Dual-Gradient Drilling in Ultradeepwater Gulf of Mexico
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166272, ’First Successful Commercial Application of Dual-Gradient Drilling in Ultradeepwater Gulf of Mexico,’ by Robert Ziegler, Mohd Saiful Anuar Sabri, and M Ramdan B Idris, Petronas, and Roar Malt and Roger Stave, AGR Enhanced Drilling Solutions, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-2 October. The paper has not been peer reviewed.

A previous attempt to drill an exploration well in ultradeep water in the Gulf of Mexico (GOM) did not reach its objective because of an inability to maintain a water-based-mud system light enough to maintain circulation. For the next round of exploration drilling, a controlled-annular-mud-level-type dual-gradient-drilling (DGD) system was applied successfully on this well in 2260-m water depth. This method compensates for the annular friction pressure by reducing the riser level according to the circulation pump rate.

Introduction

A specially designed drilling system was built for PC Gulf in 2011. This DGD system was to be used on Saipem’s Scarabeo-9 semisubmersible drilling rig. On the basis of experience gained with the previous offset well, Y-1, the system was considered a key tool to reach the planned total depth of Well C-1 by maintaining full mud returns should a narrow margin and a low-pressure scenario be encountered.

Given the challenging metocean conditions in the area of operation, another major advantage of this deepwater managed-pressure-drilling (MPD) system is that the surface components of the riser system are not modified, so the weather capability of the rig is not changed, which often is a problem with rotating-control-device (RCD) -based systems. Also, because of the fluid level in the riser remaining unaltered by rig movements, there is no masking of influxes or losses by heave.

Delivery of the DGD system to the Scarabeo-9 occurred in October of 2011, and installation and site-based commissioning took place while the Scarabeo-9 transited from Singapore to the GOM over a 2-month period. The system was first used on the J-1 well for another operator from mid-February to mid-May of 2012. Modifications as a result of lessons learned were implemented before spudding of Well C-1.

DGD System

This DGD system is designed for drilling post-blowout-preventer (BOP) sections. The system uses a subsea pump installed on a modified riser joint (MRJ) to manipulate the height of the drilling fluid in the riser annulus. By manipulating the fluid level in the riser, it is possible to alter the hydrostatic pressure seen by the wellbore, thus controlling the bottomhole pressure (BHP) while drilling.

The DGD system’s effect is best compared to that of a conventional, RCD (backpressure) -based MPD system: While the RCD system virtually extends the height of a light fluid column by applying backpressure, leading to a steep mud-pressure gradient, the DGD system reduces the actual height of a heavier fluid column, which allows for a significantly flatter mud-pressure gradient much closer to nature. This leads to a significantly lower pressure exerted on the often weak casing shoe while maintaining the required pressure to control moveable formation content deeper in the drilled interval.

For this specific operation, a three-stage subsea pump module (SPM) was installed on an MRJ to pump the mud returns back to the surface from a depth of 400 m. Fig. 1 presents a system overview.

The MRJ was run as a single joint (50 ft) through the rotary table (the other joints were run as 100-ft doubles) without assistance from the DGD-system crew. After preparations were made in the moonpool (creating space among line cables and sheaves), operations to make up the SPM to the MRJ took place. Fig. 2 illustrates the MRJ being interface tested with the SPM onboard the Scarabeo-9 during transit.