Summary
It is projected that by the year 2015, Alaska North Slope (ANS) crude-oil
production will decline to such a level (200,000 to 400,000 B/D) that there
will be a critical need for pumping additional liquids through the pipeline to
maintain economic operation of the Trans-Alaska Pipeline System (TAPS). The
estimated proven and recoverable gas reserves of 38 trillion cubic feet (Tcf)
on the ANS can be converted to a high-premium liquid product using
gas-to-liquids (GTL) technology. The GTL product can be transported from the
ANS to the southern port of Valdez, Alaska through the TAPS.
One of the proposed modes of transportation of the GTL products from ANS to
market is by commingling it with crude oil as a single phase and pumping the
mixture through the TAPS. This mode of transportation changes the properties of
the GTL as well as the crude oil. The focus of this work, therefore, was to
determine the physical and chemical properties of GTL and its blends with ANS
crude oil, analyze the energy requirements for flowing the mixture through
TAPS, and determine the optimum blend ratio to maximize the transportation
economics. Four blends of GTL/crude-oil mixture in the ratios of 1:1, 1:2, 1:3,
and 1:4 were prepared for their rheological evaluation and pressure drops at
different temperature conditions.
The results of the analysis show that flow behavior of the GTL and GTL
blends are temperature sensitive. Viscosity and density of the blends were
found to decrease with increasing amounts of GTL and increasing temperature.
The optimum GTL/crude-oil blend ratio of 1:2.5 (28%:72%) was determined to take
full advantage of the economics of commingled flow of GTL/crude-oil mixture
through the TAPS.
Introduction
The ANS has a huge amount of natural-gas reserves, which can be exploited in
many different ways. It is seen as an excellent economic potential for the
state of Alaska. The estimated ANS proven and recoverable reserves of natural
gas in known oil and gas reservoirs is approximately 38 Tcf (Thomas et al.
1996). Currently, the natural gas in the ANS is used primarily for pressure
maintenance, miscible injectant (MI), running gas turbines in some of the pump
stations to support liquid movement in TAPS, and power oil-production
facilities. Most of the gas may remain unused upon depletion of ANS recoverable
oil and thus may be stranded, unless an option for transportation is developed
to make it marketable. The various options being examined for operational and
economic feasibility for the optimum use of ANS natural gas include
constructing a new Alaska Natural Gas Transportation System (ANGTS) to
transport the gas to the lower 48 states by way of Canada; constructing a
shorter pipeline to Valdez, from where the gas would be converted to liquefied
natural gas (LNG) and shipped to the markets; and converting the gas to
gas-to-liquid products (GTL), blending it with the ANS crude oil, and
transporting the resulting liquid through the existing TAPS. Thomas et al.
(1996), in their previous study, concluded that “state-of-the-art GTL
conversion technology appears to be feasible and could be deployed within a
meaningful time frame to sustain ANS and TAPS oil operations for 20 or more
years beyond what might be anticipated without GTL.” The GTL technology might
also prove to be the best method for monetizing other stranded natural-gas
reserves worldwide.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
25 April 2006
- Revised manuscript received:
3 August 2006
- Manuscript approved:
16 August 2006
- Version of record:
20 December 2006
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