A Simplified Method of Hydraulics Analysis for Riserless Drilling System

摘要

Hydraulic calculation plays an important role in accurately predicting downhole pressure in deepwaterrnareas. Unlike conventional onshore drilling, riserless drilling faces with many challenges inrnDeepwater, such as the changeable marine environment, shallow flow, shallow gas and other naturalrndisasters. Riserless Drilling System (RDS) has been devoted to overcoming shallow formationrndrilling difficulties, however a fast and efficient hydraulics model for RDS becomes the key factorrnto restrain the development of riserless drilling at deep water environment. As hydraulics analysis isrnclosely related to flow regime of drilling fluid. According to the characteristics of RDS, this articlernestablishes a set of whole flow regime hydraulic analysis method by Robertson-Stiff model. Basedrnon this theory, circulation pressure drop of RDS at three different flow regimes (laminar flow,rntransition flow, turbulent flow) could be obtained from a special Darcy Equation. Besides, there isrnno simplification process during the derivation of modeling process and thus this RDS hydraulicsrncalculation method has a high accuracy. The application of this RDS method for South China Searnshows that it can exactly predict the downhole pressure as long as rheological equation of drillingrnfluid is known. With the help of this model, RDS could go through narrow density window formationrnsmoothly. Cases study shows that circulating pressure drop both in pipe and annulus increase slowlyrnat laminar, and go faster at transition flow and turbulent flow. In addition, there is a critical pumprnflow in RDS, only when flow rate isn’t less than critical pump flow can ensure the continuity ofrnwellbore circulation. Otherwise, the u-tube effect will generate, and a vacuum volume will exist inrndrilling strings. Hydraulic calculation is an essential process in petroleum engineering, it involvesrndrilling and completion process, well cementing and well kill etc. How to accurately analyze it is stillrna long way to go. This hydraulics method proposed in this paper can provide valuable references forrnfollow-up researches of riserless drilling in deep-water.