| Shale gas is an unconventional natural gas with its large reserves,higher virgin shale content,long production cycle and other advantages.Shale gas is one of the most realistic alternative resources of conventional oil and gas.Shale gas resources in Guizhou province reached 13 trillion and 540 billion cubic meters,recoverable resources of about 1 trillion and 950 billion cubic meters,ranked the third in the country and has rich resource.Guizhou Feng'gang No.3 block is one of the land and natural resources of the second round of shale gas exploration area,with good prospects for shale gas exploration and development.In the exploration and development of shale gas,the fractured area with good preservation condition is the area of shale gas enrichment.The fractures in shale reservoirs control the gas bearing and gas permeability of the reservoir,which is one of the important reservoir space of shale gas.The fractures in shale reservoirs provide an important basis for the layout design of shale gas exploration,the design and deployment of horizontal wells,the design of hydraulic fracturing and the optimization of water injection scheme,which has important theoretical significance and engineering value.This paper carried out shale reservoir petrology,pore structure,pore size distribution,rock mechanics characteristics,fracture characteristics,find out the main controlling factors of fracture development based on the Guizhou Feng'gang No.3 block of the Lower Cambrian.Then based on geological mechanics,rock mechanics,elastic mechanics and fracture mechanics theory,the prediction method of fracture distribution of shale reservoir under the influence of multi period tectonic stress field was established.The research results have an important guiding role for the shale gas pressure cracking in the north area of Guizhou.The research results are as follows:Through the X ray diffraction analysis of the whole rock and clay mineral,the Lower Cambrian shale reservoir,the quartz content is 36% ~ 92%,with an average content about 78%.The content of pyrite is from 2% to 25%,with an average content about 7.4%,and the content of plagioclase is from 3% to 23%,with an average content about 15%.The clay mineral content is 2% ~ 30%,with an average content about 8%,and the clay minerals is illite,with an average of 87%.The area of the black shale in Niutitang Formation of brittle mineral content is high,it is easy to form cracks under the action of external force.SEM observation showed that: the development of the Lower Cambrian shale pore include intergranular pore,intragranular pore,dissolution pore,organic pore,special pore(framboidal pyrite internal pores),pore connectivity is poor,resulting in very low permeability.Through NMR test and analysis of a large number of nano pores,shale pore throat mainly between 0-0.1?m,which belongs to the nano pore,there are a few microns.The pore size distribution of shale is mainly between 0.001?m-0.01?m and 0.01-0.4?m,which showed that the pore of shale in the study area is dominated by nanopore.Through the rock stress-strain permeability test,confining pressure and shale permeability under different osmotic pressure was measured,the result showed that the relationship of shale permeability and confining pressure was nonlinear,through numerical simulation,found that the cubic function fitting degree is the highest,reaching 0.99145.With the increased of confining pressure,the permeability of the shale was nearly cubic function in the lower confining pressure.With the increased of confining pressure,the permeability of shale decreased,and the relation between the permeability of shale and the osmotic pressure was approximately linear.Under low confining pressure,the relationship between permeability and shale osmotic pressure was approximately linear positive correlation.Under the low confining pressure,the permeability increased with the increase of the osmotic pressure,and the permeability increased with decreasing amplitude under high confining pressure.By the numerical test shale with filling of calcite,in different loading directions of the failure mode,that the final failure mode of shale uniaxial damage after induction for four: inverted V(? =0°?30°?45°),inverted Z(? =15°),linear(?=75°),V shape(? =60°?90°).Observation of the numerical experiments of acoustic emission distribution,the distribution characteristics and macroscopic failure mode were similar.Based on the fractal geometry theory,the calculation of different stress levels,different loading conditions of shale damage fractal dimension,the fractal dimension D can reflect the failure mode of the specimen.From the trend of the failure mode,the inverted Z shape damage sample D to a maximum value of 1.688016,linear damage sample minimum D was 1.481904,the inverted V shape and V shape failure samples D between these two.Therefore,the bigger the D,the failure mode is more complex,the smaller the D,the failure mode is more simple.From the macro to the micro perspective to observed the characteristics of fractured shale reservoir,fractures developed in the study area,in which mainly tectonic fractures,tension fracture proportion is greater than the shear fractures in tectonic fractures.From the outcrop,core,microscope and scanning electron microscope observation showed that the shale crack filling degree is high,the most common fillings in the micro cracks are calcite,followed by a small amount of quartz and pyrite.The lithology,tectonic stress field,rock mechanics properties and organic carbon content are important factors to control shale reservoir fracture.Among them,the influence of tectonic stress field on the crack degree is the highest.Quartz,dolomite,calcite and other brittle mineral content is higher,the more developed cracks.The content of TOC is positively correlated with the degree of development of cracks.The study area of Yanshan period and Himalayan period after the superposition of stress field simulation results showed that the high stress located the end of the near fault zone,fault and fracture junction,the low stress area located between the fracture and fracture.The fracture developed areas(Ic>1.11)ars mainly distributed in the Hejiaba fault zone,Suiyangshan fault zone,Dangwan fault zone,Tao Ping fault zone and Fengyan fault zone and the locations where the fault orientations change,as well as the intersecting areas of the fault.The relatively developed fracture areas(1<Ic <1.11)are mainly distributed in the Yuquangou fault zone and west of Suiyangshan fault zone,Yonghe fault zone,west of FC-1 wells and east of Fengyan fault zone.The distribution of the fracture development zones was mainly controlled by the fault zones.The next step of exploration is to focus on the development of fracture zone?... |
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