渤海低渗储层液氮压裂技术可行性研究

Feasibility Study on Liquid Nitrogen Fracturing Technology for Low-Permeability Reservoirs in Bohai Oilfield

渤海低渗储层改造面临产量递减快和稳产期短的问题。液氮压裂技术可以通过形成复杂裂缝以增强地层能量,提高油气井产量并延长稳产期,但是该技术在海上应用的可行性有待研究。为此,通过建立液氮在大尺度裂缝内的流动传热模型,分析了液氮压裂在渤海低渗储层中的造缝能力,结合常规水力压裂的携砂效果,评估了液氮作为携砂液需要的排量和液量,并综合考虑液氮压裂的施工压力和施工规模,分析了液氮压裂在海上的适用性。分析结果表明:提高液氮注入排量是扩大复杂裂缝造缝范围的有效手段,以4.32 m^(3)/min的排量注入液氮60 min,可以形成半长为44 m的复杂裂缝;液氮作为携砂液,达到常规水力压裂携砂铺置效果所需的排量为9~14 m^(3)/min。针对海上作业特点,提出了液氮携砂压裂和液氮复合压裂2种海上液氮压裂工艺,液氮复合压裂工艺具备海上实施条件,有望成为渤海低渗储层开采的重要技术储备。

The stimulation of low-permeability reservoirs in the Bohai oilfield faces the problems of rapid production decline and short stable production period.Liquid nitrogen fracturing technology can be used to create complex 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 nitrogen in large-scale fractures was built to analyze the fracture initiation capacity of liquid nitrogen fracturing in lowpermeability 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 evaluated.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 m^(3)/min for 60 minutes,complex fractures with a halflength of 44 m is created.As a proppant carrying fluid,liquid nitrogen requires a displacement of 9-14 m^(3)/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 proppant 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 technical reserve for the exploitation of low-permeability reservoirs in the Bohai oilfield.