Abstract

As the industry seeks out exploration prospects in deeper water, the complexity of drilling in tight pore-pressure and fracture-pressure windows has become increasingly challenging. Dual-gradient drilling (DGD) offers a promising solution through more closely matching the natural pressure gradient of the water depth.

Coupled with imprecise nature of pore pressure prediction and formation strength, deepwater operations are subject to well control issues and losses due to fracture propagation. It is not uncommon to experience both influx and lost returns whilst drilling due to the nature of geological uncertainty.

This paper examines simulation results specific to a ‘Controlled Mud Level’ (CML) system. This subclass of DGD enables operators to adapt bottomhole pressure (BHP) through adjusting annular mud height. The system relies on a subsea pump module (SPM) attached to the riser at a given height. Drilling fluid is diverted to the SPM and pumped via a return line instead of the riser. The inlet pressure required to maintain the desired annular mud height governs circulation rate.

During transient events, such as a connection, the annular height is raised thereby accounting for loss of annular frictional pressure (AFP) once circulation stops. Conversely, when the connection resumes, riser level decreases to compensate for the AFP gain encountered. Mud height is raised to increase BHP during subsequent kill operations.

This paper presents simulated transient behavior using commercial drilling software while concurrently highlighting the importance of understanding dynamic hydraulics in a low-price environment where deepwater wells show progressively diminishing operational margins.

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