PetroChina’s Tarim Oilfield Company (TOC) operates the Tazhong field, which is located in the central part of the Taklimakan Desert in the Tarim basin of Xinjiang, China. The Tazhong Field is a marginally over-pressured, fractured limestone reservoir that produces sour oil and gas from depths ranging between 4000 to 6500 m.

Conventionally drilled wells in this field frequently experience severe lost circulation, which leads to significant reservoir damage due to the drilling fluid, and often, suspension of drilling operations due to the loss of measurement/logging-while-drilling (M/LWD) signals and directional drilling control. Additionally, frequent gas-kick events create high-risk well control situations, and often, they require drilling operations to be terminated earlier than planned. Additionally, the large amount of non-productive time (NPT) and associated costs combined with the incapability to deliver the proposed laterals negatively impacts the economics of the project and recovery of reserves.

Recently, a new managed pressure drilling (MPD) system designed to resolve the drilling challenges of critical pressure environments was applied in the Tazhong Field. The technology provides the capability to maintain a constant bottomhole pressure (BHP) throughout all of the drilling stages, and enables rapid, automatic BHP adjustments according to well conditions. The advantages reduce the risk of combined losses and kick situations while drilling through narrow pressure-margin zones. The application of this new MPD technique in the Tazhong field has had an immediate impact on drilling operations, and its successful implementation in this field will be described in this paper.

The automated MPD System deployed in this project included a rotating control device and an autochoke system, a backpressure pump (BPP) to maintain back pressure and constant bottomhole pressure (BHP) while rig pumps are turned off, a pressure-while-drilling (PWD) sensor for real-time downhole pressure and temperature readings, and non-return valves (NRV) in the bottomhole assembly (BHA) to prevent backflow during MPD operations using a statically underbalanced mud system.

This paper will discuss the challenges, drivers for MPD, pre-well engineering, equipment, execution, and best practices derived from the implementation of this new closed-loop, pressure-optimization MPD system.

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