DE-FC26-08NT0005641

Goal
The goal of this project is to develop a robust reservoir model to test possible oil recovery methods that do not use steam or a liquid capable of freezing for the Umiat and similar frozen reservoirs. This will be accomplished by collecting the data needed to develop robust geologic and engineering models. The results will provide important information concerning production methods for this and similar frozen reservoirs in northern Alaska and other arctic regions.

Performers
University of Alaska Fairbanks, Fairbanks, AK 99775-7320
Renaissance Alaska LLC

Background
The Umiat oil field in the Brooks Range foothills of northern Alaska contains light oil in a shallow, frozen reservoir. The Umiat field was discovered in the 1940s but was never considered viable because it is shallow, in the permafrost, relatively small, and far from any transportation infrastructure. Initial estimates of recoverable reserves in the Umiat field ranged from 30 to over 100 million bbl, with an average of about 70 million bbl. However, recent reserve estimates by private industry suggest that the accumulation may be considerably larger than originally thought, and modern horizontal drilling techniques now enable development of shallow reservoirs. This makes Umiat and similar fields in northern Alaska attractive exploration and production targets.

Little is known about how to produce conventional oil from a frozen reservoir. Most prior work has been on developing production techniques for heavy oil in unconsolidated but unfrozen sands, or for gas hydrates. There is no information available describing the behavior of a rock/ice/light oil system at low pressure. This information, along with a robust reservoir model, is needed to accurately model and evaluate the effectiveness of different production methods.

Impact
The Umiat field contains significant albeit unconventional (i.e., frozen) potential energy resources. This is a particularly attractive target considering the decline in the production of conventional oil resources from Alaska’s North Slope. Development of production methods and strategies for these shallow, unconventional resources will promote economically viable resource extraction. This project will encourage involvement of smaller exploration and production companies in Alaska by providing critical information not currently available to the public and by evaluating the applicability of existing production techniques. This information will increase the likelihood of successfully bringing smaller fields into production.

Accomplishments

Project Management Plan
The Project Management Plan was revised to account for a delay in the Renaissance drilling program (which has been postponed until a partner can be found). Due to this delay, and to take advantage of existing geologic information, an additional graduate student was added to build and test a reservoir model based on existing data. Preliminary results from the rock property work has also led to two additional students being added to the project to examine in more detail both the pore structure and the behavior of oil in the presence of ice.

Develop sedimentologic and stratigraphic model of the reservoir interval
Description of core taken during the original exploration program during the 1940s and ’50s was completed during the summer of 2010. Two weeks of field work during June and July 2010 focused on describing outcrops of the Umiat reservoir in the vicinity of Umiat field. Samples were also collected for diagenetic studies and pore structure analysis. Horizontal and vertical permeability was measured in major lithofacies in both core and outcrop.

Results of this work indicate that the Umiat reservoir is a complex system of shallow marine and distributary mouth bar sands with a strong vertical permeability anisotropy. Current work is focusing on linking permeability patterns with the observed lithofacies and facies associations in order to better predict permeability and permeability anisotropy across the field.

Develop structural model of reservoir
Examination of the existing Umiat core during the summer of 2009 showed that natural fractures are not abundant in these old vertical wells, probably because they are sparse, widely spaced and/or steeply dipping. To address this issue, more emphasis is being placed on developing a structural model for the distribution of fractures by examining their structural context in more detail. Fieldwork conducted during June and July 2010 examined both the structural geometry and the distribution of fractures on the Umiat structure and on a similar structure that is nearby and well-exposed, the Big Bend anticline.

Results of this work indicate that there are at least two major fracture networks present at the Umiat structure—one approximately perpendicular to the structure and a second approximately parallel to the structure. These fractures could play a major role in enhancing vertical permeability and vertical connectivity in a reservoir that is otherwise highly compartmentalized.

Characterization of reservoir and fluid properties
Data collection, including field work and description of remaining slabbed legacy Umiat core, has been completed.

An experimental apparatus was designed and constructed to conduct relative permeability experiments of oil and gas in the presence of ice. Preliminary results of experiments on both Berea Sandstone and samples of Umiat core demonstrate a significant reduction of oil relative permeability at irreducible water saturation caused by freezing of connate water. For Umiat samples, this decrease ranges from 39.3–91.5% with an average decline of 76.22%. This is significantly higher than the average 27% reduction obtained by the Federal Bureau of Mines. The average decline of relative oil permeability at irreducible water saturation has increased almost three times by changing the freezing temperature from 26°F (-3.3°C) to 14°F (-10°C). This suggests that the reduction of the relative permeability is strongly dependent on the freezing temperature.

The relative permeability studies on the existing Umiat reservoir strongly indicate that freezing temperature and the void space structure play a large role in the permeability of the reservoir in the presence of ice. To further explore this issue, the experimental determination of the relative permeability of the Umiat reservoir is being augmented by petrographic analysis of the pore structure of the Umiat reservoir, and Nuclear Magnetic Resonance (NMR) is being used to study the pore-scale distribution of ice as temperature is incrementally decreased. These studies began during spring 2010 and will clarify when and where ice forms in the pore structure of the reservoir and have implications for the gas/oil relative permeability throughout the reservoir.

Compositional analysis and flow assurance studies on available samples of Umiat oil began in fall 2009. Analyses indicate that the total asphaltene content measured for Umiat dead oil is not significant, which suggests that Umiat oil does not have a high propensity for asphaltene precipitation. Lower asphaltene precipitation will lead to smooth production, no pipeline deposition and plugging (resulting in less pressure drop along the pipeline), and no change in composition.

The phase behavior of the Umiat fluid needs to be well understood in order for a reservoir simulation to be accurate. This analysis of the phase behavior was compared to theoretical Umiat composition derived using the Pedersen method with original Umiat fluid properties published in the original reports. This comparison allowed estimation of the ‘lost’ light hydrocarbon fractions. A‘pseudo-live’ reservoir oil sample was physically created by adding the lost light ends to the weatherized Umiat dead oil sample.The phase behavior of the ‘live’ oil was also simulated using the Peng-Robinson equations of state (EOS). The EOS model was tuned with measured experimental data to accurately simulate the differential liberation tests in order to obtain the necessary data for reservoir simulation studies.

A major part of predicting how the Umiat reservoir will perform is determining the relative permeability of oil in the presence of ice. Early in the project, UAF work on samples of the Umiat reservoir indicated that there is a significant reduction in the relatively permeability of oil in the presence of ice. However, it was not clear as to why this reduction occurred or where the ice resided. To explore this further, additional experimental and theoretical work was conducted. Core flood experiments were performed on two clean Berea sandstone cores under permafrost conditions to determine the relative permeability to oil over a temperature range of 23ºC to -10ºC and for a range of connate water salinities. Both cores showed maximum reduction in relative permeability to oil when saturated with deionized water and less reduction when saturated with saline water.

Desktop reservoir simulation of proposed production strategies
A preliminary petrophysical model of the Umiat reservoir based on available geologic data has been completed and will be used to run multiple realizations of the field and test various production strategies.

The petrophysical model was also used to calculate the amount of stock tank oil-in-place to verify the numbers reported in a contract report to Renaissance Alaska. Porosity values and net/gross ratios for each interval were derived from the reservoir model; water saturations, the oil/water contact, and the gas/oil ratio were reported by the original USGS studies. The resulting total hydrocarbon pore volume for the modeled horizons in the Umiat area is 1.4 billion barrels. The total stock tank oil-in-place is estimated to be ~ 1.2 billion stock tank barrels, and the associated gas is 84 billion standard cubic feet.

Technology Transfer
Throughout this project, UAF researchers have worked closely with Renaissance Alaska personnel, sharing both data and interpretations. Teleconferences involving the entire research team are held quarterly.

During fall 2010, all UAF faculty and students participated in weekly one hour seminar focusing on the implications of the ongoing geologic and engineering research on Umiat reservoir performance, drilling optimization, and production. Renaissance Alaska personnel participated in the seminasr via teleconference.

During the second year of the project, members of the team presented talks at the national meeting of the American Association of Petroleum Geologists (AAPG) and at the AAPG International Conference and Exhibition (ICE). In addition, a paper was submitted and accepted for the Arctic Technology Conference to be held in Houston, Texas in February 2011.

One M.S. thesis was completed in August 2010 and a second was completed in December 2010.

Current Status (October 2011)
In order to determine the permeability distribution and anisotropy in the reservoir, additional horizontal and vertical permeability measurements of key lithofacies will be collected. Integration of this information into a revised sedimentologic and structural model of the reservoir will continue. Additional petrographic work on the Umiat reservoir began in July 2010 and will focus on the diagenetic history and pore structure of the reservoir.

In addition to developing a descriptive fracture model of the reservoir, a fault seal analysis of the faults that bound the north side of the Umiat structure was initiated during fall 2010. This analysis will clarify the likelihood of a trapping mechanism existing in the footwall north of the faults.

Current research is focused on understanding the relative permeability of oil and water in the presence of ice. The work will consist of conducting relative permeability studies using a range of water salinities. NMR studies will begin in winter 2011 to ascertain where free water is located in the pore structure as the reservoir is progressively frozen. Equation of State modeling will be used to determine phase behavior of the Umiat fluid at expected reservoir conditions.

Legacy Umiat oil compositional analysis results in combination with the documented original gas/oil ratios were used to reconstitute an original Umiat oil composition. The reconstituted Umiat oil will be used in further PVT analyses to determine key properties needed for subsequent reservoir simulations, including Formation Volume Factor and bubble point.

The static petrophysical reservoir model is being refined by incorporating the new geologic data interpretations. This revised model will be used to develop both pattern and full field simulations in order to evaluate well placement, simulate potential production strategies, and test various production methods.

Project Start: October 1, 2008
Project End: September 30, 2012

DOE Contribution: $1,350,652
Performer Contribution: $1,530,000

Contact Information
NETL - Chandra Nautiyal (chandra.nautiyal@netl.doe.gov or 281-494-2488)
UAF – Catherine Hanks (chanks@gi.alaska.edu or 907-474-5562)
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