Deepwater Well-Control Aspects for Slender-Well Drilling
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 94852, "Well-Control Aspects Regarding Slender-Well Drilling With Surface and Subsea BOPs," by C.S. Avelar, SPE, and S. Ohara, SPE, Shell Brasil; O.L.A. Santos, SPE, Petrobras; and P.R. Ribeiro, SPE, Unicamp, prepared for the 2005 SPE Latin American and Caribbean Petroleum Engineering Conference, Rio de Janeiro, 20-23 June.
The 2003 Brazilian deepwater drilling record was achieved with a slender well in 9,475-ft water depth in Campos basin. The well was drilled safely with a surface-blowout-preventer (SBOP) system. The full-length paper details two discussions. The first is on technical issues concerning well-control operations on the drilled well and on a hypothetical conventional well. The second is on aspects of the SBOP technology. The main objective was to broaden discussion regarding application of the SBOP concept to explore and develop deepwater and ultradeepwater reserves in Brazil, with emphasis on well control.

Introduction
A combination of SBOP and a dynamically positioned (DP) semisubmersible drilling rig was used. The SBOP system included a surface-mounted BOP, a 13 3/8-in. high-pressure casing riser, a seabed shutoff and disconnect system (SDS), and control systems for these components. This SBOP system may be used with older-generation drilling rigs in ultradeep waters. Associated benefits can include reduced environmental damage because of the smaller volumes of drilling fluid and additives, compared with that used with a subsea-BOP system. The most important part of this work presents a comparison of a gas-kick circulation using the driller's method between the 9,475-ft-water-depth well drilled with an SBOP and a DP vessel and a hypothetical well drilled under the same conditions, but with the use of a subsea BOP.

Well Scenario
Fig. 1 shows the wellbore configurations for the two scenarios investigated in this paper. The system on the left shows a conventional situation having a 300-ton, 18 3/4-in. subsea BOP on the seabed with a 21-in. marine riser attached on top of it. The system on the right uses a 40-ton SBOP on top of a 13 3/8-in. high-pressure casing riser. The bottom of the casing riser is connected to the SDS at the seabed; the latter is used in case of an emergency disconnection. Below the mudline, both wells have the same design.

Well-Control Simulation
The well-control simulations were conducted with existing software that handles kick circulation by use of the driller's method. The theoretical basis involves a slug-flow model for the two-phase-flow region and a Bingham plastic model for the water-based mud. The bottomhole pressure was assumed constant and equal to the value of the formation pressure that generated the kick downhole, with a 50-psi safety margin added. Additional drilling parameters used in the simulations were as follows.Kick volumes: 5, 10, and 15 bbl.Shut-in drillpipe pressure: 650 psi.Flow rate: 450 gal/min.Shut-in time: 3 minutes.Reduced circulation rate: 100 gal/min.Surface temperature: 68°F.Temperature at mudline: 41°F.Bottomhole temperature: 234°F.Mud density: 10 lbm/gal.Mud rheology: shear stress=10 lbf/100 ft and viscosity=20 cp.