Targeting deepwater wells

Jon Lawson

Deepwater drilling is demanding, technically, financially and in human terms. Drilling and completion now account for around 50% of deepwater field development costs.

As oil and gas production move into deeper waters the technical challenges and risks increase. When drilling through water depths of 5,000 feet and beyond, it is not unusual to have as many as nine casing strings running through each other so the tolerances between casings, and between the centralizer and casings, are incredibly fine.

Centek’s UROS-CT and Centraliser Sub are specifically designed for use in deep water with tight tolerance casings, where the centralizer must compress completely to travel through a series of tight casing strings but still be capable of fully expanding to the designed open-hole size.

Once in position these centralisers must provide optimum stand-off and fluid displacement to improve well clean-out. The low friction coefficient of Centek centralisers allows casing to be reciprocated and rotated, improving mud removal and cement displacement, contributing to a good cement bond and long term zonal isolation.

A Norwegian customer in the Ekofisk field needed a centraliser system for a particular close tolerance operation in a deepwater well with a total vertical depth of 9,778 RKB ft and a measured depth of 16,614 RKB ft. (Rotary Kelly Bushing is a reference point for measuring well depth).

The centralisers were required to provide reduced insertion and low running forces in the restricted section of the well which was assessed at the outset using Centek’s Latload modelling software. In addition correct standoff and a high axial stop collar load capability was needed to meet to meet close tolerance requirements – and all this within a tight budget. For this particular well the drill team needed to run a 17-in liner through a wear bushing restriction with an internal diameter of 17.75-in to 3,295 TVD. Close tolerance applications of this type usually require an expensive centraliser sub.

Centek and Halliburton recommended using the Centek UROS-CT centraliser and a new type of stop collar from Ace Oil Tools. The UROS-CT close tolerance centraliser is specifically designed for use in tight annulus applications, where the margins between pore pressure and the fracture gradient are narrow. In deepwater operations the UROS-CT can compress virtually flat to allow travel through a series of tight casing strings but once in the open hole it is still capable of expanding to the designed hole size.

In this instance, the UROS-CT was used with the Ace Ratchet Collar because it offers high axial load capability along with a slim design. The stop collar is in two parts which when pushed together are locked in place with a ratchet mechanism. The collar is simple and quick to install and gave an axial holding force of 103,000 lbs in this particular design and size. The combination of the UROS-CT centralizer and Ace Ratchet Collar met the customer’s requirements and the 17-in liner passed safely through the tight restriction without problems.

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