| Radial drilling-fracturing,combining radial jet drilling with hydraulic fracturing,is an revolutionary technology for efficient development of unconventional oil and gas resources.At present,the mechanism of controlling fracture initiation and guiding fracture propagation by radial horizontal wellbore(radial wellbore)is not clear,and there is no corresponding optimization method of fracturing parameters.In this paper,theoretical analysis,numerical simulation and experiments are used to study the three key problems,including controlling multi-point fracture initiation,guiding multi-fracture propagation and optimizing radial wellbore parameters.The main research work and results are as follows:(1)An analytical fracture initiation model for single radial wellbore and a finite element fracture initiation model for multiple radial wellbores are established.Besides,fracturing experiments are conducted.The effects of in-situ stress,radial wellbore parameters(orientation,number of branches,length,and diameter)and weakness plane on fracture initiation pressure and location are analyzed.The results show that: under the conditions of this paper,in-situ stress,number of radial wellbores and radial wellbore orientation(included angle between radial wellbore and maximum horizontal in-situ stress)are the main controlling factors;when the number of radial wellbore is greater than 3 and radial wellbore orientation is larger than 45°,fracture initiation pressure can be significantly reduced;and radial wellbore can activitate the weakness plane,which increases stimulated reservoir volume.The key parameter for controlling multi-fracture initiation in multi-branch radial wellbore is the radial wellbore orientation,and the multi-fracture synchronous initiation can be achieved by the same radial wellbore orientation.(2)A radial wellbore guiding multi-fracture propagation model is established by using finite element-meshfree numerical method.The mechanism of multi-branch radial wellbore guiding fracture propagation is revealed.The effects of different number of radial wellbore branches,horizontal in-situ stress difference and orientaiton of radial wellbore on the propagation of multiple fractures are analyzed.The results show that the stress field changes due to the pressure in the wellbore,which makes the rock near the wellbore easier to meet the criterion of fracture propagation,thus inducing the propagation of multiple fractures along the radial wellbore direction.Horizontal in-situ stress difference and the included angle between two radial wellbores are the main controlling factors affecting the guiding ability.Increasing the horizontal in-situ stress difference and reducing the included angle make the guiding ability drop.(3)Using the method of discrete fracture,a semi-analytical fluid flow model of complex fracture is established.The flow characteristics of radial drilling-fracturing are analyzed,the productivity is simulated,the effects of different branch number and angle on productivity are analyzed,and the parameters of radial wellbores are optimized for maximizing productivity.The results show that radial drilling-fracturing significantly reduces the flow pressure drop in the fracture and the productivity is twice that of conventional fracturing;the productivity of radial wellbore increases significantly after fracturing,which is five times that of pre-fracturing;the productivity enlarges with the increase of radial wellbore branches;and the maximum productivity of four branch radial wells can be achieved after fracturing when the included angle is 90 degrees.This paper presents a method of synchronous fracture initiation from multi-branch radial wellbores,reveals the mechanism of multi-fracture propagation guided by radial wellbores,and finally optimizes the radial wellbore parameters.The research results are expected to provide support for the popularization and application of radial drilling-fracturing technology in unconventional oil and gas development. |
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