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November 2008

Vol. 13, No. 46 Week of November 16, 2008

40 Years at Prudhoe Bay: Mid-life transition for a giant

40 years after discovery the Prudhoe Bay field continues to produce oil while looking forward to future natural gas sales

Alan Bailey

Petroleum News

At a time when daily oil production from the giant Prudhoe Bay on Alaska’s North Slope has declined from a peak of about 1.6 million barrels in 1987 to about 400,000 barrels, it is tempting to think of the field as being in terminal decline.

Not so, BP’s Greater Prudhoe Bay Waterflood Resource Manager Scott Digert told the 2008 Alaska Geological Society Technical Conference on April 17, 2008. The AGS conference was celebrating the 40th anniversary of the discovery of Prudhoe Bay.

“This is a very exciting time to be working on Prudhoe,” Digert said.

Digert also reminded the audience that, despite major discoveries in places like the Gulf of Mexico, Prudhoe Bay remains the top producing oil field in the United States.

And the field has exceeded all expectations for oil production — originally, estimated recoverable oil reserves were 9.6 billion barrels.

“As we stand today we’re at 11.7 (billion barrels) and counting,” Digert said.

Moreover, Prudhoe Bay and the associated trans-Alaska oil pipeline have proved to be the anchor for about 25 producing North Slope fields.

“If it weren’t for Prudhoe and TAPS none of those fields would be in production either,” Digert said.

Ivishak reservoir

The Prudhoe Bay reservoir occupies an area of about 254 square miles. The main section of the reservoir lies in the Triassic Ivishak formation, consisting of sandstone and coarse pebbly rocks called conglomerates, laid down by an ancient river and river delta system, Digert said. In fact, information about the subsurface rocks obtained from the 2,500 or so well penetrations in the reservoir has enabled a detailed reconstruction of the geography of the ancient landscape.

The Ivishak is a massive 450-foot thick rock unit. And a separate 35-foot shallow marine sandstone layer known as the Sag River formation creates a second, much smaller reservoir above the main reservoir. The reservoir strata have been bent into an arch-shaped anticline, cut off at the top by a major geologic discontinuity termed the lower Cretaceous unconformity — the oil and gas lie trapped against that unconformity.

To plan field development, petroleum engineers subdivide the Ivishak reservoir into four stratified zones, Digert explained. Differences in the grain sizes between the rocks in the different zones cause major differences in the permeability, or ability to flow fluids, within the zones. Zone 3, for example, consists of massively coarse conglomerates formed when the ancient delta was at its maximum extent. The conglomerates allow fluids to flow particularly easily — Digert recalled seeing 50,000 barrels of water per day being injected into just one foot of these rocks.

Layers of impervious shales are interstratified with the sandstones and conglomerates. The shales inhibit the vertical movement of fluids within the reservoir, but can be used to support the transmission of pressure across the reservoir to enhance oil recovery, Digert said.

Gas cap and oil rim

When discovered 40 years ago, the Prudhoe Bay field contained a huge gas cap sitting in the reservoir on top of an equally huge oil rim. A thin tarry zone of heavy, impermeable oil marked the base of the oil rim and separated the oil from underlying water, Digert said.

At field startup in 1977, pressure in the gas cap forced oil up through the producing oil wells. Gas produced through the wells was injected back into the gas cap to maintain reservoir pressure. And in the part of the oil pool where the pressure from the gas cap transmits into the oil, gravity also causes the oil to flow down the gently sloping reservoir rock layer toward the production wells. This gravity flow effect gave rise to the term “gravity drainage area” to describe this part of the oil pool.

The lower section of the oil pool is too distant from the gas cap to be in significant pressure communication with the gas.

“We don’t see that same gas-cap drive — the gas can’t expand directly into those areas,” Digert said.

Consequently, massive volumes of water have been injected into the periphery of the oil pool to maintain reservoir pressure in this waterflood area of the reservoir. Waterflood started in 1981, Digert said.

So, the basic mechanism for production at Prudhoe Bay involves the driving of oil under pressure up production wells by continuously cycling vast quantities of gas from the reservoir back into the top of the gas cap, while simultaneously pumping equally vast quantities of water into the peripheral part of the reservoir.

Between 1990 and 1995 the gas cycling process necessitated a progressive expansion of the gas injection capabilities in the field, with gas production starting at around 4 billion standard cubic feet per day and eventually topping out at 8.5 billion standard cubic feet per day. The central gas facility at Prudhoe Bay is now the largest plant of its kind in the world and compresses the gas to about 4,000 pounds per square inch for injection into the reservoir.

Waterflood

Water for the waterflood operations consists in part of water separated from the produced oil. However, a major component of the injected water consists of seawater that has been processed through a seawater treatment plant. Initially, water was injected at the rate of about 500,000 barrels per day, but that rate has now increased to about 1 million barrels per day, Digert said.

Fluids flowing from the production wells pass through six flow stations and are separated into water, crude oil and gas, Digert said. The separated water is used in the waterflood operation, while the crude oil transits through pipelines to the trans-Alaska oil pipeline for export from the field.

The gas is piped to a major facility that chills the gas to minus 40 degrees Fahrenheit. The chilling causes the heavier hydrocarbons such as propane, butane and pentane to drop out of the gas as natural gas liquids.

Some of the natural gas liquids are mixed with the crude oil for export. In all, more than 500 million barrels of natural gas liquids, or NGLs, have been shipped to market via the trans-Alaska oil pipeline. Much of the NGLs, however, are used for enhanced oil recovery operations both at Prudhoe Bay and in the nearby Kuparuk River field.

All but about 500 million cubic feet per day of the dry natural gas from which the natural gas liquids have been extracted goes to the compressors in the gas plant, for re-injection into the reservoir, Digert said. The gas that is not compressed is used as the primary fuel source for North Slope operations, with the bulk of this fuel gas being used to drive the huge gas compressors.

But because of factors such as the use of gas for fuel, more material leaves the Prudhoe Bay reservoir than is pumped back in, Digert explained. As a consequence, the gas pressure declined significantly over the years after the field went into production. In 2002, to stem this pressure decline, BP started injecting water as well as gas into the gas cap.

Further enhanced recovery

In addition to upping the gas and water injection capacity to maximize field production, several state-of-the-art enhanced oil recovery techniques have come into play to extend oil recovery from the field way beyond the initial recovery estimate of 9.6 billion barrels.

In what is known as tertiary extended oil recovery, a solvent known as miscible injectant removes some of the residual oil left behind after a waterflood operation. Miscible injectant consists of a mixture of natural gas and some of the natural gas liquids obtained from the gas chilling plant.

When the miscible injectant is swept through a waterflood area of the reservoir, lighter elements of the residual oil left behind from the waterflood operation tend to adhere to elements of the miscible injectant, Digert explained. As a consequence the fluid recovered from the reservoir becomes richer in oil components than the miscible injectant fluid that is injected into the reservoir, he said.

In 1999 the massive $80 million MIX module was barged to the North Slope to expand the field’s miscible injectant usage.

The use of 400 million cubic feet to 500 million cubic feet per day of miscible injectant is increasing oil production by about 50,000 barrels per day, Digert said.

Meanwhile, the multi-year use of gas injection and waterflood has resulted in some fragmentation of the oil, gas and water pools within the Prudhoe Bay reservoir. For example, the relatively gentle slope of just two to three degrees of the reservoir strata has allowed gas from the gas cap to flow under the impervious shale layers to leave small, disconnected pools of oil above the shales, Digert said.

“The gas tends to run under some of these continuous shales,” Digert said.

State-of-the-art, high-tech drilling techniques have proved critical in extending field life by enabling access to these small, disconnected pools.

“So these are the targets that we’re still drilling for today,” Digert said.

One technique involves the use of sidetrack drilling, the drilling of new low-angle or horizontal well bores out from an existing steep well penetration, to thread directly through the isolated pockets of oil.

And what people refer to as quaternary extended oil recovery, a technique called miscible injectant sidetrack, or MIST, uses miscible injectant in combination with a specially shaped sidetrack well to target remnant oil pools in worked-out waterflood zones. Essentially, a sidetrack well is designed to wrap horizontally around an existing injector well at a distance of about 1,000 feet. Bubbles of miscible injectant released into the reservoir at different points on the horizontal sidetrack then expand and herd the residual oil toward production wells.

Future gas sales

Despite all of these ingenious ways of producing yet more oil from Prudhoe Bay, the field continues along a slow path of declining production. However, the really big prize in the future is gas production from the field — the field originally contained equal volumes of oil and gas, Digert said. There is probably about another 2 billion barrels of viable oil production left in the field, but the field could produce gas equivalent in energy to about another 4 billion barrels of oil, he said.

The gas treatment plant for gas export would be sited next to the existing central gas facility, Digert said. Exporting gas from the field rather than re-injecting it would reduce the potential ultimate oil production because of a loss of gas pressure. However, the volume of the resulting gas sales would more than offset the loss of oil, Digert said.

“Our game now is going to be continued reservoir management strategies. … We have an enormous opportunity to optimize (oil) production and maximize that production ahead of gas sales,” Digert said.

And there is also the prospect of continued future oil production in the Prudhoe Bay unit from horizons above the Prudhoe Bay field, such as viscous oil from West Sak (also known by BP as Schrader Bluff).

Meantime, the production of Prudhoe Bay oil is already accompanied by the production of huge volumes of gas for fuel use and recycling.

“In our gravity drainage area they’re starting to look like gas wells with a little bit of oil, more than oil wells with a little bit of gas. It’s mostly gas coming out,” Digert said.

But Digert feels optimistic about Prudhoe’s future.

“We’ve had massive development of a massive field,” Digert said. “We think we’re very well positioned for the next 40 years … both in oil and then the gas sales that will follow.”






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