Prediction Of Reasonable Injection Pressure For Stimulation Of Water-injection Wells In Poorly Consolidated Sandstone Reservoir

There are many unconsolidated and poorly consolidated sandstone reservoir in Bohai Bay.However,during long term of waterflooding,formation plugging often occurs.To study how to improve the water absorbing ability by enhancing the water injection pressure under the condition of safety water injection is very meaningful.However,there're few study on calculation fracturing pressure for unconsolidated sandstone and safety water injection.It is necessary for us to study the reasonable water injection pressure for unconsolidated sandstone reservoir.Rock mechanics tests are done for unconsolidated sandstone.Based on seepage mechanics,elastic-plastic mechanics and rock mechanics,fracturing pressure during water injection for poorly consolidated sandstone is studied.The forming process of big channel is modeled by numerical method,and some influencing factors are analyzed.Meanwhile,the safety water injection is also considered to determine the reasonable water injection pressure.And some water injection tests are also done indoor.Several aspects of research are studied:(1)Fracturing pressure during water injection for poorly consolidated sandstone is established.For the problem of fracture initiation for poorly consolidated sandstone during water injection,the analytical stress solutions are obtained under 6 different stress state near the wellbore.There are two fracturing mode on borehole wall: shear fracturing and tensile fracturing.The effective stress path on the borehole wall should be considered while calculating the fracturing pressure.The fracturing modes under different stress state are analyzed under different stress path.The results show that tensile fracturing is more likely to occur under low in-situ stress,and shear fracturing is more likely to occur under high in-situ stress.If the rate of injection pressure is too fast,the fracturing pressure is increasing.The strength of poorly consolidated sandstone also affects the fracturing mode and fracturing pressure during water injection pressure.Tensile fracturing tends to occur for the high strength rock under the same in-situ stress.(2)The numerical model to model the forming process of big channel in unconsolidated sandstone under the under the effect of hydraulic scouring is established.The erosion and deposition are considered in the model.The effects of bottomhole injection pressure,inhomogeneity,plugging extent and critical flow rate on the initiation of erosion are studied.The results show that big channel is more likely to occur under high bottomhole injection pressure and low critical flow rate on the initiation of erosion.The inhomogeneity of reservoir is essential the forming of big channel.As the plugging extent increases,the required bottomhole pressure to remove the plugging is also increasing.(3)The plugging removal and safety water injection are both considered to determine the reasonable injection pressure during water flooding.The critical injection pressure calculation model for caprock safety and fault safety are established.The effects of caprock strength,natural fracture and fault on critical injection pressure is analyzed.The range of reasonable injection pressure is designed for B15 well in SZ36-1 oilfield,after field test,the water injection rate increase from 380m3/d to 470m3/d.and no other unsafety problems occur,The field test verify the feasibility of the technology to remove plugging.(4)The plugging removal water injection simulation experiments of artificial rock are done.The failure law of unconsolidated and poorly consolidated sandstone is studied.The results show that the fracture is more likely to occur after plugging.The fracture propagates along the maximum horizontal stress and stop at the interface between high permeability zone and low permeability zone.The fracturing mechanism and fracturing pressure during water injection is studied.The method of determing the reasonable injection pressure for poorly consolidated sandstone reservoir considering both plugging removal and safety is developed.This provides a feasible prediction method for determing the reasonable injection pressure for pooly consolidated sandstone during waterflooding.