Fractures Propagation And Propping Mechanism In Fractured Tight Oil Reservoir

The world's tight oil is rich in resources,which is about 690 billion ton.The tight oil in the Bakken and Eagleford area of the United States have been effectively developed.The prospect tight oil resources in China is about 7-9 billion ton.With the gradual depletion of conventional oil and gas resources,tight oil has become one of the hot spots of unconventional oil exploration and development in the world.Taking the second member of Dagang Oilfield as an example,some of the tight oil reservoirs in China have the characteristics of deep,rich in natural fracture,complex lithology and poor physical properties.It is necessary to carry out hydraulic fracturing to form effectively supportted fracture network.At present,there are few studies on the law of hydraulic fracture propagation in fractured tight reservoirs,and the effective supporting mechanism of fractured network is less studied.Both of them are the key factors affecting the exploration and development effect.Based on the characteristics of tight oil reservoirs,this paper studies the fracture propagation of tight oil reservoirs,and analyzes the factors of affecting crack propagation.The fracture propagation law is studied by combining the theoretical model of interaction between hydraulic fractures and natural fractures.The fracture propagation simulation models of vertical well and horizontal well are established to simulate and analyze the law of the crack propagation and the effect on in-situ stress.Based on the analysis of mechanism of the fracture support and the shear mechanism of the natural fractures,experimental analysis of self-supporting fracture conductivity and fluid-solid coupling support fracture conductivity capacity are carried out to establish multi-level seam sewn conductivity optimization model.effectively supported fracture network fracturing technology is suggested and has good effect in practice.The main research results are as follows:?1?Take the Kong2 member(E_k2)of Dagang Oilfield as an example.Mineral composition test,rock mechanics test and stress test show the characteristics of tight oil reservoirs in China which shows that rock Young modulus of tight oil reservoir in Ek2,Poisson ratio and mineral composition distribution are significantly different.Sandstone,shale,dolomite have a certain degree of natural fractue fractures.The reservoir has strong heterogeneity.The difference coefficient of reservoir horizontal principal stress is between 0.2-0.3.From the results show,Ek2 reservoir has the ability to form the network fracture.?2?CT scanning before and after fracturing and true three axis fracturing simulation experiment are carried out.The influencing factors and their regularity of fracture propagation are studied.Based on fracture mechanics theory,the interaction characteristics of hydraulic fractures and natural fractures are analyzed combing with experiment results.The results show that the natural fractures of the reservoirs are mainly horizontal joints and low-angle fractures.The expansion of hydraulic fractures is mainly controlled by natural fractures.The interaction between hydraulic fractures and natural fractures is mainly controlled by the approximation angle and in-situ stress difference.?3?Based on the characteristics of tight oil reservoir,rock mechanics data and core structure in the Ek2 formation,a numerical simulation model of fracture propagation in the two segment of tight oil reservoir is established and fracture propagation law and characteristic of induced stress is analyzed.Results show that the smaller the horizontal stress ratio is,the larger the displacement is,the greater the viscosity of the fracturing fluid and the larger the damage range of the natural fracture,the larger the range of the stress inversion,which is to say greater possibility of forming network fractues.The optimization method of cluster spacing between horizontal wells is built.The optimized cluster spacing is 30-40m meters,which is conducive to the uniform expansion of fractures and the complexity of fractures.The hydraulic fractures formed in the reservoirs will lead to induced stress field and reduction of the stress difference,which can improve the complexity of fractures.Hence the secondary fracturing technique is proposed.?4?Based on the analysis of the characteristics of propped fracture network,the shear model of natural fractures is established.The experiments of self-supporting fractures and fluid-solid coupling support fractures are carried out,and the mechanism of the crack damage is experimented.The conductivity optimization method is built.Results show that the optimized conductivity of main fracture is 6.29 D·cm,and 1^st sub-stage fracture conductivity 1.55 D·cm,2^nd sub-stage fracture conductivity 0.13 D·cm.Shear fracture is formed in the Ek2 formation.The conductivity of propped fracture is mainly affected by proppant embedding and the residual damage of the fracturing fluid.Under the reservoir condition,the self-supporting fracture conductivity satisfies the requirements of secondary fractures,40/70 mesh and 30/50 mesh ceramsite meet the requirements of the main fracture and the primary crack conductivity.Hence,the multi-stage supported fracture network technology is suggestted.?5?Based on the comprehensive research results,the fracturing technology of secondary fracturing,multi-stage multi-fracture support and horizontal well cluster optimization is proposed,which has achieved good results in practical application.The results of this paper has deepend the development of the hydraulic fracturing network and the high efficient support theory of the tight oil reservoir in China,which provides theoretical and technical support for the development of the tight oil reservoir in Dagang Oilfield.