Diagnosis And Inversion Of Waterflood-Induced Fracture In Low-Permeability Reservoir

The pressure behaviors of water injectors and producers are quite different in waterflooding process of low-permeability reservoir.Without fracturing or only mini-fracturing,the water injection rate is very high,while with large-scale fracturing,the oil production is usually small.Field tests in Changqing Oilfield show that the interpreted effective permeability around water injector is much larger than that around producers and core analysis result.Meanwhile,the water breakthrough in oil wells is unidirectional.These phenomena are closely related with waterflood-induced fractures(WIFs)caused by continuous water injection.Based on the development of Ordos Basin,the dynamic characteristics of WIFs are analyzed.Then,we proposed the pressure models by considering the influence of fracture closure during field-testing period.Lastly,we presented a comprehensive method of WIF monitoring,diagnosis,and inversion.Key results are obtained as follows:(1)The mechanism of WIF under formation fracturing pressure in low-permeability reservoir is revealed.The development practice of low-permeability reservoirs show that WIFs still exist in most injection wells even though the injection pressure is controlled below the formation fracturing pressure.This illustrates that the WIFs are not caused by formation fracturing,but activates and induces the closed natural fractures,and then extend,communicate with each other,and finally form fracture channel.(2)Pressure models and inversion methods for single WIF and WIF channel are proposed by taking the WIFs’ variations into consideration.Different with proppant fractures,the WIFs close gradually during the field-testing period when the pressure inside fracture is below fracture closing pressure because they are not strengthed by proppants.As a result,the water that stored in the WIFs would be squeezed into the formation,which behaves similar with wellbore storage effect – which is called “fracture storage effect”.This is the reason that the interpreted wellbore storage coefficients increased by up to several orders of magnitude in so many water injectors.If there is no extra pressure drop between wellbore and WIF,the WIF can be regarded as a “expanded wellbore”,which leads to a prolonged storage period with a considerably large storage coefficient.Otherwise,two unit slope in pressure derivative curve may be identified,which is a peculiar “bi-storage effect” in water injectors: wellbore and fracture storage.Since the fracture walls would not contact all at once,but contact mechanically at the tips first,then subsequently towards the wellbore during the fracture closing period,a new variable fracture storage regime is identified in water injectors.The decreasing fracture conductivity behavior could also cause an upward of pressure derivative curve at late times,which is traditionally recognized as the presence of a closed boundary when it may not be the case.(3)Pressure models and inversion methods for multiple WIFs are proposed by impacts of multiple closures on bottom-hole pressure(BHP)responses.For multiple WIFs,they would not close all at once,but successively,due to different stress conditions perpendicular to the fracture walls,which behaves as multiple unit slopes in pressure derivative curve.(4)A comprehensive workflow,including real-time monitoring,formation test,and dynamic production analysis,is proposed to characterize the performance of WIFs,which can identify the initiation of WIF timely,investigate the formation fracturing pressure,and uneven waterflooding performance in the areal and vertical directions,analyze the waterflooding direction and obtain the dynamic parameters in the formation and fractures.This workflow can help us obtain the formation fracturing information timely and effectively,and then provide a useful guidance for the adjustment and managing the WIFs.