Research On Fracturing Blockage Relieving Mechanism Of Water Injection Well In Unconsolidated Sandstone Reservoir

Water injection development is widely used in unconsolidated sandstone reservoirs in China.However,in the process of water injection,due to poor quality of injected water and unreasonable setting of injection parameters,plugging often occurs around the injection wells,resulting in decreased injections,reduced oilfield production and poor development effect.In order to solve this problem,many blockage relieving measures have been developed at home and abroad.Fracturing blockage relieving is one of many blockage relieving methods for injection wells in unconsolidated sandstone reservoirs.Because the unconsolidated sandstone reservoir is different from the conventional reservoir,it has the characteristics of low bond strength and high porosity and permeability.The change of pore pressure and the deformation and failure mode of reservoir rock are complex during fracturing.The mechanism and rule of rock fracture initiation and propagation in unconsolidated sandstone reservoir under different fracturing technologies are complex,and there is no clear cognition at home and abroad.The lack of theoretical basis makes the selection of fracturing technology and the optimization of operation parameters rely too much on field experience in the process of fracturing blockage relieving,and there is no clear guiding principle,which limits the application effect and scope of fracturing blockage relieving in water injection wells in unconsolidated sandstone reservoirs.Therefore,in this thesis,aiming at the fracturing blockage relieving mechanism of water injection wells in unconsolidated sandstone reservoirs,laboratory experiments,theoretical analysis and numerical simulation are carried out comprehensively to form a fracture pressure calculation method suitable for water injection wells in unconsolidated sandstone reservoirs.The main fracture mechanism and influence law of reservoirs under different fracturing technologies are revealed.The research results can provide certain theoretical reference for the optimization design of fracturing blockage relieving of water injection wells in unconsolidated sandstone reservoirs.In view of the fracture pressure of unconsolidated sandstone reservoir,this thesis comprehensively considers the factors such as formation pressure change and borehole wall plugging,and considers the characteristics of low strength of unconsolidated reservoir and the possibility of tensional fracture and shear fracture.The calculation model suitable for the fracture pressure of injection wells in unconsolidated sandstone reservoir is derived.The quantitative influence of effective stress coefficient,wellbore blockage coefficient,reservoir damage coefficient,formation strength parameters and deviation angle on the fracture pressure is analyzed.In view of the fracture mechanism and influence law of unconsolidated sandstone reservoirs,this thesis first carried out the physical simulation experiment of fracturing,and found that the filtration of low viscosity injection fluid was serious,and the pore pressure around the well increased to form a shear microfracture zone.High viscosity injection fluid filtration loss is reduced,easy to hold pressure,forming tensile cracks.Based on the injection of low viscosity liquid,the numerical simulation research was carried out on the blockage relieving mechanism and influence law of pressure-increasing water injection in injection wells of unconsolidated sandstone reservoirs.The results show that the formation mainly occurs shear fracture during the pressure-increasing water injection process,and shear dilation is the main mechanism of blockage relieving.The lower the original permeability of the formation,the higher the injection pressure,the longer the injection time,the smaller the degree of borehole wall plugging and near wellbore pollution,the larger the extension range of the fracture zone,and the better the effect of pressure-increasing water injection.Based on the experimental results of injection of high viscosity liquid,the mechanism and influence of fracturing filling and blockage relieving of injection wells in unconsolidated sandstone reservoirs are studied by numerical simulation.The results show that during the fracturing filling process,by controlling the filtration of fracturing fluid,the formation can mainly undergo tensional fracture and form simple and straight tensile crack.Reservoir permeability,fracturing fluid filtration coefficient and displacement have great influence on fracture extension.Fracture width increases with the increase of permeability,the increase of displacement and the decrease of filtration coefficient.Fracture length decreases with the increase of permeability,the decrease of displacement and the increase of filtration coefficient.