HOME PAGE SUBSCRIPTIONS, Print Editions, Newsletter PRODUCTS READ THE PETROLEUM NEWS ARCHIVE! ADVERTISING INFORMATION EVENTS PETROLEUM NEWS BAKKEN MINING NEWS

Providing coverage of Alaska and northern Canada's oil and gas industry
May 2011

Vol. 16, No. 21 Week of May 22, 2011

There is still life in aging CI fields

Although oil and gas production rates from Cook Inlet have declined significantly, fields still hold some significant resources

Alan Bailey

Petroleum News

Despite steadily declining production rates, there are some significant remaining oil and gas resources in Alaska’s aging Cook Inlet oil and gas fields that first went into production several decades ago. And at the annual meeting of the Pacific Section, American Association of Petroleum Geologists, in Anchorage on May 10 Cook Inlet producers talked about the history and current status of three of those fields: the Beluga River gas field, the Grayling Gas Sands gas field and the Granite Point oil field.

Beluga River

Richard Levinson from ConocoPhillips spoke about the Beluga River gas field that his company operates on the west side of the inlet, about 40 miles west of Anchorage. This field, co-owned by ConocoPhillips, Chevron and Municipal Light & Power, is the prime source of natural gas for the co-located, gas-fired Beluga power station, operated by Chugach Electric Association. Municipal Light & Power, a division of the Municipality of Anchorage, bought its share of the field in 1996, to secure gas supplies for its power generation facilities.

The field was discovered in 1962, went into production in 1968 and has so far delivered more than 1.19 trillion cubic feet of natural gas, Levinson said. Development of the field has reached maturity, with production peaking in the mid-2000s and gas pressure in the field now depleted to below 40 percent of its original level, he said.

In common with other legacy gas fields of the Cook Inlet basin, the Beluga River gas resource was discovered while drilling for oil in exploration targets below the gas sands. The field is very large, with reservoirs in sand bodies in the Sterling and Beluga formations in a geologic structure 7.25 miles long by 2.5 miles wide. The field structure, typical of the established Cook Inlet fields, consists of a broad north-northeast trending, razor-clam-shaped fold in the rock strata, bounded on the west side by a steeply sloping geologic fault, Levinson said. The rock formations containing the reservoir sands are, in total, up to 3,200 feet thick and were deposited from a river system, with distinct river channels accumulating the sand that now contains gas.

Gas from coal

The gas originated from the numerous coal seams found in the area, with the coal seams, together with rocks formed from silt and mud, sealing the gas in the sands and sometimes limiting the flow of gas from one sand unit to another. And because individual sand-filled river channels can run between gas wells in the field and thus escape well penetration, ConocoPhillips has been reducing the spacing between the wells, as part of its field development program, Levinson said.

Reservoir quality is variable and the sand units tend to contain a certain amount of pore-clogging clay, presumably derived from volcanic ash that has been deposited in the Cook Inlet basin from the system of volcanoes that has been adjacent to the basin for millions of years. Unfortunately, as water encroaches into the reservoir sand, the clay tends to absorb the water and expand, breaking up the rock and thus increasing sand production through the gas wells, Levinson said. Sand production, in part also a result of gas pressure depletion, has become a major issue in the field, with a technique involving the placement of gravel inside the well casing to act as a sand filter proving particularly helpful for sand control, he said.

The compaction of the sands in the deeper reservoir sections, where the load exerted by thousands of feet of overlying rock is highest, is problematic, with the clay content of the rock tending to seal the spaces between rock pores, inhibiting the flow of gas through the reservoir. As a consequence, these lower sections, in the lower part of the Beluga formation, have remained largely untouched by field development, although they contain significant gas volumes, Levinson said. Modern techniques for producing so-called “tight gas” are now coming into play to develop this part of the field, he said.

Grayling Gas Sands

The Grayling Gas Sands gas field, one of the three largest Cook Inlet gas fields along with the Beluga River and North Cook Inlet fields, lies directly over the McArthur River oil field, offshore the west side of the inlet. The field directly accounts for about 18 percent of Cook Inlet gas production, Chevron staff geologist Brent Voorhees told the AAPG conference. The field was discovered in 1965, with production starting in 1968 and peaking in November 1997 at a rate of 225 million cubic feet per day, Voorhees said.

Over the years more than 200 oil and gas wells have been drilled from the four offshore platforms that support the Grayling Gas Sands and McArthur River fields. Today, the Grayling Gas Sands production rate is about 75 million cubic feet per day through 20 wells on one of those platforms, the Steelhead platform, with two gas wells on the Grayling platform supplying fuel gas for field operations, Voorhees said.

In a similar setting to the Beluga River field to the north, the Grayling Gas Sands reservoir is trapped in a major upward fold of the rock strata, in a structure associated with a north-northeast trending geologic fault. The more than 7,000-foot thick system of gas reservoir rocks in the field consists of multiple, sand-filled river channels, stacked vertically to form more than 40 gas pay sands, most with individual gas-water contacts rather than existing as a continuous, single reservoir. Some of the sand bodies have not yet been tapped, Voorhees said. Gas in the field originated from the multiplicity of coal seams that exist in the rock sequence in the area.

After 40 years of production, the gas pressures in different sand units have been differentially depleted, Voorhees said. And with a variety of grain sizes, and with varying amounts of clay and other impervious materials, reservoir quality in the gas sands is quite variable, he said.

Since 2007 Chevron has been undertaking a renewed phase of field development, including the drilling of wells into isolated sand bodies, drilling into some reservoir zones that had been prematurely abandoned, and the drilling of horizontal wells to access single reservoir zones, Voorhees said. This program has involved the conversion of some oil wells into gas wells. Two new horizontal wells and two well workovers are planned for 2011, he said.

Granite Point

The Granite Point oil field, also offshore the west side of the inlet, was discovered in 1965, with oil tested in the Tyonek formation at depths ranging from 8,000 to 9,000 feet, James Waugaman of Chevron North America told the AAPG conference. Oil was later also found in the Hemlock formation, he said.

Field development from three offshore platforms began in early 1967, with production peaking at 50,000 barrels of oil per day. Water injection began in 1970, with phases of re-drilling in the field following, to slow the production decline rate. The decline rate has hovered around 10 percent per year since the late 1990s, with current production being somewhat over 2,000 barrels per day, Waugaman said.

To date the field has produced a total of more than 145 million barrels of oil, out of an estimated more than 600 million barrels of oil originally in place, thus perhaps leaving some additional development potential.

The structure of the Granite Point field is a look-alike to the Beluga River and Grayling Gas Sands fields, being a north-northeast trending fold in the strata, bounded by a fault on the west side. Analysis of the chemistry of the oil indicates that it originated in marine shales of the Jurassic Tuxedni group, with the oil having migrated upwards through a system of geologic faults and sand layers into the younger and shallower Tyonek and Hemlock formations of Tertiary age, Waugaman said.

The Tyonek at Granite Point consists of river-lain sands, pebbles and silt, interspersed with coals seams. Ancient river channels containing sand range from 10 to 100 feet in thickness. The sands in two reservoir zones, the C5 and the C7, are reasonably continuous, but other sand units tend be discontinuous across the field, Waugaman said.






Petroleum News - Phone: 1-907 522-9469 - Fax: 1-907 522-9583
[email protected] --- http://www.petroleumnews.com ---
S U B S C R I B E

Copyright Petroleum Newspapers of Alaska, LLC (Petroleum News)(PNA)©2013 All rights reserved. The content of this article and web site 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.