Fracture Mechanism And Characterization Of Low-permeability Reservoir

Tight sandstone reservoirs usually presents low porosity and permeability,but the development level of natural fracture systems can greatly improve the reservoir properties.Fractures not only control the seepage system of low permeability reservoirs,but also the main storage sites of oil and gas.The degree of fracture development is a key factor for the high yield and stability of low permeability reservoirs.The degree of fracture development is a key factor for the high yield and stability of low permeability reservoirs.In this paper,fracture systems are the main object of study,by carrying out the study of fracture identification,fracture characterization,mechanism,prediction and evaluation of the distribution.Based on field outcrop,core,micro section observation and logging analysis,this paper summarized the characteristics of the development of low permeability reservoirs in the southeastern part of the Chezhen sag.It is concluded that the types of cracks in the study area are mainly shear fractures,followed by tension fractures and non-structural fractures.The high-angle oblique fractures are the most;horizontal fractures are relatively rare,generally diagenetic shrinkage fractures or layer fractures.The crack is mainly formed in three phases,one is controlled by the early Yanshan movement and developed the near EW and NW-SE shear fractures.And then controlled by the late YanShan movement which provided EW squeeze and transformed the early formation of the EW to the near SN to the orthogonal shear joint.And finally the NE-SW and EW to the crack formed by the extended stress field of the Himalayan period.The local stress field is mainly caused by the change of folds and faults,which also has some influence on the formation of cracks.According to the statistical analysis of the core data stratification,it is found that the fracture development section is affected by the distance between the fault distance and the vertical distance from the fault plane,and has the obvious zonation,the closer the fault plane is,the higher the fracture line density,the stronger the control and the impact of the fault,and vice versa.The description of the cracks in the rock slab shows that the micro cracks in the macroscopic fracture development area of the study area are also very developed,and the extension of the micro cracks is long,which often serves two barrels of throat and communicates with each other.According to the micro section observation,the micro cracks are widely distributed under the microscope,and the observed micro fractures often contain traces of oil and gas migration.Therefore,it is considered that the micro fracture is very useful for the increase of permeability.The core fracture density is the largest in the limestone section,followed by the sandstone section,and the slime density in the mudstone section is the smallest.Formation of low-permeability reservoir fractures in addition to being influenced by lithology,rock structures and thickness also affect the development of fractures.Under the same stress,if the thickness is the same,the finer the thinning layer of the low permeability reservoir,the more the fracture develops.Also in the stress under certain,if the same lithology,then the thinning of the rock more fractures developed,and vice versa.The heterogeneity of the rock also has an important effect on it.Such as near-EW,NW-SE,near SN and NE-SW fractures were observed in the Mesozoic stratum.But in different parts,some of them performed X-shaped shear joints,and some were nearly orthogonal form of the distribution of cracks,After the staging,it was found that the two groups of fractures with nearly orthogonal checkerboards were not synchronic cracks,but rather"superimposed cracks"caused by formation heterogeneity.By combining the formation fracture evaluation index with the rock deformation energy value,the quantitative prediction formula of fractures suitable for the study area is fitted as ?=-14.8×Fy~2+128.6×W~2+25.7×Fy-135.6×W+38.2.And the calculated data are compared with the core observation data.It is found that the prediction error is less than20%,which is basically consistent with the core data.It can be considered that the prediction result is ideal.In this paper,the comprehensive identification standard of fracture area and the evaluation system of fracture prediction are established,which is highly consistent with the actual measurement.Fracture identification and prediction results has highly reliability.The fracture comprehensive recognition standards and fracture prediction system for low-permeability reservoir.And it is a good reference for the same type of gas field home and abroad.