共和盆地东北部花岗岩型干热岩井下裂缝系统及其构造成因

Characteristics and tectonic origins of underground fracture systems of granitoids HDR in the northeastern Gonghe Basin

花岗岩型干热岩中的先存天然裂缝系统是影响地热能开发的重要因素之一,其识别对于地热资源的有效利用至关重要。由于花岗岩类具有高强度和低孔隙度,裂缝系统往往成为干热岩地热开采中热流体的主要渗流通道及储集空间,储层裂缝系统的表征一直是干热岩有效开发利用的关键,也是共和盆地干热岩勘查开采示范工程遇到的难题。本文采用构造地质学和地球物理学等多学科方法,以共和盆地东北部DR3和DR8全井段连续取心井、GR1和GR2分段取心井的岩心为研究对象,开展了先存裂缝系统的观察识别和测井、地震资料响应研究,明确了深部埋藏区花岗岩型干热岩天然裂缝系统发育特征,探讨了构造作用对裂缝系统的控制作用。通过对岩心裂缝类型和空间展布特征的详细解剖,发现研究区井下裂缝系统主要由单缝、网状缝和破碎带组成。单缝大多为剪切缝,少量拉张缝,单缝的产状受临近断层和裂隙的控制;网状缝多伴随破碎带;破碎带明显受断裂及较大规模隐伏裂隙控制。岩石岩性对裂缝发育程度有一定影响,在相同背景下,抗剪强度较低的正长花岗岩、二长花岗岩较花岗岩、花岗闪长岩和闪长岩更易产生破碎带及网状缝。通过连续取心与多种常规测井方法对比分析,声波测井对于花岗岩类裂缝响应较为明显,不同深度裂缝发育带声波时差值随着深度及破裂程度不同而发生变化。干热岩裂缝系统的形成与研究区多期区域构造活动有关,大量不同角度剪切缝的存在也反映了多期裂缝形成应力场的最大主应力方向发生过多次偏转。多尺度、多方法花岗岩型干热岩的先存裂缝系统识别,可为后续的储层改造及规模化地热能开发提供重要依据。

The pre-existent natural fracture system in granite-type Hot Dry Rock(HDR)is one of the important factors affecting the development of geothermal energy,and its identification is crucial for the effective utilization of geothermal resources.Due to the high strength and low porosity of granitoids,fracture systems often become the main permeable and storage channels for hot fluid in HDR geothermal exploitation.The characterization of fracture reservoir has always been a key problem in the effective development and utilization of HDR,and is also a major challenge encountered in the exploration and exploitation demonstration project of HDR in Gonghe Basin.In this study,we used geological and geophysical disciplines,and took four hot dry rock drilling cores with full-well continuous coring(DR3 and DR8)and segmented coring(GR1 and GR2)in the northeastern Gonghe Basin as the research objects,conducted observation and identification of the existing natural fracture systems,and analyzed the response of seismic and logging data,to determine the characteristics of deep buried fracture systems and the controlling effect of tectonic action on fracture system is discussed.The types,vertical distribution and spatial distribution of core fractures are dissected in detail.It is found that the underground fracture system in the study area is mainly composed of single fractures,meshed fractures and broken zones.Single fractures are mostly shear fractures,with a small number of tensile fractures,and the dips of single fractures are controlled by nearby faults and fractures.Meshed fractures are often accompanied by broken zones.Broken zones are obviously controlled by faults and larger-scale buried fractures.Rock lithology has a certain influence on fracture development degree,syenite granites and monzonitic granites with lower shear strength are rupture easily than granites,granodiorites and diorites,so that fracture zones and network fractures are more development.Through the comparison between continuous coring and many conventional logging methods,the responses of acoustic logging to granite-type fracture systems are obvious,and the horizontal fractures is more prominent,and exhibiting the cyclic wave jumping;the acoustic time difference of fracture development zone at different depth varies with depth and fracture degree.The formation of the fracture system in HDR is related to multi-stage regional tectonic activities in the study area,and the existence of a large number of shear fractures at different dip angles also reflects that the maximum principal stress direction of the multi-stage fracture formation stress field has been changed.The identification of the pre-existing fracture system in granite-type geothermal reservoirs at multiple scales and using multiple methods can provide important basis for subsequent reservoir modification and large-scale geothermal energy development.