| Fractures in shale not only provide accumulation space for shale gas,but also provide migration channels for its production.Accurate description of shale fractures is an important basic work in the development and utilization of shale gas.Carboniferous shale in Qaidam Basin has good resource potential.In this paper,the distribution characteristics of natural fractures in Carboniferous shale in eastern Qaidam Basin are described qualitatively and quantitatively,and how the existence of natural and/or artificial fractures in shale will affect rock mass structure is discussed.The research content of this paper is of guiding significance for evaluating the benefit of shale gas development and utilization in the study area.In this paper,three areas of Shihuigou,Dulan and Wanggaxiu and in eastern Qaidam are selected for field geological survey,and basic data for fractured rock mass research are obtained.The Carboniferous shales in Shihuigou,Dulan and Wanggaxiu are dominated by high-angle fractures,while the NNW,near SN and near EW directions are the dominant orientations of shale fractures in that three areas respectively.Shale fractures in Dulan and Wanggaxiu both are divided into three groups according to direction.Fisher distribution is used to fit the fracture occurrence of each group in the that two areas,and the optimal fitting distribution parameters of the fracture occurrence are obtained.By using the inverse modeling method(Joint-OKY2 software),the diameter mean,standard deviation and three-dimensional density(Ci value)of each group of fractures in the Dulan and Wanggaxiu are obtained,and the three-dimensional discrete fracture network model of shale in that two areas is established.The Goal values of shale model are 0.1728 and 0.0908 respectively.At the same time,the actual and simulated distribution of fractures on the same sampling surface(outcrop)are compared.The results show that the difference of the actual and simulated parameters of fractures distribution in a reasonable range.The model established in the paper can better reproduce the actual distribution of field fissures and will be used for shale blocking research.In the study of blocking,the paper not only explores blockiness of shale rock mass in the presence of natural fractures,but also discusses the effect of increasing artificially induced fractures on the blocking of Wanggaxiu shale.According to block recognition by General Block software,under natural conditions,The block percentage of Dulan shale is 0.00194%,and the block density is less than 1(block/m~3).the percentage of Wanggaxiu shale block is 2.8665%,and the block density is about 124(block/m~3).The blockiness of Dulan and Wanggaxiu shale both are very low and can be classified as non-massive rock mass.It is suggested that hydraulic fracturing and other methods should be adopted to generate artificial fractures to communicate with natural fractures and destroy rock mass structure so as to ensure the production efficiency of Carboniferous shale gas wells in the study area.Twenty-nine kinds of structural models of Wanggaxiu fractured rock mass with different artificial fracture directions have block percentages ranging from 4.3279%to 6.8705%and block densities ranging from 432 to 832 blocks/m~3.Through analysis,when the Dip-direction of artificial fracture is in the range of 45°~90°and the Dip is about 30°,the blockness of Wanggaxiu shale model is the highest.Therefore,in order to prevent the collapse of the shale gas wellbore block,drilling in this direction should be avoided as far as possible during the drilling process,and more fractures in this direction should be controlled in the hydraulic fracturing design to ensure the fracturing effect. |
PDF Full Text Request