Shallow oil reservoirs, commonly associated with heavy oil, require massive completion programs. The reason for these huge completion programs is due to the need for larger hole sizes compared with standard borehole developments. The commercial feasibility of these developments relies on repetitive and predictable drilling performance in large hole sizes and high-angle curvatures to reach very soft formations. One such development is the Mariner field in the UK sector of the North Sea, which was discovered 30 years ago. This field, located 140 km east of the Shetland Islands in 110m water depths, is the largest new field development in the UKCS. Two targeted reservoirs contain heavy oil in shallow unconsolidated sands in the Maureen and Heimdal formations. Field objectives included the following: 1. 100+ well targets at the rate of ± 12 wells per year 2. Timely well delivery schedule 3. Technology application to lower Drilling &Well operations sanctioned cost by at least 25% The construction program for the Mariner field required the 20-in. casing shoe to be set at a shallow true vertical depth subsea (TVDSS) of ±750m, at inclinations between 35 and 55°, and build rates of 3 to 5°/30m in the ultra-soft formations with an unconfined compressive strength (UCS) of < 1000 psi. Although the advancements made in rotary steerable system (RSS) technologies are capable of generating 15 to 18°/30m dogleg severity (DLS) output, these technologies have historically been confined to small hole sizes extending from 57/8 in. to 91/2 in. This paper describes the development of the world’s first RSS bottom hole assembly (BHA) design, capable of delivering repetitive and predictable performance to drill large hole sections to high inclinations in shallow, ultra-soft formations.
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