| It is a well-established fact that there are huge oil and gas resources in the global deep water area.At present,conventional riser drilling technology used in deep water drilling has a great challenge in the control of wellbore pressure,which seriously limits the exploration and development process of deep water oil and gas resources.By controlling the inlet pressure of the subsea pump equals to the sea water hydrostatic pressure,riserless drilling technology simulates the dual gradient effect,avoids the pressure loss in the riser acting on the bottom hole in the conventional riser drilling,thus reducing the bottom hole pressure,which is conducive to the safe drilling in the deep water narrow density window formation.Meanwhile,riserless drilling can also effectively control the bottom pressure by adjusting the subsea pump speed which makes it advantageous in deep water drilling.However,the basic hydraulics theory of riserless drilling is not perfect enough and needs further research.An accurate calculation of the bottom hole pressure requires a more accurate model to compute the pressure loss in annuli.The traditional calculation model of the pressure loss in annuli omits the high order of the ratio between the flow core and the annular clearance,which has high precision when the flow rate is large or the yield value of the drilling fluid is small,but there is a large error when the flow rate is small or the yield value is large.A parallel plate general flow equation is established,and by equivalent analogy with the general flow equation in annuli,the geometry conditions of the parallel plate ensuring equal flow rate and pressure loss are obtained,and the concrete forms of the parallel plate general flow equation of different rheological models are derived.Because the parallel plate general flow equation does not omit the ratio between the flow core and the annular clearance,it is more accurate than the traditional calculation model of pressure loss in annuli.The calculation models using a micro-element analysis method and a overall analysis method of U-tubing effect in riserless drilling are established.Micro-element analysis method derives the motion differential equations of the drilling fluid in the drill pipe and in the annuli by force analysis of the fluid micro-element,and considers that the total flow is one-dimensional,the motion equation of the total flow of the U-tubing effect can be obtained.The overall analysis method uses the energy equation for the total flow of one-dimensional unsteady flow of incompressible fluid to calculate andsolve the U-tubing effect.Using the developed analysis models of the u-tubing effect,the variation of the return flow rate,the drop height of the fluid level in the drill pipe,the mud pit gain,the duration and the bottom hole pressure are studied,and the sensitivity analysis of the factors affecting the duration and the bottom pressure are conducted.The characteristic of the narrow density window of deep water high pressure and high temperature well requires the effect of temperature be taken into account when calculating the bottom hole pressure.A transient wellbore temperature calculation model with the drill stem valve in use and a pseudo steady temperature calculation model without the drill stem valve which means the drill pipe is not filled up with the drilling fluid are established.Considering the influence law of temperature and pressure on the density of drilling fluid,a calculation model of ECD in riserless drilling is established,which provides a theoretical basis for the accurate calculation of the bottom hole pressure.Based on the narrow slot flow model,new calculation models of steady surge and swab pressure of Casson and Robertson-Stiff rheological models in the concentric annulus of a vertical well are established,respectively,taking into account the pipe movement velocity as a moving boundary condition.Compared with Burkhardt model which is applied in the field,the accuracy of the new models are verified.The surge pressure based on Casson and Robertson-Stiff rheological models in riserless drilling are calculated using the new calculation models of surge and swab pressure. |
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