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 operati.
Highlights
Due to the depletion of onshore oil and gas resources, the oil-gas industry has extended its search for resources to deep-water areas
The mud-lift pump is placed in water and return mud and cuttings to surface through a mud return line (MRL)
This study provides the fundamentals for designing a safe system for controlled mud-cap drilling operations
Summary
Due to the depletion of onshore oil and gas resources, the oil-gas industry has extended its search for resources to deep-water areas. A new mathematical model describing the unsteady flow after surface pump shutting down for CMC drilling is developed and validated with experiment from the literature (Ogawa et al, 2007). A numerical simulation based on the explicit Euler was performed to investigate the transient behaviors of mudflow velocity, mud level in the annulus and BHP. The uncontrolled unsteady flow can cause a significant problem during drilling in CMC drilling, especially in deepwater narrow-margin formations. Modeling and Analysis of Unsteady Flow Behavior in Deepwater Controlled Mud-Cap Drilling right-hand side term takes into account the boundary pressure at the mud level in the drill string and the annulus. The annulus pressure at subsea level is approximately equal to the seawater hydrostatic pressure; the length of the mud column within the annulus can be calculated using the following equation: Lann
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