Research on Gas-Charging Water Depths and Gas-Charging Rates of Dual-Gradient Drilling with Gas-Charging in Risers

To determine the gas-charging water depths and gas-charging rates of dual-gradient drilling with gas- charging in risers, a differential element model was built for calculating the annular pressure of risers. The model was built according to the drift flow theory of gas-liquid two-phase flow, considering the influence of annular pressure variations of risers on the gas volume during gas flow. Then the model for calculating gas-charging rates was derived for dual-gradient drilling with gas-charging in risers, and field tests were used to verify the results of model. Taking a deep-water well in the South China Sea as an example, the effects of operating parameters on the gas-charging water depths, annular pressure at the mudlines, and gas-charging rates were analyzed by the model proposed for dual-gradient drilling with gas-charging in risers. The results showed that in dual-gradient drilling with gas-charging in risers, the regulation effects of bottom-hole pressure by gas charging are most significant in a water depth within 300 m, with the fastest response. In a water depth range of 300.00–1 100.00 m, gas charging only played an auxiliary role in bottom-hole pressure regulation. When the water depth exceeded 1 100.00 m, a high gas-charging rate was required for dual-gradient drilling by gas charging. When the wellhead back pressure was small, gas charging was highly efficient in regulating annular pressure of risers at the seabed mudlines. The research indicated that in the dual-gradient drilling with gas-charging in risers, gas-charging points could be selected according to water depth, and gas-charging pipelines should be properly deployed.