Department of Administration

Alaska Oil and Gas Conservation Commission

Alaska Department of Administration, Alaska Oil and Gas Conservation Commission
Administration >  AOGCC >  Statistical Reports >  Current Oil & Gas Pools Statistics >  Kuparuk River Unit, West Sak Oil Pool
AOGCC Pool Statistics
Kuparuk River Unit, West Sak Oil Pool
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Operator: ConocoPhillips Alaska, Inc.
Location map for West Sak Oil Pool with respect to other North Slope oil pools.
Discovery Well: ARCO Alaska Inc.
  West Sak River State No. 1
  Permit #171-003
  API No. 50-029-20090-00-00
  Sec. 2, T11N, R10E, UM

Depth: 10,290’ MD / 10,290' TVD

April 26, 1971

Status: Producing
Location: Central Arctic Slope Area Location Map Pool Location Map DNR Unit Map
References: List of Orders Summary - Annotated Reference List  
Summary:

Billions of barrels of oil have accumulated in shallow reservoirs on the North Slope of Alaska. These oil accumulations are located within the Kuparuk River Unit (“KRU”) and within the adjacent Milne Point, Nikaitchuq and western Prudhoe Bay Units. These reservoirs lie roughly 3,000’ to 4,500’ below sea level, and beneath about 1,800 feet of permafrost. As a result, the temperature of the reservoirs is relatively cold (only 40° to 90° F), which decreases mobility of the oil and hinders production. In addition, the oil itself is quite viscous. According to ConocoPhillips, the current operator for the pool, West Sak oil has about the same viscosity, or ability to flow, as olive oil. In contrast, oil from deeper reservoirs within the KRU and at Prudhoe Bay has about the same viscosity as water. The shallow reservoir sandstones containing the oil are also poorly consolidated, making it difficult to produce the viscous oil without large quantities of accompanying sand. As a consequence, these shallow oil accumulations are challenging to produce economically.

Within the KRU, this viscous oil has accumulated in the West Sak sands, an informally named member (subdivision) of the Late Cretaceous-aged Colville Group. The West Sak sands are roughly the age-equivalent of the Schrader Bluff sands that contain similar oil deposits at Milne Point, Nikaitchuq and in the western Prudhoe Bay Unit.

The West Sak Oil Pool is located largely within the boundaries of the KRU, but it laps over into the Milne Point and the Prudhoe Bay Units to the northeast and also extends to the south, beyond the southern boundary of the KRU. The pool occupies about 163 square miles within the KRU.

After its discovery in 1971 by the West Sak River State No. 1 well, the extent of the West Sak Oil Pool was determined through drilling of nearly 35 additional exploratory wells and hundreds of development and service wells that targeted the underlying Kuparuk Oil Pool. Production of oil from the West Sak Oil Pool was proved feasible by a 45-acre pilot project located in Section 35 of T11N, R10E, UM that was undertaken from June 1983 through December 1986. This project consisted of fifteen wells drilled in a nine-spot pattern with five-acre spacing. It employed conventional slant-wells (inclined less than 31° from vertical) and implemented a water-flood to enhance oil recovery (8 producers, 5 water injectors, 1 water supply well and 1 disposal well). During 3-1/2 years of operations, this pilot project produced a total of 750,000 barrels of oil at an average rate of about 800 barrels of oil per day (“BOPD”). The maximum monthly production rate achieved by any well occurred during June 1985, when well WSP-03 averaged 227 BOPD.

Regular production from the West Sak Oil Pool began from 1D-Pad in December 1997, and for 1998 production averaged about 1,550 BOPD. Since then, West Sak production has climbed steadily because of the nearly constant addition of new wells from other drilling pads, the use of horizontal wells and extended-reach drilling technology, the implementation of multi-lateral wells (multiple wellbore segments within the reservoir that branch out from a single well), and improvements in well completion, water-flood and production technology. In contrast with pilot project well WSP-03, mentioned above, well 3K-102 and its multi-lateral component segment 3K-102L1 averaged 1,213 BOPD during their first full month of production in February 2009.

The West Sak Oil Pool development is currently restricted to five pads in the southeastern KRU (1B, 1C, 1D, 1E, and 1J) and one pad in the northern KRU (3K), which recently opened a new portion of the pool to production. For the first five months of 2011, the West Sak Oil Pool averaged approximately 15,050 BOPD, with an average water cut of 25%.

Geology:

This vast pool has accumulated within the West Sak sands, an informal member of the Late Cretaceous-aged Colville Group. These sands are the stratigraphic equivalent of the Schrader Bluff Formation, which contains oil in the adjacent Milne Point and Nikaitchuq Units and in the western portion of the Prudhoe Bay Unit. The pool is currently defined by Conservation Order No. 406 as the accumulation of hydrocarbons common to and correlating with the interval between 3,742' and 4,156' measured depths in the West Sak No. 1 well. The West Sak Oil Pool lies at depths ranging from about 3,000’ to 4,500’ true vertical depth subsea, beneath about 1,800’ of permafrost. Consequently, the reservoir temperature is low (about 40° to 90° F).

The West Sak and the equivalent Schrader Bluff sands were deposited along an inner marine shelf that, over time, shallowed and became a delta-front environment. The gross thickness of the West Sak interval ranges from 700 feet in the southwest portion of the KRU to 350 feet in the northeast; its average thickness is 450 feet. Individual reservoir sandstone layers range up to 40 feet in thickness, and are separated layers by non-reservoir quality siltstone and mudstone. The reservoir layers consist of very fine-grained, moderately sorted, poorly consolidated sandstone that generally is extensively bioturbated.

Three types of petrofacies are recognized in the West Sak interval based on porosity and permeability distributions: Rock Type 1 and 2, which constitute potential pay intervals, and Rock Type 3 consisting of non-pay mudstones. Rock Types 1 and 2 are litharenites and lithic wackes, respectively, and are evenly distributed throughout the West Sak net sand intervals. Rock Type 1 is dominated by macro porosity; the average grain size is very fine sand. Characteristics of this rock type are 25 to 35% porosity, unstressed air permeability between 200 to more than 1,000 millidarcies, oil saturation from core plugs (not normalized to 100% fluids) from 40 to 75%, water saturation from 15 to 30%, and grain density averaging 2.65 g/cm3. Rock Type 2 contains both micro and macro porosity; the average grain size is coarse silt. Characteristics of this rock type are 20 to 30% porosity, unstressed air permeability from 15 to 200 millidarcies, oil saturation from core plugs (not normalized to 100% fluids) from 20 to 60%, water saturation from 25 to 75%, and grain density averaging 2.71 g/cm3.

The regional structure of the West Sak interval is a north-northwest striking monocline, with northeast dip of 1 to 2 degrees. The top of the West Sak ranges from 1,300’ below sea level in the southwest KRU to 4,200’ below sea level in the eastern KRU. The West Sak is densely cut by north-south and east-west trending post depositional normal faults with throws averaging 50’, and ranging to 200’. Faults with vertical displacements as small as 50’ may segment the reservoir into blocks that have discrete oil/water contacts.

Seven regional hydraulic blocks with oil/water contacts, ranging from 2,150’ below sea level in the western KRU to 4,050’ below sea level in the northern KRU, have been identified to date. The trapping mechanism of the West Sak Oil Pool includes a combination of structural, stratigraphic and thermal elements. The thermal elements are related to proximity of permafrost to high viscosity oil.

West Sak oil is characterized by biodegradation, with API gravities that vary from 22 degrees in the deeper, eastern part of the KRU to 10 degrees in the shallower western side of the KRU.


  Strat Column Type Log
 
Production: Prod Chart          
  Oil (bbls) NGL (bbls) Gas (mcf) Water (bbls)
Cumulative 57,394,563 0 27,930,497 17,468,179
2005 Total
4,174,669 0 2,742,940 1,326,696
2006 Total
6,678,977 0 5,093,911 1,858,791
2007 Total
6,415,316 0 4,124,544 1,184,642
2008 Total
6,165,572 0 2,530,596 1,823,367
2009 Total
6,885,632 0 2,384,261 1,935,608
2010 Total
5,730,371 0 2,169,345 2,455,823
2011 Total
5,737,229 0 3,536,361 1,705,780
2007 Rate (b/d)
17,576 0 11,300 3,246
2008 Rate (b/d)
16,892 0 6,933 4,996
2009 Rate (b/d)
18,865 0 6,532 5,303
2010 Rate (b/d)
15,700 0 5,943 6,728
2011 Rate (b/d) 15,718 0 9,689 4,673
2008 Change (%)
-4 0 -39 54
2009 Change (%)
12 0 -6 6
2010 Change (%)
-17 0 -9 27
2011 Change (%)
0 0 63 -31

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