Modeling and Analysis of Unsteady Flow Behavior in Deepwater Controlled Mud-Cap Drilling

Abstract

-A new mathematical model was developed in this study to simulate the unsteady flow in controlled mud-cap drilling systems. The model can predict the time-dependent flow inside the drill string and annulus after a circulation break. This model consists of the continuity and momentum equations solved using the explicit Euler method. The model considers both Newtonian and non-Newtonian fluids flowing inside the drill string and annular space. The model predicts the transient flow velocity of mud, the equilibrium time, and the change in the bottom hole pressure (BHP) during the unsteady flow. The model was verified using data from U-tube flow experiments reported in the literature. The result shows that the model is accurate, with a maximum average error of 3.56% for the velocity prediction. Together with the measured data, the computed transient flow behavior can be used to better detect well kick and a loss of circulation after the mud pump is shut down. The model sensitivity analysis show that the water depth, mud density and drill string size are the three major factors affecting the fluctuation of the BHP after a circulation break. These factors should be carefully examined in well design and drilling operations to minimize BHP fluctuation and well kick. This study provides the fundamentals for designing a safe system in controlled mud-cap drilling operatio．

Development and application of transient gas‐liquid two‐phase flow model considering sudden density change

“…It is directly related to safety and efficiency, and also to the success of drilling operations . In order conduct safe drilling for a narrow pressure window in deepwater, research institutions and scholars studied several dual‐gradient drilling technologies, such as dual‐gradient drilling with light medium (gas, hollow sphere), submarine pump lifting drilling, mud cap control drilling, and deepwater multi‐gradient drilling, while developing corresponding core technologies, matching equipment, and drilling theories. In recent years, based on the idea of injecting a light medium (gas, hollow sphere) into the annulus to realize dual‐gradient drilling, a new deepwater dual‐gradient drilling technology based on downhole separation was developed and a new downhole cyclone separator was designed .…”