Preparing for pre-salt

Jerry Lee

October 1, 2016

Jerry Lee speaks with Seadrill to discover how the firm prepared for drilling operations on the giant Libra field offshore Brazil.

RGH (riser gas handling system) riser joint in the moonpool during running. Photos from Seadrill.

Drilling in pre-salt geology presents challenges on which Brazilian operator Petrobras has spent years working. When selecting the approach to the giant Libra field, offshore Brazil, the operator chose sister drillships from Seadrill, to assist in unlocking the field’s potential.

The Libra field, 183km south of Rio de Janeiro, covers 1547sq km in the ultra-deepwaters of the Santos Basin pre-salt. Discovered in May 2010, in about 2000m water depth, the field contains an estimated 8-12 billion boe of reserves in pre-salt limestone coquina reservoirs. Though the prize is large, getting to it is a challenge.

Before the salt layer, the drilling challenges are the very narrow drilling margins resulting from the difference in the fracture- and pore-pressure, says Jay Lancaster, managed pressure drilling (MPD) and well control technical superintendent, Seadrill. Due to this narrow window, applying too much bottomhole pressure (BHP) can cause fractures and losses of expensive drilling fluid; applying too little BHP can cause influx or even kicks, which may lead to blowouts.

Following the post-salt section, the well will have to pass through a thick salt layer, which can be a problem because it creeps, so drilling and casing must be done quickly. After the salt layer, the challenges of drilling in the pre-salt section become two-fold: large vugular carbonate sections, which are known to be loss zones, and greater pressure uncertainties.

With these challenges in mind, the Libra consortium – Petrobras (40% interest, operator), Shell (20%), Total (20%), CNPC (10%) and CNOOC (10%) – offered drillship tenders in 2014 and awarded two, two-year contracts, with the option for two-year extensions, to Seadrill’s West Tellus and West Carina ultra-deepwater drillships.

The West Tellus and West Carina are sixth generation DP3 drillships built by Samsung Heavy Industries in 2013 and 2014, respectively. Both are capable of operating in 12,000ft water depth. But, what sets these vessels apart are that the MPD systems are fully integrated as part of the vessels’ drilling systems. The equipment offers greater efficiencies, technically and financially, than temporary MPD kits, and was the main driver behind Petrobras awarding the contracts, says David Gouldin, drilling and well control manager, Seadrill. With their integrated MPD systems, the West Tellus and West Carina, will be able to convert between the two MPD variations essential to drilling the field: constant bottomhole pressure (CBHP) and pressurized mudcap drilling (PMCD).

MPD can help address both the post-salt and pre-salt challenges that Libra faces. In the post-salt section, CBHP can prevent influxes and losses while navigating through the narrow drilling windows. The MPD system compensates for the loss of frictional pressure when pumps are turned off while drillpipe connections are being made. The MPD choke can be adjusted to decrease or increase BHP, to stay within the drilling window. In the pre-salt section, the vessels can switch to PMCD to address loss zones. With PMCD, sacrificial fluid (e.g. sea water) is pumped down the drillpipe to fill the pores of the loss zone, while mud is simultaneously pumped into the annulus to keep it full – to maintain communication with the bottom of the well and the necessary hydrostatic pressure – allowing drilling operations to continue.

Seadrill’s West Tellus drillship, which was built by Samsung Heavy Industries in 2013.

To enable these capabilities on the West Tellus and West Carina, Seadrill contracted Weatherford to supply the equipment needed for the MPD systems, particularly an MPD riser joint, which is comprised of a rotating control device, an annular isolation device and a MPD flow spool, as well as the associated MPD ancillary equipment and piping needed to be able to perform MPD operations. Seadrill also invested more than 5000 man-hours into training the crew.

“Everyone had some form of training on the rig so there was full understanding of what we were doing,” Gouldin says. “At the time, it seemed a bit over the top, but understanding the campaign we were going to start gave a lot of confidence to the crews.”

With the MPD equipment fully integrated in Q2/Q3 2015, the West Carina began drilling its first well in August 2015, Lancaster says. The well, 3-RJS-739A, is south of the Libra discovery well, 2-ANP-2A-RJS, in the northwest area of the Libra block. While the newbuild had only recently had the equipment integrated into its system, it was immediately put to use due to the narrow drilling margins at that well. Pre-drill estimates put the pore pressure at about 10ppg, however, when the mud weight was increased to an equivalent circulating density of 10.2ppg, losses were seen, so the crew knew they were dealing with about a 0.2ppg window. As a result, the equivalent mud weight was reduced to 10.16ppg which reduced losses and allowed drilling to continue, Lancaster says.

Petrobras announced in a February 2016 that the well intersected ca. 270m column of 28° API oil. The reservoir had excellent productivity and was in communication with other wells in the area.

The West Tellus began drilling its first MPD well in September 2015, Lancaster says. Also located in the northwest area of the Libra block, the well, 3-RJS-741, is 8km northeast of the discovery well. While drilling the well, high loss conditions came on unexpectedly, and light annular mud was not available on the field. With the high losses, the well would have been a prime candidate for PMCD, Lancaster says.

“Though 30,000bbl of mud were lost, the well had good show and is expected to be put online,” Lancaster says. “Without the MPD system that well would not have been drillable.”

In March 2016, Petrobras announced that the well encountered a 301m column, the biggest discovered on the field at that time, of 28° API oil and excellent productivity.

Together, the vessels have completed operations on four wells on the field, with and without MPD.

Since the Libra field was discovered in 2010, seven wells have been drilled with the seventh well, appraisal well 3-RJS-742A (NW2), completed in June 2016, Petrobras announced that same month. NW2 is currently the largest discovery on the field to date, with 410m of net pay, showing 27° API oil. Petrobras also announced in June that the eighth appraisal well, 3-RJS-743A, is being drilled in the northwest region of the block.