Investigation on the propagation characteristics of pressure wave during managed pressure drilling

The small difference between formation pressure and fracture pressure in offshore oil and gas reservoirs poses a huge challenge to drilling. Managed pressure drilling (MPD) technology, as a drilling technique that can accurately control bottomhole pressure, is an effective technique to solve this challenge. In MPD technology, the pressure wave propagation behavior and mechanism in the wellbore induced by wellhead backpressure are crucial for parameter design and efficient application. In this paper, pressure wave propagation characteristics and mechanism in gas-liquid flow were investigated with a new proposed pressure wave velocity model that considers inter-phase mass transfer effect. This new model and its solution algorithm were verified with experimental data in literature. The influence of gas invasion stage, drilling fluid type, drilling fluid density and backpressure on pressure wave propagation characteristics were investigated. Results show that the time for pressure wave induced by wellhead backpressure in the wellbore cannot be ignored in the design of the backpressure value during MPD. This propagation time increases with occurrence of gas invasion. Moreover, the propagation time in water-based drilling fluid is longer than that in oil-based drilling fluid, which is because the interphase mass transfer between invaded gas and oil-based drilling fluid. The influence mechanism of high drilling fluid density and wellhead backpressure on pressure wave propagation characteristics is due to the suppression of gas invasion process. These findings could be used as guides in parameters design and optimization in MPD

Dynamic well killing method based on Y-tube principle when the drill bit is off-bottom

The design of dynamic well killing parameters lacks theoretical support because the U-tube principle is not applicable while drilling bit is not at the well bottom. We present the Y-tube principle and establish a gas and liquid two-phase flow model with the killing fluid backflow and zero net liquid flow theory in wellbore below the drill bit. A dynamic well killing theory is proposed, and the variations of the well killing parameters are revealed. The results were as follows. (1) The Y-tube was composed of an annular part above the bit, a drillstring part, and a wellbore part below the bit. Gas–liquid flow models in those three parts were established to accurately simulate the dynamic well killing process when the drill bit was off-bottom. (2) Based on these models, the dynamic inflow and outflow curves during the killing period were obtained. The variations of the dynamic well killing parameters were analyzed by comparing the two curves. (3) With the increase of pressure, the critical gas velocity of blowout decreases, while the temperature, density and viscosity of the killing fluid do the opposite. With the increase of temperature, density and viscosity, and the decrease of the wellbore diameter, the bottom hole pressure increases. And it decreases with the decrease of the distance between the bit and wellhead. (4) Considering the actual well killing conditions, it is recommended to increase the critical blowout velocity by increasing the killing fluid surface tension and density, and to change the killing fluid density and viscosity so that the dynamic outflow curve is always above the dynamic inflow curve or the dynamic outflow curve is always above the formation pressure. As a result, the dynamic well killing will be successful when the bit is not at the well bottom