Feasibility Study on Liquid Nitrogen Fracturing Technology for Low-Permeability Reservoirs in Bohai Oilfield
The stimulation of low-permeability reservoirs in the Bohai oilfield faces the problems of rapid pro-duction decline and short stable production period.Liquid nitrogen fracturing technology can be used to create com-plex fractures to enhance formation energy,increase well production and extend stable production period,but the feasibility of its application in offshore oilfields needs to be studied.A heat transfer model for the flow of liquid ni-trogen in large-scale fractures was built to analyze the fracture initiation capacity of liquid nitrogen fracturing in low-permeability reservoirs in the Bohai oilfield.Combined with the proppant carrying effect of conventional hydraulic fracturing,the required displacement and liquid volume of liquid nitrogen as a proppant carrying fluid were evalua-ted.Finally,the operation pressure and scale of liquid nitrogen fracturing were comprehensively considered,and the applicability of liquid nitrogen fracturing in offshore oilfields was analyzed.The results show that increasing the injection rate of liquid nitrogen is an effective means to expand the fracture initiation range of complex fractures.When the liquid nitrogen is injected at a flow rate of 4.32 m3/min for 60 minutes,complex fractures with a half-length of 44 m is created.As a proppant carrying fluid,liquid nitrogen requires a displacement of 9-14 m3/min to achieve the proppant carrying placement effect of conventional hydraulic fracturing fluid.Moreover,based on the characteristics of offshore operations,two offshore liquid nitrogen fracturing processes,i.e.liquid nitrogen prop-pant carrying fracturing and liquid nitrogen composite fracturing,were proposed.The liquid nitrogen composite fracturing process has the conditions for offshore implementation,and are expected to become an important techni-cal reserve for the exploitation of low-permeability reservoirs in the Bohai oilfield.
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