Continuous-Circulation Technique Drills Narrow-Margin Deepwater Wells
Authors: Carpenter Chris 
Published: February 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 192749, “Deepwater High-Pressure/High-Temperature Drilling Through an Ultranarrow Pore-Pressure Fracture Gradient Window: A Case Study,” by Enrico Squintani, Andrea Uslenghi, Susanna Ferrari, and Luca Affede, Eni, prepared for the 2018 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 12–15 November. The paper has not been peer reviewed.

Ultradeepwater wells are commonly characterized by a narrow margin between pore and fracture gradients. In these wells, even a small variation in pressure profile may lead to severe operational issues. This paper describes the drilling of high-pressure/high-temperature (HP/HT) deepwater wells through an ultranarrow pore-pressure fracture gradient (PPFG) window by means of technology application and strict procedural control.

Introduction

An exploratory well with a narrow drilling margin can be realized successfully by use of a customized design concept. Such a concept may not follow standard procedures strictly as ordinarily defined but still can fulfill the guidelines and concepts behind conventional approaches.

To apply such a customized concept, a multidisciplinary team was created to design the well through all the possible expected scenarios. An integrated assessment was prepared, evaluating how real-time monitoring could mitigate the operating risk of the exploration activity.

The operator’s guidelines used for standard parameters such as kick tolerance and choke margin were reviewed and re-interpreted for this specific application, because the introduction of new technologies could allow different design factors to control the bottomhole pressure (BHP) without affecting the margin of risk.

Well A

Planning. This was the first exploration well drilled in a Mediterranean block with the use of a dynamically positioned rig with continuous-circulation technology installed. The estimated water depth of the area was greater than 2000 m. The distance of the nearest reference well from Well A was more than 90 km. Because of this long distance, the degree of uncertainty of the predicted PPFG was very high. The well was planned to explore sands targets at a total depth (TD) of approximately 6000 m.

From a well-design viewpoint, attention was paid mainly to identify the casing points; otherwise, because the narrow margin between pore and fracture gradients was so tight, the targets could require reaching the TD in 6-in. hole instead of the planned 8½-in. hole. This development could compromise data collection.

Because of these factors, it was decided to use the operator’s proprietary device and continuous-circulation system to add an additional safety margin to the geopressure fluctuation. Continuous circulation was planned to be used from the drilling of the intermediate hole size until the well reached TD and also while running the casing/liner. This project saw the first installation of the system on a drilling ship. Uncertainties were expected to be managed primarily with standard operational precautions; in fact, the ability to place the casing shoes inside the right formation was very important and the correct depth had to be determined while drilling the well.

The salt was supposed to be drilled with a 17½-in. bottomhole assembly. The casing point of this section was planned at approximately 4000 m measured depth, approximately 100 m above the bottom of the theoretical evaporitic sequence but before entering the possible overpressured zone.