Drill string mechanical behaviors of large-size borehole in the upper section of a 10000 m deep well
The oil and gas exploration has been advancing to the new fields in the more complex ultra deep strata of 10 000 m deep,while the upper large-size borehole poses great challenges to the drilling of 10 000 m deep wells.Hard rock and low drilling fluid return speed result in low rate of penetration(ROP),and the severe vibration in the large-size borehole induces multiple cracks in drilling tools.When the weight on bit(WOB)is small,the ROP is low;and when the WOB is slightly large,the vibration of the bottom hole assembly(BHA)is intensified rapidly,which causes the service life of the large-size drilling tool much shorter than expectation.To address this problem,this paper comparatively researches the string vibration in the upper section of Well SDCK-1 and its neighboring ultra-deep well(8 000 m deep),and then specifically analyzes the drill string mechanical behaviors of large-size borehole by means of full-hole string system mechanical model and numerical simulation.And the following research results are obtained.First,the larger the borehole size,the more severe the vibration of the bit and the BHA.The vibration intensity of the bit and BHA at the depth of 500 m in the second-spudding large-size borehole of Well SDCK-1 is about 48.0％and 41.5％respectively higher than that at the same depth in the medium-size borehole of its neighboring well,and about 29.0％and 2.9％respectively higher than that at the depth of 3 000 m in the third-spudding medium-size borehole of Well SDCK-1.Second,under the same borehole size and rock behaviors,BHA has greater influence on bit vibration intensity than the whole length of drill string.Therefore,optimizing the BHA can significantly reduce the bit vibration intensity,thereby protecting the bit,and increasing the bit's rock breaking energy utilization efficiency can improve the ROP.Third,in the large-size borehole,the rock breaking excitation of the bit propagates upward,and the transverse vibration attenuation is slower than the axial vibration attenuation,so the more effective way to protect the BHA is to suppress the transverse vibration at the lower BHA in the large-size borehole.The torque of large-size bit is larger,and the fluctuation of WOB and torque is more obvious.Fourth,the bending moment of the BHA in the large-size borehole and its fluctuation are larger.In addition to the large overall vibration intensity,the alternating bending moment is also an important cause of frequent crack initiation in the drilling tools on site.In conclusion,the research results reveal the mechanical behaviors of the drill string in the large-size borehole of an ultra-deep well,and suggest to control transverse vibration intensity and alternating bending moment,which can provide a guidance for the drilling of an large-size hole in the upper part of the ultra-deep well.
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