|dc.description.abstract||A concept review of concentric coiled tubing drilling (CTD) system is performed in this Master thesis, hereafter named DualCTD. The main purpose is to investigate and present the advantages, limitations and applications for the DualCTD system. A feasibility study has been carried out for subsea drilling of drainage holes from an existing well, and drilling of subsea production wells in the Barents Sea.
The DualCTD system consists of two concentric coiled tubing (CT) strings that form a separate circulation system for the drilling fluid. Drilling fluid is pumped down the annulus between the two CT strings to the bottom hole assembly (BHA) where a mud motor generates rotation of the drill bit. The drilling fluid cleans the bit for cuttings and transports the cuttings through a circulating sub/dual float valve and into the inner string. This separated circulating system provides effective hole cleaning from the bottom of the well. The drilling fluid can be a light fluid, that is optimized for hole cleaning capabilities.
A secondary annulus, formed between the DualCTD string and the borehole, is filled with a barrier fluid (BF). Viscous BF is used to separate the two fluid systems in the secondary annulus. Placing the BF in the secondary annuls below seafloor results in an optimized stabilized hydrostatic head. The BF can also be optimized for formation preserving properties. A choke valve in the return fluid line is used to control the back pressure and match the downhole pressure for the two fluid systems with the formation pressure. A light drilling fluid and a heavy BF will also increase the buoyancy of the DualCTD string.
Buckling calculations conducted show that longer horizontal sections could be drilled due to increased buckling resistance and reduced friction drag for the buoyant DualCTD string. Horizontal sections of up to 2300 m can be drilled with a 3,5 x 2,375 DualCTD setup with a 6 bit for vertical kick-off points of 2000 m and deeper. This is more than three times as long as for conventional CTD.
A hydraulic model for calculating the pressure loss in the circulating system is developed. Burst and collapse pressure was found to be limiting for the maximum flow rate due to high frictional pressure loss in the circulation system in deep wells. Cutting transport capacity of the circulation system was found to be low due to the low acceptable flow rates. Cutting transport capacity will therefore limit the maximum rate of penetration.
The DualCTD system will also make it possible to drill through challenging pressure regimes, depleted reservoirs and problematic zones with its unique potential for managed pressure drilling.
Well control approach for the DualCTD concept will be much of the same as in underbalanced-/managed pressure CTD operations The DualCTD blow out preventer needs to be verified for cutting of the DualCTD string. Running of casing and cementing operations may have to be performed on drill string due to the large weight of the casing and the low axial load capacity of the DualCTD string.
Significant development work is needed to bring the DualCTD to a field proven metho||