Gullfaks Satellite First Successful Rat Hole Elimination Design in Tight Anti Collision Scenario
Authors: Khalil Haitham , Blackburn Jason , Beeh Hany A. , Skofteby Geir , Kallestad Richard 
Published: August 2016

Abstract
The main objective of this paper is to share the experience of the first Dual reamer bottom hole assembly (BHA) design implemented off-shore Norway, Gullfaks field, 17 ½ × 20-inch section. It presents the drilling challenges, innovative bottom hole assembly and the first world wide application for the Electro-magnetic receiver sub fully integrated with rotary steerable system (RSS)
Hole enlargement while drilling (HEWD) became a well-known application, and they are widely used to support several well intervention objectives like; i) Accommodating un-common casing design. ii) Reduce operational risk such as high equivalent circulating density (ECD). iii) Optimized casing and completion programs. There are two main types on hole enlargement tools, based on activation mechanism: ball-drop using a ball to Activate/De-activate the reamer, and hydraulic on demand triggered by changing flow rate on a predefined specific range, so called ‘Indexing’ for Activation/De-activation of the reamer. Both carrying a common implicit risks and limitations, where reamers have to be positioned above logging while drilling tools (LWD) so that the enlarged hole does not impair the quality of the formation evaluation measurements, or compromise the bottom hole assembly stabilization. This results in a rat-hole of 40-50 meters at the section target depth (TD), consequently challenges the casing running, casing cementing job, and drilling next sections with potential risk of cement pack-off around bottom hole assembly. Today in the industry, these challenges are usually addressed by an extra dedicated run for opening the rat-hole.
Collaborative efforts between operator in the North-sea and a Service Company to address the risk and limitations associated to the hole enlargement while drilling design. Dual Reamer System developed to reduce the rat-hole length to minimum instead of 40-50 meters, and to eliminate the extra dedicated run for opeing the rat-hole. The innovative approach planned to drill to section target depth (TD) using upper ball drop reamer, tool positioned 45 meters behind the bit for Hole Enlargement While Drilling, then pull back to position the bit at rat hole shoulder, de-activate upper reamer (ball drop system), and activate the lower hydraulic on demand reamer to eliminate the rat-hole. A Gyro while drilling integrated into the BHA along with 9-in world first electro-magnetic receiver sub mounted on the top of the hydraulic on demand reamer. Providing a full integration, and securing a real time communication with the RSS in a critical and challenging Anti-collision situation.
The unprecedented approach successfully implemented on Gullfaks field, 17 ½ × 20-inch section drilled to target depth (TD) in one run, with all objectives met on directional control in tight Anti-collision scenario, and measurements and logging while drilling.
The Dual reamer BHA along with the Electro-magnetic receiver sub proven efficient steering capability and reliability, which led to significant improvement in the drilling, casing running and cementing operations