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Providing coverage of Alaska and northern Canada's oil and gas industry
December 2013
Copyright Petroleum Newspapers of Alaska, LLC (Petroleum News)(PNA)©1999-2019 All rights reserved. The content of this article and website may not be copied, replaced, distributed, published, displayed or transferred in any form or by any means except with the prior written permission of Petroleum Newspapers of Alaska, LLC (Petroleum News)(PNA). Copyright infringement is a violation of federal law subject to criminal and civil penalties.
Vol. 18, No. 52 Week of December 29, 2013

Declining oil flow in TAPS lowers North Slope NGL export limits

Although designed as a means of shipping crude oil from the North Slope to the Valdez Marine Terminal, and hence by tanker to market, the trans-Alaska pipeline also carries modest quantities of another material called natural gas liquids, or NGL, to add value to the export stream. But declining oil throughput in the pipeline also reduces the amount of NGL that the line can carry, Mike Malvick, senior process advisor with Alyeska Pipeline Service Co., explained to Petroleum News.

Essentially, NGL, being more volatile than crude oil, increases the vapor pressure in the line, thus potentially increasing the gaseous hydrocarbon emissions from crude oil relief tanks along the line. So, to ensure compliance with its Environmental Protection Agency air permits, Alyeska has to pose a vapor pressure limit of 14.2 pounds per square inch to the fluids delivered into the line at pump station 1 on the North Slope, Malvick said. And that vapor pressure limitation typically limits the NGL content to around 7 to 8 percent of the volume of fluids in the line: Reduce the rate of flow of oil, and the rate of flow of NGL also must drop.

The oil stream flowing from each of the five pipelines that deliver oil to pump station 1 from the various North Slope oil fields has to meet that 14.2-pounds-per-square-inch limitation, Malvick said.

Light hydrocarbons

NGL consists of a series of relatively light, volatile hydrocarbons such as butane, pentane and octane that contain multiple carbon atoms per molecule but fewer carbon atoms than the typical hydrocarbons in crude oil. The NGL hydrocarbons have a higher market value than methane, the hydrocarbon that forms the primary component of natural gas and that contains just one atom of carbon per molecule. The North Slope oil producers recycle natural gas produced from the oil fields back into field reservoirs, to maintain reservoir pressure and hence sustain oil production — natural gas is too volatile for exporting mixed with oil and would have to be exported through a specialized gas line.

Some NGL, separated from the fluids flowing from oil wells, is used in a material called miscible injectant, a hydrocarbon solvent injected into oilfield reservoirs for enhanced oil recovery. The remainder is mixed with the sales oil streams delivered to pump station 1.

And, to maximize the volume of NGL that can be shipped down the pipeline without hitting the vapor pressure limit, it is possible to distil out the heavier and less volatile NGL components, preferentially mixing these with the exported crude oil, rather than exporting the lighter fractions. The lighter components, including propane, are preferentially used to make miscible injectant, Malvick said.

The granddaddy of the NGL processing systems on the North Slope is in the massive Prudhoe Bay field Central Gas Facility, the facility that compresses huge volumes of natural gas for re-injection into the field. A unit within the facility separates the heavier NGL components for injection into the Prudhoe Bay sales oil stream at a facility adjacent pump station 1 — a monitoring system ensures that the vapor pressure of the resulting fluid does not exceed the 14.2-pounds-per-square-inch limit, Malvick said. And some of the NGL from the Central Gas Facility goes to the Kuparuk field for enhanced oil recovery, he said.

—Alan Bailey






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Copyright Petroleum Newspapers of Alaska, LLC (Petroleum News)(PNA)©1999-2019 All rights reserved. The content of this article and website may not be copied, replaced, distributed, published, displayed or transferred in any form or by any means except with the prior written permission of Petroleum Newspapers of Alaska, LLC (Petroleum News)(PNA). Copyright infringement is a violation of federal law.