Two years ago, as drillers approached the bottom of a deepwater well in the Lula field, they were stopped again and again by total drilling fluid losses. After a series of frustrating efforts to finish the well, drilling had proceeded only 27 m and it was obvious the hole needed to be plugged and temporarily abandoned.
It was one of a worrisome subset of wells in the Brazil’s rich subsalt trend plagued by high costs, delays, and safety concerns. Conventional drilling methods could not navigate the tight pressure limits in these rich but difficult formations. Rather than limiting drilling in locations with reservoirs that get thousands of feet thick, Brazil’s national oil company Petrobras began looking for a better way to deal with this significant minority of troublesome wells, said Jose Umberto Arnaud Borges, offshore well project manager of exploratory well construction at Petrobras.
The company concluded that managed-pressure drilling would be a key technology. It was needed to drill in formations demanding extremely tight pressure control. In some, the safe drilling margin was so narrow that the effective mud weight would need to remain within 0.4 lb/gal, he said. Pressure management in the pre-salt is complicated by the added weight of the fluid in a tall riser, often more than 6,500 ft long. Adding to the hazards are fractures and cavernous features, such as karsts, that could lead to severe fluid losses if the pressure moved outside that narrow window.
“Both geological environments—narrow drilling margins in HP/HT (high-pressure/high-temperature) wells and severe or total lost circulation—require the use of managed-pressure drilling for mud-cap drilling and safe drilling of these intervals,” said Humberto de Oliveira Maia Neto, who headed this effort as the leader of the Offshore Exploration Drilling Group at Petrobras.
Applying managed-pressuring drilling required adapting a method commonly used on land, and sometimes offshore, but never in water so deep. “We found that there was nothing developed for floating rigs,” Borges said. That led to a couple years of work to adopt the available tools to floating rigs, where the motion of the sea complicates the task of measuring and controlling fluid flow. Petrobras had to develop new procedures for deepwater managed-pressure drilling and train its crews.
Petrobras now has two drilling rigs working offshore Brazil equipped to do managed-pressure drilling, and plans to expand this total to 12 by the end of next year and to 16 by 2016. So far, managed-pressure drilling has been used on seven wells, including the one left unfinished in the Lula field.
“We re-entered the same well using managed-pressure drilling and were able to drill 89m with minor (fluid) losses. Then we started to get full losses, with no circulation at all,” Borges said. At that point, the crew turned to another form of managed-pressure drilling: pressurized mud-cap drilling.
This approach, which was developed on land and used in shallower water, reverses the normal order for fluid flow in a well. Fluid with a heavy mud weight is pumped down the riser annular into the well and held in place by the closed choke on the managed-pressure system. The mud creates a barrier to control the well while drilling continues using a lighter fluid, commonly seawater. The water, cuttings, and some of the mud flow into the openings in the formation. “We drilled another 70 m and were able to cover the complete reservoir zone,” Borges said. “It was a very successful operation.”
While Petrobras’ initial focus is on dealing with wells in spots likely to cause trouble, its expansion plans see managed pressure as a money saver in wells that could be drilled conventionally but can be done better and more efficiently using managed-pressure drilling.
The company projects the total average cost of drilling a well using a drilling rig equipped for managed-pressure drilling is USD 19 million, but it expects that will reduce other expenses by USD 25 million, for a net saving averaging USD 6 million per well, Borges said.
Savings come in the form of reduced time lost to deal with pressure management problems, such as lost circulation, savings on drilling mud, and avoiding well-design contingencies, such as adding casing strings to deal with unexpectedly difficult well sections.
Expanding its managed-pressure fleet will require installations costing about USD 17 million per vessel as well as training crews how to drill differently. “They are used to doing it a certain way for decades and we are saying, ‘OK, now learn how to do this a different way,’” Borges said, adding, “We are getting more and more confident with our methodology and our people.”
