Bottomhole Pressure Inversion Method for Open-Circuit Drilling Based on Drilling Fluid Return Height

In the process of shallow deepwater drilling, due to the narrow safety window of drilling fluid density and the frequent drilling in shallow gas, shallow flow, gas hydrate and other factors, the risk of drilling accidents increases, and kick and even blowouts are easy to occur, which seriously endangers the safety of operators. In the absence of risers and subsea blowout preventers, open-circuit drilling is usually used to drill deep water shallow drilling. In the process of open-circuit drilling, drilling fluid directly returns to the seabed, so it is impossible to shut in the well when kick or even blowout occurs, and it is difficult to obtain the shut-in vertical pressure and casing pressure through traditional methods, and it is difficult to calculate the bottomhole pressure, which cannot provide a basis for the selection of parameters such as density and displacement of subsequent kill fluid. Aiming at this problem, based on Fluent numerical simulation software, this paper simulated and studied the return form of drilling fluid at the bottom mud line of open-circuit drilling by solving the VOF model, and analyzed the influence of different ocean current velocity distribution, drilling fluid displacement, gas penetration on the return height of drilling fluid. It is found that ocean current velocity distribution has little influence on drilling fluid return height, and mainly affects the diffusion dilution degree of drilling fluid. Drilling fluid displacement, gas penetration and wellhead pressure have great influence on drilling fluid return displacement and height. On this basis, according to the principle of underwater jet, the relationship between drilling fluid return height and wellhead pressure is established. On the premise of known return height, the wellhead pressure can be calculated, and the bottomhole pressure can be retrieved from the wellhead pressure. This study reveals the regularities of drilling fluid return flow in open-circuit drilling, provides theoretical guidance for the prediction of bottomhole pressure when kick occurs and the well cannot be shut in, provides support for the effective implementation of subsequent well control measures, and ensures the operation safety of deep-water shallow open-circuit drilling.