Flow Characteristics of UBD Applied to Gas Reservoirs
By: Zongyu Lu, et al (2021)
This paper discusses the use of Underbalanced Drilling (UBD) technology in high pressure, low permeability gas reservoirs. While UBD can protect gas formations from drilling fluid damage and improve drilling speed, the invasion of gas into the wellhole can lead to a two-phase flow state that complicates well control due to gas phase compactness and slippage effects.
To prevent violent slippage and rapid gas expansion, it's essential to understand two-phase flow laws. These laws can help in determining the right backpressure to apply, the process of gas distribution in the borehole, and the bottom-hole pressure, thereby informing the design of drilling fluid density, well backpressure, and equipment selection. The paper uses the numerical difference method to solve two-phase flow one-dimensional equations. Using a drilling example, it presents the distribution of gas void fraction, migration, and bottom-hole pressure change laws, providing a theoretical basis for designing boreholes in low permeability gas reservoirs using UBD.