The objective of this paper is to share the results obtained from in-depth analysis using a down hole mechanics and dynamics monitoring sub, which lead to the first successful Gullfaks Satellite 14 ¾ x 17 ½ -in Rathole elimination operation through challenging lithology.
Shallow gas is encountered in the Gullfaks South area at a true vertical depth of between 335-339m (from MSL), and a casing design incorporating a 16-in liner is utilized to isolate this interval. A 14 ¾ x 17 ½ -in section is then drilled to isolate weak and unstable formations, and in all previous wells this section was drilled with a two run strategy. The first run to drill to section target depth (TD) while hole opening, followed by a dedicated second run to open the rathole. An opportunity was identified to improve well construction performance by eliminating the dedicated run to open the rat-hole, and tailoring the bit design to overcome the challenging lithology and high levels of shock and vibration seen on offset wells.
Detailed pre-job planning and BHA design analysis was combined with new downhole technologies to overcome these challenges. Dual reamers were used within a Rotary Steerable System (RSS) BHA to drill the 14 ¾ x 17 ½ -in section and eliminate the rat-hole in one run. This innovative approach involved drilling to section target depth (TD) using an upper ball drop reamer, tool positioned 45 meters behind the bit for hole enlargement while drilling, and then pulling back to position the bit at rat-hole shoulder. At this point the upper reamer was de-activated and a lower on demand hydraulically activated reamer, mounted directly above the RSS was opened to eliminate the rat-hole. Due to excessive shocks & vibrations experienced on offset wells, a down hole drilling mechanics & dynamics sub was utilized to provide real-time information about downhole forces and BHA motions, combined with a tailored bit design to control depth of cut in sandy intervals. This unprecedented approach resulted in 2.5 days saving.
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