Drilling Underbalanced From a Drillship in the Santos Basin
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 17459, "Drilling Underbalanced From a Floating Unit in a 1,500-m of Water Depth Exploratory Well: Planning, Equipment and Rig Modifications," by A.C.V.M. Lage, E.F. Nogueira, G.S. Vanni, L.H.S. Vitullo, and and E.T.M. Fartes, Petrobras, prepared for the 2005 Offshore Technology Conference, Houston, 2–5 May.
The full-length paper describes steps taken to drill the final section of an exploratory well in 1500 m of water in the Santos basin. The first well in the area reached a fractured carbonate reservoir where massive circulation losses caused the well to be abandoned. The full-length paper discusses planning activities, additional equipment, riser modifications, and drilling procedures for a new well to be drilled underbalanced from a drillship to access the target reservoir.

Background
Despite its use for many years, under-balanced drilling (UBD) is challenging to implement from floating units. Although thousands of wells worldwide have been drilled underbalanced, UBD is used almost exclusively onshore. The literature reports some UBD from fixed platforms or jackup rigs, but operational difficulties associated with safety, logistics, and equipment have limited UBD from floating rigs.
At the end of 2000, Petrobras, along with other operators, drilled a well with aerated mud in the Albacore field in 974 ft of water from a semisubmersible, the Petrobras 17. Although not underbalanced, a two-phase mixture composed of nitrogen and water-based mud (WBM) was pumped down the drillstring and returned to surface under pressure. Returns were taken to a typical UBD surface system.
After this milestone, Petrobras began development of technology for dual-gradient drilling (DGD). In DGD, gas is used to dilute the mud returns from the seafloor. From the point of gas injection to the surface, the drilling-fluid density is less than the effective mud density below the seafloor. In ultradeep water, DGD provides an effective approach to manage challenges associated with the narrow operational margin between pore pressure and formation fracture pressure. Fig. 1 shows the effective-circulating-density (ECD) vs. true-vertical-depth (TVD) profile for dual- and single-gradient drilling. Despite the advantages of DGD, most equipment currently available on floating rigs has not been designed to handle the returning liquid and gas flow.
Drilling Experience. First Well. In 2003, an offshore exploratory well, targeting a carbon-ate reservoir, was drilled vertically in the Santos basin. The well was a slender design using 36-in. followed by 13 3/8-in. casing. After cementing the 13 3/8-in. casing, the 12 1/4-in. hole section was drilled, and the 9 5/8-in. casing was set at 4940 m, just above the zone of interest. The final section was to be drilled with 8 1/2-in. bits through the carbonate rocks from 4940 to almost 5900 m. A kick occurred while drilling the upper part of the carbonates. After controlling the well and increasing mud weight, the drilling operation resumed until the lower part of the carbonates was reached. The well had to be abandoned because of severe lost circulation.