Stripping the liquids from the gas

Decades of gas cycling have made North Slope gas less rich, but the fields should still have enough liquids to compete favorably

By Bill White

Researcher/writer for the Office of the Federal Coordinator

Editor’s note: This is a reprint from the Office of the Federal Coordinator, Alaska Natural Gas Transportation Projects, online at www.arcticgas.gov/alaska-lng-could-have-right-heat-content-asia-buyers

Alaska gas is getting drier

The Alaska North Slope’s roughly 33 trillion cubic feet of proven gas reserves are wet.

But not as wet as when they were discovered 45 years ago.

The main field is Prudhoe Bay, the nation’s largest oil producer and one of its largest gas reservoirs. Prudhoe would be the anchor source of gas for a large-scale Alaska LNG project.

When oil production was about to start back in 1977 and a gas pipeline project was being planned, Exxon, one of the producers, estimated the gas was 73 percent methane, 6.9 percent ethane, 3.72 percent propane, 3.23 percent butane or heavier gases (more carbon atoms).

Oh yes, plus a nuisance ingredient: 12.71 percent carbon dioxide. CO2 is a nuisance gas because it doesn’t burn and it can corrode pipelines. So gas producers strip out most CO2 before piping the gas to market and essentially all of it before making LNG.

Prudhoe’s 12 percent carbon dioxide is a big proportion. Gas in the other big North Slope gas field — Point Thomson — is about 4 percent CO2, according to ExxonMobil. But 12 percent isn’t off the scale. Australia’s Gordon LNG project under construction will process some gas that is 14 to 16 percent CO2. Gas at the proposed East Natuna LNG project in Indonesia is an estimated 70 percent CO2.

So, through a muscular process, carbon dioxide and other impurities would be stripped from the North Slope’s gas stream. The plant handling this scrubbing chore would be one of the largest of its kind in the world — removing as much as 500 million cubic feet of CO2 each day from about 3.5 billion cubic feet a day of raw, produced gas.

Nobody will buy LNG loaded down with CO2. It would be like buying a holiday turkey loaded with water to boost its weight. The water adds no benefit, just cost. Besides, during cooling CO2 would become solid — think dry ice — and clunk up the LNG machinery well before the methane vapors get cold enough to become liquid.

In 1977, Exxon estimated that Prudhoe’s gas composition would be as follows after cleaning:

•85.11 percent — Methane.

•7.7 percent — Ethane.

•3.99 percent — Propane.

•1.23 percent — Butane.

•0.22 percent — Pentane and heavier.

At the time, a project was being planned to pipe Prudhoe gas to the U.S. Lower 48 states. (Economics killed that project in the 1980s.) Federal gas regulators estimated the gas heat content per thousand cubic feet at 1.139 million Btu.

That would have been a rich gas indeed. Only Libya’s LNG is comprised of a higher percentage of high-Btu gas liquids today, according to the International Group of Liquefied Natural Gas Importers. Back in the early 1980s, the gas liquids carried with the Alaska pipe’s methane would have been extracted somewhere along the pipeline route, probably in Alberta, Canada, where a petrochemical industry was sprouting up and in need of gas liquids feedstock.

But as was mentioned, Prudhoe’s gas is less wet today than in 1977. Depending on what mixture of methane and gas liquids the North Slope producers decide to ship, the gas stream that would support an LNG export project likely would boast a Btu content of between 1.07 million and 1.13 million per thousand cubic feet.

What happened?

They anticipated doing this early on. In October 1977, four months after the first oil flowed from Prudhoe Bay, an Atlantic Richfield (now ConocoPhillips after a series of mergers) executive told a congressional committee, “The most efficient way of getting natural gas liquids from Prudhoe is by blending them into the crude stream.”

In 1986, the producers executed that idea. They christened the world’s largest gas processing plant at Prudhoe Bay. The plant extracts butanes and heavier gases — the high-Btu liquids that will blend with crude oil and stay liquid in the trans-Alaska oil pipeline.

Through 2011, they produced over 635 million barrels of gas condensate and gas liquids. Only two of the North Slope’s two dozen oil fields have yielded more barrels of liquid product over the years: Prudhoe Bay and Kuparuk River, according to state of Alaska statistics. The state profited by collecting tax and royalty on those gas liquids.

Gas liquids production peaked in 1997 at 95,000 barrels a day. Because so much of the heavier liquids are gone and the oil flow is lower, the plant produces about 30,000 barrels a day of gas liquids now, according to BP, which runs the plant.

An ExxonMobil executive explained the removal of heavier gas liquids during a February 2013 presentation to legislators in Juneau, Alaska.

Prudhoe Bay’s gas “has been cycled for 30 years,” said Steve Butt, ExxonMobil senior manager for the proposed Alaska LNG project. “When you talk about Prudhoe, the Prudhoe Bay operators have done an excellent job of managing an oil field in the way one manages an oil field. Keeping the reservoir pressure up (by reinjecting produced natural gas), so that the oil will move and can be produced and all the stakeholders in that project may enjoy those benefits.

“In doing that, however, that gas has been cycled, almost three times — meaning it’s been produced out of the ground, taken to a compression facility, the liquids can be removed, and put back in the ground. Every time you move that gas you remove the liquids, which means you no longer have those liquids to sell. The good news is you’ve already sold them and already generated all the benefits that those liquids have.”

Selling the heavy liquids is part of the reason the gas stream’s Btu content has declined. But the stream is still rich.

Alaska’s rich gas

The North Slope producers considering an LNG export project have divulged little detail about the makeup of gas that would be piped from the North Slope to a liquefaction plant. Other than to say the gas stream would contain some ethane and a little propane.

Those lighter liquids would make the gas piped from the North Slope too rich to use in existing Alaska or U.S. gas systems without modifying the gas to dilute the Btu content.

“It would be too hot to burn in a traditional residential environment because of the ethane,” Butt of ExxonMobil said in his Juneau presentation. “You’d have to blend it with something like air.”

Would the Prudhoe Bay producers continue diverting some gas liquids to make a cocktail called miscible injectant that helps scour more oil from Prudhoe Bay and nearby fields? Propane, ethane, methane and carbon dioxide are the main ingredients of miscible injectant. Would state oil-field regulators allow a rolling back of the miscible program?

The declining richness of Prudhoe Bay’s gas stream over time is evident in changing state estimates of the heat content.

In 1996, four state department heads, in a memo to their boss, Gov. Tony Knowles, said LNG from an export project conceived back then would be steeped in gas liquids. The North Slope producers estimated the heat content would be about 1.17 million Btu per thousand cubic feet for the first 10 years, then average 1.1 million Btu for the remainder of the project’s life, the department heads said.

In 2004, a state petroleum manager said the heat content of pipeline gas — with small amounts of CO2 — leaving Prudhoe Bay for North America markets would range between 1.067 million and 1.119 million Btu per thousand cubic feet. The higher number would occur if the miscible injectant program was scaled back or ended, leaving more ethane and propane for the gas stream. (With no CO2, as would be the case for an LNG export project, the Btu range would have been a little higher.)

Fast forward to today. The raw, out-of-the-ground gas production that enters Prudhoe’s Central Gas Facility has 1.002 million Btu per thousand cubic feet, according to data from the U.S. Energy Information Administration.

After the central gas plant removes butane, pentane and heavier liquids for shipment in the oil pipeline, the heat content drops to about 0.955 million Btu per thousand cubic feet, according to BP, the plant operator.

This low-Btu gas — carbon dioxide and all — is what gets burned to power the Prudhoe Bay field and a couple of nearby utilities. This gas is about 81 percent methane, 5.3 percent ethane, 1.7 percent propane and traces — about 0.12 percent — butanes and heavier.

Scrub carbon dioxide from this field gas would lift its energy content to about 1.08 million Btu per thousand cubic feet.

Other factors could tweak the Btu content of Alaska LNG exports.

That said, the methane, ethane and propane blend comprising Alaska export LNG likely would be comparable to the LNG that plants in Indonesia, Malaysia and Brunei ship to utilities in Japan, South Korea and Taiwan.

The LNG’s heat content — likely between 1.07 million and 1.13 million Btu per thousand cubic feet, probably closer that range’s bottom than its top — also would make it marketable as rich gas in those key Asian markets.

Part 1 of this story appeared in the Aug. 18 issue.