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Vol 21, No. 23 Week of June 05, 2016
Providing coverage of Alaska and northern Canada's oil and gas industry

Replenishing resource

Further evidence hydrates bolstering East Barrow natural gas production

ALAN BAILEY

Petroleum News

For several decades gas fields near Barrow, in the extreme northwest of Alaska, have provided the town with a convenient fuel supply at a stable cost. But the fields present an enigma: Despite continuing gas production, the reservoir gas pressures have not declined as would have been predicted, and the quantity of gas produced has exceeded expectations.

Could this curious phenomenon point the way towards the future development of a huge natural gas resource on the North Slope?

Scientists investigating the fields have suggested two possible explanations for the pressure maintenance in the fields. Either an influx of subterranean water into the reservoir is bolstering the gas pressure, or methane hydrate deposits in the reservoirs are dissociating, continuously releasing methane, the primary component of natural gas, and thus continuously replenishing the gas resource in the reservoirs. Low water production from the fields tends to support the methane hydrate theory. And recent research is bolstering that view.

Methane hydrate is a naturally occurring ice-like material that concentrates methane in a lattice of water molecules within a specific range of relatively low temperatures and high pressures. In certain situations methane hydrate deposits could become a prolific source of natural gas for use as a fuel, if viable ways of producing gas from the hydrates can be developed. The phenomenon observed in the Barrow gas fields is particularly intriguing since, if the reservoir pressure maintenance does result from the presence of hydrates, the observations from the field would demonstrate the possibility of producing gas from hydrate deposits elsewhere.

Huge quantities of the hydrates exist around the base of the permafrost under the North Slope.

Drilling program

The U.S. Department of Energy had planned to support a hydrate research project in the Barrow gas fields, with the drilling of a couple of test wells in conjunction with a major development drilling program that the North Slope Borough had planned. But in early 2010 DOE dropped its funding for the Barrow research. However, the field development drilling did go ahead in 2011 and 2012, to boost and maintain gas supplies for local communities. The drilling, while not specifically targeting methane hydrate research, did provide some additional insights into the Barrow gas fields enigma.

Consultancy firm Petrotechnical Resources of Alaska funded an intern project to investigate what new light on the gas production puzzle could be gleaned from the results of the development drilling. And on May 24, during the Society of Petroleum Engineers 2016 western regional meeting, Nolan Youngmun from the PRA team presented a paper, reviewing the results of earlier research into the gas fields and the findings from the development drilling results in the East Barrow field.

Youngmun said that despite an original estimate of 6.2 billion cubic feet of original gas in place, with 5.7 bcf of that being recoverable, the East Barrow field has to date produced about 9.1 bcf. And, despite the volume of gas extracted from the field, the pressure of the gas in the reservoir has remained somewhat constant.

Field performance simulation

A study of the field conducted a few years ago simulated the expected field performance under various combinations of gas, water and methane hydrate in the field reservoir. This study showed that the observed reservoir pressure history could best be matched by a scenario in which the dissociation of hydrates and the influx of some water both occurred, Youngmun said.

Moreover, the computer modeling of the East Barrow reservoir and the likely zone of methane hydrate stability in the field indicated that hydrates would be stable within the upper part of the field’s pay zone and that these hydrates could be in communication with the field’s underlying gas pool. A comprehensive reservoir simulation for the field confirmed that a combination of free gas, hydrate dissociation and some water influx most accurately replicated the East Barrow gas field’s performance history. A particularly astonishing finding from the reservoir simulation was a revised estimate of nearly 42 bcf of original gas in place, including gas held in the hydrate deposits. That is an increase of 677 percent relative to the original gas in place estimate, Youngmun commented.

Savik wells

An analysis of the results of the development drilling conducted in 2011 and 2012 focused on two wells: the Savik No. 1 and the Savik No. 2. These gas production wells were directionally drilled through the Upper and Lower Barrow Sandstone reservoir rocks in the East Barrow field.

Log analyses for the No. 1 well indicate that the well encountered hydrate in the upper sands while encountering gas in the lower sands. Flow tests for the well indicated a gas flow rate suitable for commercial operation, with adequate maintenance of the well head pressure.

Savik No. 2, which also penetrated both sand bodies, proved more problematic, with the wellhead pressure declining rapidly during flow testing, probably because of hydrates plugging the downhole section of the well. After several unsuccessful attempts to sustain a gas flow through the well, including the use of a coiled tubing unit to unplug the well, it was concluded that the downhole end of the well must lie entirely within the pressure and temperature stability zone for methane hydrate, rather than in free gas - the deepest section of the No. 2 well was shallower than the producing section of the No. 1 well. The No. 2 well was shut in, Youngmun said.

Unexpected recovery

However, in July 2014 the wellhead pressure in the Savik No. 2 well suddenly rose, without any stimulation being applied to the well. And gas could subsequently be flowed from the well at commercial rates. Although the research team has recommended some further tests on the well, to better understand the well’s behavior, it appears that as gas production and associated hydrate dissociation have continued, the base of the hydrate stability zone has risen above the level of downhole end of the No. 2 well, thus putting the well in direct communication with free gas in the reservoir.

In addition to well logging and drilling operations confirming the presence of methane hydrate deposits in the East Barrow gas field, the results of field modeling and the observed performance of the two Savik wells all support the concept that hydrate dissociation combined with a small amount of water influx are recharging and maintaining the pressure in the gas pool, Youngmun said.



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