Through-Tubing Rotary Drilling From Njord Floating Platform

Abstract

Summary The densely-faulted Njord reservoir in the Norwegian North Sea is considered one of the most complex reservoirs in the world. The field is developed from a semi-submersible platform with 15 subsea-completed wells drilled in a pre-drilling campaign in 1996 to 1997 and two major platform drilling campaigns, one in 1997 through 2000 and the other in 2002 to 2003. Drilling of two conventional sidetracked oil producers in the last campaign was challenging and costly. As the field matures, the need for a cheaper way of drilling sparsely located smaller undrained compartments became essential. This led to initiate an ambitious campaign called the low-cost infill targets (LIFT) for identifying and drilling those targets using a cheaper drilling technique called the through tubing rotary drilling (TTRD). TTRD is a hugely demanding task especially, from a floating platform as any economic rationale will be lost if completion accessories and well integrity are compromised through TTRD. To the best of our knowledge, no TTRD operations have previously been executed from a floater. The severity of depletion, especially with depletion and re-pressurization (Huff'n Puff) of parts of the reservoir provides a significant technical test and challenge for TTRD on Njord. The relative movement of the floater also presents extra operational challenges, which requires accurate measures to prevent damage to the tubing hanger, Christmas tree (XMT), downhole-safety valve, and existing completion string. Issues related to bottomhole assembly design to meet drilling and production needs, mud rheology, equivalent circulating density (ECD) management, rock mechanics, and completion techniques are critically analyzed and risk-reducing or eliminating measures are put in place through extensive research and development for each of the prospective targets. This paper is intended to give a comprehensive description on the technological challenges of the TTRD technology from a floating platform, research and development activities to qualify the technology on Njord, screening of drilling targets and the drilling experiences from two TTRD wells on Njord.