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Abstract
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A
process and system are described for producing a synthetic crude oil by
contacting a synthesis gas with a combination of a synthesis gas conversion
catalyst and a hydroconversion catalyst in a synthesis gas reactor. The
synthesis gas can be obtained from gas associated with crude oil production,
i.e., associated gas, in a synthesis gas generator. The synthetic crude oil
can be blended with a natural crude oil to produce a blended stabilized crude
oil having 2 wt % or more of the synthetic crude oil. The resulting blended
stabilized crude oil has improved flow characteristics including a pour point
of 30° C. or less.
BACKGROUND
OF THE INVENTION A stream of produced fluids containing hydrocarbon products
produced from a subterranean reservoir contains several components that must
be separated: a stabilized crude oil generally having a vapor pressure of
14.7 psia or less, condensate, liquefied petroleum gas (LPG) and methane.
Condensate refers to a light hydrocarbon mixture that is separated from
stabilized crude oil. It typically contains pentane, hexane, and can contain
small amounts of butane. These more volatile condensates are often shipped
separately from stabilized crude oil. LPG refers to propane, butane and
mixtures thereof. In addition to these components, other components that are
frequently separated are ethane and water. Further, contaminants such as
sulfur and other non-carbon and non-hydrogen elements may also be separated
out of the crude oil and gases. A significant amount of capital must be spent
for facilities to separate hydrocarbon containing produced fluids into these
components. Heavy and/or waxy crude oils are found in many locations around
the world, and are attractive for their low cost per barrel and impressive
yield of high-value products. However, high pour points and challenging flow
properties complicate the process of transporting these crudes from oilfield
to refinery. While transportation by pipeline is usually the preferred
method, most heavy crudes are extremely time and temperature-sensitive and
must be transferred via insulated or heated tankers to prevent
solidification. The addition of chemical pour point depressants or “lighter”
petroleum products is a common method of improving flow characteristics of
crude oils and thereby alleviating this problem. Pour point is the lowest
temperature at which a liquid will not flow, or the temperature at or below
which the liquid loses its ability to flow. It is desirable to keep crude oil
at least 11° C., and preferably 22° C., above its pour point during
transportation and storage. The lowest sea temperatures are typically around
10° C. To avoid having to heat ships, lines, tanks and the like, crudes
should have a pour point less than 30° C., depending on the part of the
world. Typically, the amount of methane or “associated gas” produced along
with crude oil is insufficient to justify conversion to Liquefied Natural Gas
(LNG). However options to handle associated gas are limited. Natural gas
often cannot be burned (flared) due to increasingly stringent regulations
around greenhouse gas emissions. Also, natural gas often typically is not
reinjected into a producing formation as this dilutes the crude oil and leads
to a loss in crude oil production. Local uses, such as combustion of the
natural gas for facilities uses, are insufficient to consume this gas. One
technology currently used to handle associated gas that cannot be flared,
reinjected or used in local markets is to convert the associated gas into
synthetic fuels such as diesel, jet fuel, and naphtha by a Fischer-Tropsch
process. Conventional Fischer-Tropsch processes make a waxy product that is
subsequently converted via hydroconversion into premium quality
transportation fuels. Gas conversion to these products via the
Fischer-Tropsch process is well known such as is described in U.S. Pat. No.
7,479,216. The conventional Fischer-Tropsch conversion process is an
extremely exothermic and expensive process and when used on associated gas,
facilities distinct from those for crude oil must be used to handle the
premium Fischer-Tropsch diesel, jet fuel, condensate and other products. For
oilfields with high gas/oil ratio (GOR), gas-to-liquids (GTL) conversion via
Fischer Tropsch synthesis is economically unfavorable and may require more
space and resources than are available. Likewise, the wax from the Fischer-Tropsch
process has such a high melting point that the conventional Fischer-Tropsch
product cannot be shipped in conventional crude tankers, but instead,
requires expensive ships suitable for handling this high melting temperature
material. As described in U.S. Pat. Appln. No. 2006/0069296, conventional
crude tankers are often limited to material having pour points at or below
140° F. (60° C.). Blending the wax from the Fischer-Tropsch process into
crude oil is not an option either. Blending as little as 2 wt % of a
conventional Fischer-Tropsch product into some crude oils may increase the
pour point above 60° C. Also, conventional Fischer-Tropsch products contain
substantial quantities of olefins, alcohols and acids. When blended with
crude oil these Fischer-Tropsch products can cause the crude oil to be
difficult to refine and may lead to a discount in the crude sale price. There
is a need for a low-cost process to convert associated gas from a stream of
produced fluids produced from a subterranean formation into a low-impurity
synthetic crude oil while avoiding the difficulties caused by wax content.
There is a further need for such a process to convert associated gas to a
synthetic crude oil that can be blended with natural crude oil, wherein a
blended stabilized crude oil having a pour point at or below 30° C. is
produced.
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