Study On Cuttings Transport And Pipe Stuck Mechanism In Extended Reach Drilling

The extended reach well(ERW)is an advanced well type technology for the efficient development of complex oil and gas fields in the ocean,intertidal sea,lake,mountain and polar regions.However,in the directional drilling of an ERW,the problem of pipe sticking frequently occurs due to the unclean borehole,which is one of the key factors leading to the low efficiency and even malignant pipe stuck accident.Therefore,it is of great theoretical and engineering significance to deeply reveal the mechanism of pipe stuck in an ERW and establish the corresponding design and control method for the safe and efficient control operation of directional drilling in an ERW.Firstly,a transient two-layer model of cuttings transport for extended reach drilling is modified from two aspects.On the one hand,considering the influence of the buoyancy of the cuttings bed,the momentum equations of the cuttings bed and the suspension layer are modified.On the other hand,a new method to calculate the mass exchange between the suspension layer and the cuttings bed is proposed by combining the diffusion equation with the transient two-layer model.Secondly,the cuttings transport analysis of directional drilling in an ERW under multiple operations is carried out,including alternating drilling and washing conditions,tripping and pump-off,and the cuttings distribution law and the pipe stuck mechanism in an ERW are revealed from the whole and local aspects.Finally,to reduce the risk of pipe stuck in an ERW,the calculation method of cuttings bed cleaning time before tripping is established.The results show that the modified model has good computational stability and accuracy.Under the alternating drilling and washing conditions,the cuttings beds in the wellbore show wave-like dynamic migration characteristics on the whole,and the migration direction is from the bottom to the wellhead.When the drilling and well washing cycles reach a certain number of times,the wave-like shape of cuttings beds reaches dynamic stability.Under the simulated parameters,a large sand dune is formed about every 2000-2500 m,and the risk of pipe stuck is higher in sections with dimensionless cuttings height greater than 10%.When the pump is off,the suspension force of drilling fluid is not enough to support the cuttings,so the cuttings bed in the small deviated section of slides downward and produces local accumulation at the inclination of 40°-60°.While tripping,the “bulldozer” effect causes local accumulation of cuttings in front of the connector.The accumulated heights of both pump-off and tripping are highly related to the initial bed heights.If the wellbore is sufficiently cleaned,the risk of pipe stuck can be greatly reduced under these two conditions.Cuttings on the surface show several intermittent “cuttings tide” while washing.If tripping immediately after the first “cuttings tide” comes out,the risk of pipe stuck is high because there may still be several sand dunes in the wellbore.Therefore,it is recommended to continue washing until the sand dunes in the wellbore are removed.The cleaning time of wavy cuttings bed decreases with the increase of the flow rate,and increases with ROP,initial cuttings bed height and well depth.However,the above parameters have little influence on the number of cycles,which is basically maintained at 2-3 cycles.It is recommended to refer to the cycle number to control the cleaning time.