of malti-scale fractures to reservior storage is clarified, and the spatial pattern of fracture network and its impact on productivity is analyzed. According to the results, fracture systems of different scales in the study area show similar change patterns, i.e., the increase of fracture intensity follows the power law with the decrease of fracture size. In tight reservoirs, large and well connected macroscopic fractures provide important permeability, but their contribution to porosity is limited due to low fracture density; whereas microfractures with high fracture density can provide important reservoir space, but they mainly play the role of connecting matrix pores due to their small aperture and limited connectivity. According to spatial combination of multi-scale fractures, five types of fracture networks are recognized. Among them, the multi-scale/high-density/multi-set combination type and large-scale/medium-density/multi-set type networks can form large-scale continuous high-quality reservoirs and achieve high, stable production, while small-scale/high-density/multi-set type and large-scale/low-density/multi-set type networks require hydraulic fracturing to achieve stable production; small-scale/low-density/single-set type fracture network can not improve tight reservoir due to difficulty of obtaining industrial oil flow.

Key words: metamorphic buried hill, multi-scale fracture, network structural model, distribution law

CLC Number:

GONG Lei, QIN Xinnan, GAO Shuai, FU Xiaofei, SU Xiaocen, WANG Jie. Multi-scale fracture development characteristics and fracture network patterns of buried-hill in metamorphic rocks: A case study of the Bozhong Z metamorphic buried-hill[J]. Earth Science Frontiers, 2024, 31(5): 332-343.

Figures/Tables 13

Fig.1 Location (a) and structural outline (b) maps of the study area

Fig.2 Image logs showing fracture types and fracture development characteristics

Fig.3 Fracture effectiveness statistics at different scales of measurement

Fig.4 Characteristics of fracture occurrence from image logs

Fig.5 Characteristics of fracture parameters at image log scale

Fig.6 Distribution of fracture density from image logs

Fig.7 Types and characteristics of fractures from cores

Fig.8 Types and characteristics of microfractures from thin section and SEM

Fig.9 Characteristics of fracture parameters at thin-section scale

Fig.10 Characteristics of fracture parameters on SEM scale

Fig.11 Power-law distributions. (a) Fracture density at different measurement scales. (b) Fracture porosity and permeability.

Table 1 Fracture classification and contribution to reservoir in the study area

裂缝类型		面密度/(m·m^-2)	开度/μm	连通性	孔隙度/%	渗透率/mD	贡献
宏观裂缝	成像测井	1~10	>100	好	0.1~0.2	2~20	渗流通道
宏观裂缝	岩心	10~50	40~100	好	0.2~0.5	0.2~2	渗流通道和储集空间
微观裂缝	薄片	50~300	10~40	中等	0.5~1	0.02~0.2	储集空间和渗流通道
微观裂缝	扫描电镜	300~5 000	1~10	差	1~2	<0.02	储集空间

Fig.12 Types of fracture networks in metamorphic rock reservoirs in the study area

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