Variation law of wellhead back pressure under well control during variable gradient managed pressure drilling

To accurately understand the variation law of wellhead back pressure under well control during variable gradient managed pressure drilling (MPD), based on a wellbore gas-liquid two-phase variable mass flow model considering the sudden change in density, this paper analyzes the variation of wellhead back pressure under well control during variable gradient MPD based on bottom hole constant pressure, explores the influence of the changes of different factors on the wellhead back pressure, and performs the sensitivity analysis of the factors affecting the maximum wellhead back pressure. It is found that the regulation of wellhead back pressure in variable gradient MPD is comprehensively affected by many factors, such as gas expansion, sudden change of liquid phase density at the separator, annulus diameter change at the casing shoe and seabed mud line, and its variation law is becoming more complicated. Under the scenario where other conditions remain unchanged, the smaller the spacing between the separator and the drill bit, the greater the density difference of light and heavy drilling fluids, the greater the formation pressure, and the smaller the pumping rates, the greater the wellhead back pressure in the circulating process. For the maximum wellhead back pressure, the specific variation coefficient values of formation pressure, density difference of light and heavy drilling fluids, spacing between separator as well as bit and circulating displacement decrease successively, and the corresponding sensitivity also decreases. The research results can provide theoretical support for the accurate prediction of wellhead back pressure and the fine control of wellbore pressure during the variable gradient MPD.