Effect And Mechanism Of Multiple Injection On Hydraulic Fracture Propagation

Shale oil and gas,tight oil and gas and other unconventional oil and gas play an increasingly important role in the petroleum industry.One of the core technologies of unconventional oil and gas development is large-scale fracturing of horizontal wells.It is found that multiple injection may enlarge the volume of reconstruction,enhance the complexity of fracture system,and improve the effect of reconstruction.In enhanced geothermal systems(EGS),in order to reduce the risk of induced earthquake,cyclic injection fracturing is also tried to realize the transformation of geothermal reservoir under low pressure,so as to realize "soft fracturing".Exploring a new pumping mode of hydraulic fracturing has a good prospect for the development of oil and gas,geothermal,coalbed methane and so on.However,up to now,the mechanism and law of multiple injection fracture propagation are not clear,and the applicable conditions for this technology are not clear.In this paper,based on the fracture propagation mechanism and law under the condition of multiple injection,through theoretical analysis and experimental research,the formation mechanism and influencing factors of fracture network,the mechanism of multiple injection fracturing and the fracture propagation law of multiple injection fracturing were reserached,and the application conditions of multiple injection fracturing technology were analyzed with the shale reservoir of Qingyi member in Songnan basin as the goal.The main conclusions are as follows: :(1)From case study and data analysis at home and abroad,multiple injection fracturing can reduce the fracture pressure of reservoir,increase the volume of reservoir reconstruction and the complexity of fracture network,and possibly reduce the risk of induced earthquake;(2)The formation of pressure fracture network is controlled by net pressure,weak surface strength,ground stress difference and other factors.Multiple injection fracturing strengthens the effect of fluid lubrication,reduces the weak surface strength of rock bedding,reduces the hole friction and flow resistance in the fracture,increases the net pressure of the fracture,changes the stress field around the fracture,reduces the horizontal stress difference,and the proppant bridge plug in the primary fracture increases the expansion resistance of the dominant fracture,which is the main mechanism for multiple injection fracturing to promote the formation of fracture network Reason;(3)The experimental results show that the interval time has an important influence on enhancing the effect of multiple injection fracturing,and the increase of interval time is conducive to reducing the fracture initiation pressure,increasing the fracture complexity and reducing the maximum radiation energy of acoustic emission;(4)The reservoir with natural fracture development,small in-situ stress difference and strong water sensitivity of weak surface strength is suitable for multiple injection fracturing.Taking the Qingyi shale of Songnan basin as the evaluation object,its pure shale reservoir has good applicability for multiple injection fracturing.