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Solving the Trading Bay jigsaw puzzle Hilcorp seeks linkages between complex reservoir sands as part of its efforts to produce more oil from the aging Cook Inlet field Alan Bailey Petroleum News
Since taking over a bevy of aging oil fields in Alaska’s Cook Inlet basin in 2011 Hilcorp Alaska has made some significant progress in turning around what had been a steady decline in oil production from the basin. During the 2014 Alaska Geological Society Technical Conference, Matthew Frankforter from Hilcorp talked about some of the challenges that the company is dealing with in rejuvenating one of those old fields, the Trading Bay field, offshore in the inlet.
60 wells The Trading Bay field, discovered in 1965, produces oil both from the Hemlock and the Tyonek formations, within the Tertiary rock sequence of the basin. After years of development, there are now 60 wells penetrating the field reservoirs, Frankforter said. But, to date, only about 78 million barrels of the field’s estimated 1 billion barrels of original oil in place has actually been produced, a “pretty low” recovery factor, he said. The production rate from the field has increased about five-fold since Hilcorp acquired the field, he said.
Frankforter said that the primary reason for that low recovery factor is that the field, rather than having major, connected sand reservoirs, consists of a multiplicity of individual sand bodies, with well perforations accessing some of these sands but not others. Currently production comes from about 60 of the 94 identified individual reservoir sands within a 4,000-foot thick, hydrocarbon-rich rock section.
As part of production optimization, Hilcorp is trying to correlate individual sand bodies across the field, to gain an understanding of the past performance of the various well completions and hence to figure out how best to approach future field development. The company is trying a correlation approach that keys into an understanding of the way in which the various rock units were formed.
River channels It has long been understood that the Tertiary rocks of the Cook Inlet were laid down in an ancient land environment, rather than under the sea. And the sand bodies found in fields like Trading Bay represent the in-filled channels of rivers that made their way across the ancient landscape, rather like the nearby Susitna and Matanuska rivers of present-day Southcentral Alaska. During Tertiary times, periods of lush vegetation led to the later formation of coal seams, some of which now extend many miles through the subsurface.
Frankforter said that Hilcorp has categorized the Trading Bay reservoir sand bodies into two types: a Hemlock type and a Tyonek type. The Hemlock type corresponds to a modern-day river that exhibits braided drainage, with several individual channels wending their way along a gravel and sand corridor. The Tyonek type corresponds to a meandering style of river, in which a single channel snakes its way across the landscape. In both reservoir types, the sand and more conglomeratic material that fills the ancient channels typically exhibit excellent qualities as oil reservoirs, Frankforter said.
Modern analogy There are analogies with a modern river system, such as the nearby Susitna River that drains into Cook Inlet. A braided section of the river, corresponding to the Hemlock reservoir type, might be some four miles across, a size that matches the scale of an oil field such as Trading Bay. A meandering river channel corridor, corresponding to the Tyonek reservoir type, is typically smaller, but could still fit over a field outline, Frankforter said.
But, in addition to the complexities of the sand bodies themselves, the way in which the rock strata in the Trading Bay field have been displaced by a multitude of geologic faults greatly adds to the difficulties of tracing a single sand unit from one part of the field to another, Frankforter explained.
Tracing coal seams So, rather than trying to track the individual sand bodies, Hilcorp is finding it more productive to trace coal seams across the field, lining up the patterns of coal seams and hence figuring out how the sands between the seams correlate - because of the way in which the coal formed, the coal seams tend to persist across what would have been the wide expanse of terrain within which the smaller river channel corridors lay, Frankforter said. By comparison, the basin, the broad valley, within which the current Susitna River channels lie, is about 65 miles across, he said.
In fact, it appears that the tracing of patterns of coal seams can enable correlations, not just within individual fields but between adjacent fields, Frankforter said.
Hilcorp hopes that these correlation techniques, although still in the conceptual stage, will provide new insights into field-wide reservoir continuity, aiding efforts to improve oil recovery efficiencies and the targeting of undeveloped portions of the field.
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