Abstract

Wells in challenging environments are known for their high cost with targets not reached, and often abandoned. HPHT wells fall in this category and in Lower Mediterranean Sea this was not an exception. After a few wells drilled in an offshore HPHT area, with advanced techniques and alternatives being tried on well after well, Eni through its foreign affiliate decided to use a combination of Eni's continuous circulation (CC) valves, (Eni Circulation Device - E-CD) and the Micro-Flux Control (MFC) method. Eni calls the combination of CC with the precise measurement and control of the flow and pressure from the well, within a closed loop through a rotating control device (RCD), as: Eni-Near-Balanced Drilling (ENBD). With this new drilling method, a well in the HPHT area reached all the targets for the first time.

Wells in this area face the usual kick-loss scenario, with significant non productive time (NPT) and adding hundreds of days to drill a well. The goal for using the E-CD was to avoid pressure and temperature fluctuation while drilling, by maintaining the annulus as stable as possible, and the MFC would provide a clear picture and manage the bottomhole pressure (BHP) within the downhole pressure limits, pore and fracture. Previous attempts to improve drilling were made using just RCD, then a continuous circulation system and later on the E-CD valves were used together with the RCD on other wells, but, even though improvements were seen, those were not enough to ensure reaching the final targets. It was only when the MFC method was added that success was achieved.

The paper describes the operations and results obtained by the ENBD system and its benefits in terms of NPT and kick quantity and size reduction. Comparison with the previous wells where the targets could not be reached clearly shows the significant advantage of using the system, compared to conventional drilling. Reduction in risk and increased control over the entire drilling process, in an environment with extremely narrow margins, were paramount to a successful outcome.

Introduction

Exploratory wells are in principle classified as challenging ones due to the inherent uncertainty relative to the downhole pressure limits, pore and fracture pressures, as well as formation surprises that are not clearly mapped from seismic. In addition the exploration frontier is moving increasingly towards more difficult environments, and it is no surprise that many exploratory wells have been abandoned before reaching the planned TD, therefore failing to meet its commercial or technical objectives of collecting valuable data about possible reservoirs and hydrocarbon reserves. Despite many improvements to drilling systems, such as the top drive, steerable directional systems, mud motors, Pressure and Logging While Drilling (PWD and LWD), Continuous Circulation Devices (CC), Rotating Control Devices (RCD), it has been the arrival of a closed-loop Managed Pressure Drilling (MPD) method that has provided a significant step change in drilling performance. For the first time the pore and fracture pressures can be determined very accurately and safely while drilling, in real-time, and as a consequence the mud weight can be properly adjusted and managed, avoiding loss circulations, stuck pipe, and low ROP. The industry loses an enormous amount of time trying to solve these problems, which very often leads to misunderstanding that the well's technical limit has been reached and abandoned unnecessarily.

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