Ursa TLP Well Systems
Authors: Carminati J.R. , Eaton L.F. , Folse K.A. , Sokoll R.E. 
Published: October 2008

The original Ursa TLP development plan called for 16 production wells, 11 firm and 5 contingent. Initial design was for 11 TLP wells and 3 subsea wells. The plan also included batch setting 21 well positions and pre-drilling 6 of the TLP wells in 3,950' of water in MC block 810. In late 1997 a significant shallow flow occurred at the batch set site during the final batch operations. Events proceeded such that all wells were lost and the site became unusable for the foundation support. Development plans were modified to move the installation site to an area with still significant but smaller shallow flow risk in MC block 809, in 3,800' of water. The batch set was reduced to only 12 TLP production wells. Drilling was to proceed with 1 to 2 pre-drills. Subsea wells were deleted from the plan. This paper discusses the Ursa well systems including early drilling, final batch set and pre-drills, the TLP rig, riser systems and completion systems. It presents how these areas approached shallow flow and the other challenges to produce the Ursa reservoirs.

Introduction

The Ursa field was discovered in 1990 in 4,023' of water in Mississippi Canyon (MC) block 854. The expected gross reserves are estimated to be 431 MMBOE in five reservoirs (Below Pink, Above Magenta, Lime Green, Yellow and Aqua/TerraCotta) in MC blocks 809, 810, 853 and 854. Depths of the objective sands range from 12,000' TVD to 19,000 TVD. Approximately 80% of the reserves are in the Yellow and Aqua/Terra Cotta reservoirs. Ursa is an oil field with significant gas volumes.

Following the appraisal drilling program, it was decided that Ursa development would utilize a 24 well slot TLP located in MC block 810. The initial design called for 11 TLP wells and 3 subsea wells. The original development plan included 21 batch set wells and 6 pre-drills on the TLP. The initial site location in MC block 810 was lost due to shallow flow problems during batch drilling. At a new site in MC block 809, the number of planned batch set wells was reduced to from 21 to 12, the number of predrilled wells was reduced from 6 to 1 or 2, and subsea wells were deleted. A total of 11 batch wells were successfully drilled with casing set below the base of the shallow flow sands. The reserves associated with the other 3 originally planned wells will be developed with TLP well sidetracks or future subsea wells.

The rig on the TLP will complete the predrilled wells and proceed to drill and complete the rest of the wells. The original rig rating was for 25,000' measured depth (MD) and 12,000' step-out. The MC block 809 site requires a rig rating for over 28,000' MD and 17,800' step-out at 19,000' TVD. The rig capability was evaluated and slightly modified to meet the new requirements.

The production riser system includes equipment from the connector which attaches to the subsea wellhead up to the surface tree and choke. Ursa uses a dual pipe riser system which accommodates 10,000 psi and 15,000 psi pressure ratings. The 10,000 psi system is 13-5/8" by 10-3/4". The 15,000 psi system is 14" by 10-7/8". The decision to complete the wells with 5-1/2" tubing drove the size of the dual riser system. The large size, high pressure and water depth pushed the riser loads to very high values.

The completions on Ursa are designed to deliver high rate and achieve high ultimate recoveries. Multiple techniques will be considered, including: internal gravel pack, frac pac or high rate water pack and possible horizontal completions. The system must accommodate 15,000 psi and 10,000 psi pressure ratings and 5-1/2 and 4-1/2" full bore tubing sizes. The completion type will be determined on each well as drilling is completed and well logs are evaluated.