Summary
Application of cyclic solvent injection into heavy and viscous crude oil
followed by in-situ combustion of heavy residues is explored from a laboratory
perspective. The solvent reduces oil viscosity in-situ and extracts the lighter
crude-oil fractions. Combustion cleans the near-well region and stimulates
thermally the oil production. Both solvent injection and in-situ combustion are
technically effective. The combination of the two methods, however, has never
been tried to our knowledge.
Hamaca (Venezuela) and West Sak (Alaska) crude oils were employed. First,
ramped temperature oxidation studies were conducted to measure the kinetic
properties of the oil prior to and following solvent injection. Pentane,
decane, and kerosene were the solvents of interest. Second, solvent was
injected in a cyclic fashion into a 1-m-long combustion tube. Then, the tube
was combusted. Hamaca oil presented good burning properties, especially
following pentane injection. The pentane extracted lighter components of the
crude and deposited preferentially effective fuel for combustion. On the other
hand, West Sak oil did not exhibit stable combustion properties without solvent
injection, following solvent injection, and even when metallic additives were
added to enhance the combustion. We were unable to propagate a burning front
within the combustion tube. Nevertheless, the experimental results do show that
this combined solvent combustion method is applicable to the broad range of oil
reservoirs with properties similar to Hamaca.
Introduction
This article investigates the effect of solvent injection on the subsequent
performance of in-situ combustion. The work is based on experimental results
obtained by a combination of these two successful in-situ upgrading processes
for viscous oils. It is envisioned that application in the field occurs first
by a cycle of solvent injection, a short soak period, and subsequent oil
production using the same well (Castanier and Kovscek 2005). By mixing with
oil, the solvent decreases the oil viscosity and upgrades the crude by
extracting in-situ the lighter ends of the crude oil. The heavy ends, that are
markedly less interesting, are left behind. Injection of solvent and oil
production occurs for a number of cycles until the economic limit is reached or
until the deposition of crude oil heavy ends damages production. The solvent
injection phase is followed by in-situ combustion that burns the heavy ends
left from the solvent injection. By switching from air to nitrogen injection,
the combustion is extinguished. Again, oil is produced by the same well used
for injection in a cyclic fashion. Combustion enhances the production by
decreasing thermally the oil viscosity and adding energy to the reservoir
through the formation of combustion gases. The combustion also upgrades the oil
through thermal cracking (Castanier and Brigham 2003).
For our experiments, two oils of particular interest were used. The first
experiments employed crude oil from Hamaca (Venezuela), where the field
location requires important costs of transporting crude to upgrading
facilities. The second set of experiments was conducted with viscous West Sak
oil (Alaska), where steam injection currently appears to be unsuitable because
of heat losses to permafrost.
While the presence of oil in the Orinoco heavy-oil belt, in Central
Venezuela, was discovered in the 1930s, the first rigorous evaluation of the
resources was made in the 1980s, and the region was divided into four areas:
Machete, Zuata, Hamaca, and Cerro Negro. It contains between 1.2 and 1.8
trillion recoverable barrels (Kuhlman 2000) of heavy and extra-heavy oil. The
9–11° API density crude is processed at the Jose refinery complex on the
northern coast of Venezuela. The cost of transporting heavy oils to the
northern coast provides an incentive to investigate in-situ upgrading. In 2003,
the total production from these projects was about 500,000 B/D of synthetic
crude oil. This figure was expected to increase to 600,000 B/D by 2005 (Acharya
et al. 2004).
West Sak is a viscous oil reservoir located within the Kuparuk River Unit on
the North Slope of Alaska. It is part of a larger viscous oil belt that
includes Prudhoe Bay. The estimated total oil in place ranges from 7 to 9
billion barrels, with an oil gravity ranging from 10 to 22°API. The reservoir
depth ranges from 2,500 to 4,500 feet, with gross thickness of 500 feet and an
average net thickness of 90 feet. The temperature is between 45 and 100°F, and
there is a 2,000-ft (600-m) -thick Permafrost layer. In March 2005, 16,000 BOPD
were produced and 40,000 BOPD are planned for 2007 (Targac et al. 2005). Within
the scope of this study, West Sak is of particular interest because there are
technical difficulties with steam injection that include (Gondouin and Fox
1991):
- Surface-generated steam passing through a thick permafrost layer; the well
would sink if the permafrost melted.
- The reservoirs consist of thin, medium-permeability layers.
- The formation may contain swelling clays that reduce the rock permeability
when exposed to steam condensate.
Solvent injection and in-situ combustion are effective in a variety of
fields. Both techniques upgrade the oil directly in the reservoir, thereby
making heavy resources easier to exploit. The combination of these two
processes is applicable at large scale to recover viscous oil, or in-situ
combustion could be applied on an ad hoc basis to clean the wellbore region,
increase the permeability, and thus act as a stimulation process.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
16 February 2006
- Meeting paper published:
22 April 2006
- Revised manuscript received:
17 July 2007
- Manuscript approved:
26 December 2007
- Version of record:
25 June 2008
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