Gas Influx Handling for Dual Gradient Drilling
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New drilling technology is primarily developed when limitations of existing technology makes it difficult or impossible to reach new targets. Drilling in deep waters and in depleted reservoirs is less profitable or too risky using conventional drilling methods. A better and safer alternative is to use specific managed pressure drilling (MPD) systems such as dual gradient systems. Dual gradient systems are designed for deep water operations. Drilling in hostile environments necessitates technologies and operational procedures that are thoroughly thought through. A small margin between the pore and fracture pressures implies increased need for pressure control. In this dissertation, well control of new drilling technologies has been discussed with focus on selected MPD systems. Extensive hydraulic modeling has been necessary to calculate flow and pressure variations for a drilling system using a partly evacuated marine drilling riser, i.e. where the mud level in the riser annulus is not fixed and is free to move up and down. The focus has been to simulate well dynamics when gas is flowing into the wellbore and is being circulated out. Drilling in narrow pressure margin reservoirs will increase the risk of losses and gain, and stringent well control is important. Well-control procedures have been discussed and proposed based on simulation results of different well scenarios using the Low Riser Return System (LRRS). Five papers are attached, where four of them present the main results of the research work. The results are related to the dynamic behavior of the mud level in a partly evacuated marine drilling riser. It is shown that the marine drilling riser can be used as a separator during a gas-kick circulation, and that the mud level in the marine drilling riser annulus can self-regulate as long as it is located a sufficient distance below surface. A new U-tubing model for managed pressure drilling systems like the LRRS has been developed and successfully tested in a small scale facility. It is important to take into account that the inner cross-section area of the drillstring is different from the cross-section area of the annulus. This has not been included in the existing model. The results also show that the LRRS can adjust the wellbore pressure by controlling the pumps used in the circulation system. ECD control during drillpipe connection has been studied for the LRRS, and it is shown that time is an important parameter, especially in small open-hole sections where the pressure margins are small. Simulations show that the total time needed for making a drillpipe connection is typically in the order of 5-45 minutes depending on the magnitude of the ECD.