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Dual drillstring
A technique called the Reelwell Drilling Method (RDM) intends to solve the problems of hole cleaning and managing downhole pressure. RDM is the result of a joint industry project (JIP) on ERD aimed at achieving a drilling reach exceeding 20 km (12.4 mi).
RDM uses MPD to achieve ultimate ERD. The method is based on the use of a dual drillstring (DDS), a closed-loop flow circulation system, where drilling fluid flows to the bit via the drillstring annulus, and the return flow to surface is through an inner string. Efficient hole cleaning is obtained because drill cuttings are removed from the wellbore just above the BHA and are transported to the surface via the inner string.
The DDS is a conventional 65⁄8-in. drill pipe where each joint is equipped with 3½-in. inner pipe, with a 3-in. ID. The DDS is handled according to normal procedure on the drill floor. Well control is handled conventionally by circulating a kick up the annulus and by being able to pump down both channels in the dual drillstring. Tests show that the dual drillstring can be cut off by the BOP shear ram.
RDM differs from conventional drilling in its drilling fluid circulation path. For conventional drilling the fluid returns up wellbore annulus; in RDM the drilling fluid returns to surface through the inner pipe of the DDS. RDM incorporates a special top drive adaptor, dual float valve, and flow control unit. Fundamentally, it is based on pumping the drilling fluid into the DDS annulus and transporting the cuttings back to surface inside the inner string, so that the hole remains clean at all times.
Cuttings go up the inner pipe because it is the only return flow path for the system. The well annulus is sealed with a rotating control device (RCD) on top of the BOP.
The system can also provide hydraulic weight on bit (WOB) using a piston. This also reduces buckling potential of the string and enables more precise WOB control.
Accurate pressure control enables drilling within a narrow pressure window. The system avoids dynamic pressure gradients in the open-hole section and the pressure margins remain also at long horizontal reach. The design enables the use of a passive, well-annulus fluid and an active, circulating fluid of different density. The active fluid can be underbalanced. A heavier annular fluid increases drillstring buoyancy and reduces drillstring torque and drag.
Mechanical and hydraulic calculations have confirmed the capability of RDM to drill wells reaching beyond 20 km.
ERD is a critical for breathing new life into mature oil and gas fields. ERD technology will accomplish what would not be possible through conventional directional drilling. Wellbore stability, hole cleaning, and torque and drag are still challenges, but they will be overcome.
