Note: Descriptions are shown in the official language in which they were submitted.
CA 02761048 2012-10-25
Post Cold Heavy Oil Production With Sand Microbial Enhanced Oil
Recovery Method
Field of the Invention
The present invention relates generally to a process wherein nutrients and/or
microbial organisms are injected into underground formations for microbial
processes, and in particular to a microbial enhanced oil recovery ("MEOR")
process for heavy oil accumulations.
Background
Production of heavy oil from unconsolidated reservoirs, like the ones around
Lloydminster which straddles the provinces of Alberta and Saskatchewan in
Canada, continued for decades while trying to prevent sand production with
screens or gravel packs. The oil and gas industry realized in the 1980s that
if
sand production was encouraged, that oil production also increased. A non-
thermal process was developed known as Cold Heavy Oil Production with Sand
(CHOPS) in which sand and oil were produced simultaneously under primary
conditions. Progressive cavity pumps were typically deployed in a CHOPS
process and allowed sand production and higher levels of oil production to be
reached over prior approaches.
As a result of producing sand from these reservoirs, pathways of extremely
high
permeability are generated in oil producing formations. These high
permeability
pathways are known as "wormholes". As the sand production is continued,
wormholes grow larger and extend deeper into the reservoir. The presence of
wormholes has been proposed in light of the observations in these oil fields
and
from investigations through laboratory experiments (Tremblay, B., Sedgwick, G.
and Vu, D., "CT Imaging of Wormhole Growth Under Solution-Gas Drive", SPE
Reservoir Evaluation & Engineering, Vol.2, No.1, February 1999, 35-47).
Numerous tracer surveys were conducted where rapid communication was
observed between wells confirming the existence of wormhole structures.
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Pressure-buildup analyses conducted throughout the Lloydminster area showed
in-situ permeability values on the order of tens of darcies, which was much
higher than anything measured in the laboratory (Smith, G. E., "Fluid Flow and
Sand Production in Heavy Oil Reservoirs Under Solution Gas Drive", SPE
Production Engineering, May 1988, 169-180). Such values are theorized to be
due to the flow through the high permeability channels or wormholes. The test
results also indicated that wells have very large "wellbore storage," even for
the
wells that were shut in downhole. Furthermore, laboratory experiments showed
that a stable wormhole can develop in unconsolidated heavy oil sands and that
the wormholes most likely develop in a higher porosity region with lower
cohesive
strength (Tremblay et al., supra). Tracer tests conducted by injecting in one
well
and detecting the arrival times in the surrounding wells sometimes indicated
travel times in the order of hours, lent further credence to the existence of
wormholes in the reservoir. It is thought that near the wellbore a denser
network/dilated region is formed and a few of these wormholes grow up to 50 to
200 m in length in time (Smith, supra). Figure 1 (PRIOR ART) shows a schematic
of aerial view of a CHOPS well 1 with associated wormhole network 2.
Solution gas drive in these reservoirs involves simultaneous mixture flow of
gas
as very tiny bubbles entrained in viscous heavy oil, also called foamy oil
flow.
Foamy oil flow is a result of nonequilibrium thermodynamics. Therefore, two
significant mechanisms which are theorized to affect the flow of heavy oil and
its
recovery in these reservoirs are the foamy oil flow and wormhole formation
(Sawatzky, R., Lillico, D.A., London, M., Tremblay,B.R., and Coates, R.M.,
"Tracking Cold Production Footprints"; paper 2002-086, presented at the
Canadian International Petroleum Conference, Calgary, AB, June 11 - 13, 2002).
The primary CHOPS production wells come to the end of their lives either due
to
pressure depletion or due to excessive water influx. In general, the primary
recovery in heavy oil reservoirs ranges between 3 to 10% with average of
around
5% recovery (Smith, supra). Although a few enhanced oil recovery ("EOR")
techniques have been tried, currently there are no widely applicable
commercial
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FOR techniques to increase the recovery of cold heavy oil beyond the primary
levels.
Water flooding of heavy oil is inefficient. Water will bypass the oil and
breakthrough at the producers early in the life of the flood because of
viscous
instabilities resulting from the adverse mobility contrast between water and
heavy
oil. Many of these reservoirs are relatively small or thin, and possibly have
existing primary production wormholes. Consequently, these reservoirs are not
prime candidates for expensive thermal or miscible hydrocarbon solvent FOR
technologies. Wormholes negatively affect water flood performance as well
(Bryan, J., Mai, A., and Kantzas, A, "Processes Responsible for Heavy Oil
recovery by Alkali/Surfactant Flooding", JPT, January 2009, 52-54).
Considerable water is produced sometimes in these reservoirs during primary
operations. As long as water production is low, quite high sand cuts can be
tolerated by the production system. If wormholes reach a water source, water
will
short circuit through them and the well will be suspended. Many sudden
failures
in injection schemes (firefloods, water floods, and steam floods) and in
drilling
and workover operations are also blamed on wormholes.
Summary of the Invention
According to one aspect of the invention, a post-CHOPS MEOR method
comprises selecting a well in communication with a reservoir having at least
one
wormhole and that is being subjected to or has completed primary CHOPS
production and determining whether the reservoir contains a sufficient amount
of
a gas-producing indigenous microbe to re-pressurize a drainage portion of the
well to a target pressure or to generate a target amount of gas. When the
reservoir does not contain a sufficient amount of the indigenous microbe, then
an
injectant is prepared comprising a sufficient amount of a gas-producing
microbe
to re-pressurize the drainage portion of the well to the target pressure or to
generate the target amount of gas, a nutrient suitable for the microbe, and a
fluid
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base. When the reservoir does contain a sufficient amount of the gas producing
indigenous microbe, then an injectant is prepared comprising a suitable
nutrient
for the indigenous microbe and the fluid base. The injectant is injected
through
the well and into the wormhole(s) in the reservoir, the well is shut in until
the
pressure in the well reaches the target pressure or the target amount of gas
is
generated, and the well is produced.
When the reservoir does not contain a sufficient amount of the indigenous
microbe, the injectant can comprise a gas producing microbe selected from the
group consisting of an exogenous microbe and a cultivated microbe that has
been cultivated from a sample of the indigenous microbe in the well. The
exogenous microbe can be selected from a group consisting of: clostridium,
desulfovibrio, pseudomonas, methanogens, and anaerobic fermenters. The
nutrient for the exogeneous microbe can be a carbohydrate source other than
residual hydrocarbons in the well. In particular, the nutrient can be selected
from
a nutrient group consisting of: molasses, sugar plant waste, malting waste,
and
manure. When the exogenous microbe is a methanogen or an anaerobic
fermenter, the nutrient group can further consist of NaNO3, KNO3, NH4NO3,
K2PO4, NH4CI, folic acid, ascorbic acid and riboflavin.
The injectant can be injected through the well and into the reservoir at a
temperature equal to the reservoir temperature, and at a pressure greater than
the reservoir pressure and less than the formation fracturing pressure of the
reservoir.
The sufficient amount of the microbe can be an amount required for the microbe
to re-pressurize the drainage portion of the well to the target pressure
within a
selected period of time. Alternatively, the sufficient amount of the microbe
can
be an amount required for the microbe to produce the target amount of biogas
within a selected period of time. The target pressure can be the initial
reservoir
pressure during primary production, the target amount of biogas can be
2,000,000 m3 and the selected period can be between six months and one year.
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When the reservoir does contain a sufficient amount of the indigenous microbe,
the method can further comprise selecting a sufficient amount of nutrient for
the
indigenous microbe to cause the indigenous microbe to generate a sufficient
amount of biogas to re-pressurize the drainage portion of the well to the
target
pressure within a selected shut-in period, or produce the target amount of
biogas
within the selected shut-in period. The target pressure can be the initial
reservoir
pressure during primary production and the target period can be between six
months and one year.
According to another aspect of the invention, there is provided a post CHOPS
MEOR method comprising: selecting a well in communication with a reservoir
having at least one wormhole and that is being subjected to or has completed
CHOPS production; preparing an injectant comprising a sufficient amount of a
gas-producing exogenous microbe to re-pressurize a drainage portion of the
well
to a target pressure or to generate a target amount of gas, a nutrient
suitable for
the exogenous microbe, and a fluid base; injecting the injectant through the
well
and into the at least one wormhole in the reservoir; shutting in the well
until the
pressure in the well reaches the target pressure or the target amount of gas
has
been generated; and producing the well.
According to yet another aspect of the invention, there is provided an
apparatus
for carrying out a post CHOPS MEOR method on a reservoir having at least one
wormhole and that is being subjected to or has completed primary CHOPS
production. The apparatus comprises: an injectant tank comprising a fluid
mixture
of an aqueous fluid base, a nutrient for a gas-producing microbe, and
optionally
the gas producing microbe; a fluid conduit fluidly coupling the injectant tank
to a
wellhead of a well in fluid communication with the reservoir; a pump fluidly
coupled to the fluid conduit and operable to inject the injectant into the
well at a
pressure sufficient to deliver the injectant into reservoir and at least one
wormhole. The pump can be configured to inject the injectant at a pressure
between the reservoir pressure and a formation fracturing pressure of the
reservoir.
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Brief Description of Drawings
Figure 1 is a schematic of an aerial view of a CHOPS well with an associated
wormhole network (PRIOR ART).
Figure 2 is a flowchart of steps in a post-CHOPS MEOR process according to a
first embodiment of the invention.
Figure 3 is a schematic of equipment used to carry out the post-CHOPS MEOR
process.
Figure 4 is a flowchart of steps in a post-CHOPS MEOR process according to a
second embodiment of the invention.
Detailed Description of Embodiments of the Invention
The embodiments described herein relate generally to a process wherein
nutrients and/or microbial organisms are injected into underground formations
for
microbial processes. In particular, the described embodiments provide a method
of injecting microbial organisms and/or nutrients into wormholes in a heavy
oil
formation that is undergoing or has already undergone primary CHOPS
production (hereinafter referred to as "post-CHOPS MEOR process"). Typically,
the post-CHOPS MEOR process will be carried out on the well after primary
CHOPS production has been completed. Alternatively, the post-CHOPS MEOR
process can be introduced during primary CHOPS production (i.e. after primary
CHOPS production has started but before its economic end is reached) when a
significant amount of wormhole network has been generated. One measure of
determining when such a wormhole network exists is to measure the cumulative
sand production from the well. Alternatively, the pressure build-up in the
well can
be analyzed to determine the in-situ permeability of the reservoir. In
particular, it
is expected that a sufficient wormhole network has been developed when several
hundred cubic meters of sand has been cumulatively produced, and/or when the
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in-situ reservoir permeability is determined to be in the order of tens of
darcies. In
particular, a significant network is expected to be generated when more than
200
m3 of sand has been cumulatively produced or when the in-situ permeability has
been determined to be above ten darcies.
The presence of wormholes makes it possible to inject sufficient quantities of
microbial organisms and nutrients into the heavy oil formation in a fashion
that is
concentrated around the wellbore and generate biogas in situ. Wormholes also
allow for the generated biogas to contact heavy oil over a large surface area
created by wormholes. Examples of the biogas generated can be hydrogen,
methane, or carbon dioxide. The gas generated within wormholes in the
formation re-pressurizes the reservoir providing additional energy to push
more
oil towards the producer. Some of the generated biogas in the wormholes will
be
dissolved in heavy oil and upon production enhance the solution gas drive
mechanism resulting in enhanced foamy oil flow.
High viscosity of the heavy oil presents challenges in the production of these
reservoirs. In order to avoid further degrading the heavy oil in the formation
by
the injected microbial organisms, injected nutrients are preferentially
consumed
by the injected bacteria.
If the heavy oil reservoir does not contain any wormholes because the sand
production has been prevented or because there has been no primary
production, then the injection of microbial organisms and nutrients will face
the
same disadvantages as a water flood does. Injected microbial organism
solutions
and the nutrients will have significantly more mobility than the heavy oil in
the
formation. The results will be that significant portions of the heavy oil in
the
formation will be bypassed in the form of viscous fingers because of the
adverse
mobility ratio between the heavy oil and the injected fluids. The viscous
fingers of
microbes and nutrients will penetrate into the formation far away from the
wellbore thus spreading the biogas generated over a large area. In this case,
it
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will be impossible or uneconomic to re-pressurize the reservoir over such a
large
area.
The current embodiments overcome this problem by placing the microbial
organisms and/or nutrients within the existing wormholes generated by CHOPS.
As the wormholes are mostly within a 50 to 200 m radius of the wellbore, being
denser near the wellbore and less dense away from the wellbore, the immediate
vicinity of the wellbore will largely be affected.
First, the formation water in the reservoir coupled to the wellbore is
analyzed and
then a determination is made as to whether suitable indigenous gas generating
microbial consortia are present in the reservoir. If not present, then a first
embodiment of the post-CHOPS MEOR process is performed which involves
injecting both exogenous microbes and nutrients for those microbes into the
reservoir. If present, then a second embodiment of the post-CHOPS MEOR
process is performed which involves only injecting nutrients for the
indigenous
microbes into the reservoir. If the indigenous microbes are not present in
sufficient quantities, then they can be cultivated and injected.
Referring now to Figure 2 and according to a first embodiment, a single well,
or
multiple wells in communication being operated in a huff-and-puff mode, which
have undergone a primary CHOPS process ("post-CHOPS wells") are selected
for the post-CHOPS MEOR process (step 10). Then, gas producing species of
microbial organisms ("exogenous microbes") and suitable nutrients for these
microbes are selected for the post-CHOPS MEOR process, and prepared for
injection (step 12). The exogenous microbes and their nutrients can be
selected
from those gas-producing species of microbes and their nutrients currently
used
in conventional MEOR processes and include but not limited to: Clostridium,
Desulfovibrio, and Pseudomonas. These bacteria can ferment carbohydrates to
produce biogas. Therefore, carbohydrate sources such as molasses, sugar plant
waste streams, malting wastes, manure and others that contain all the
necessary
nutritional components (e.g. carbon, nitrogen, phosphorous, etc.) are suitable
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nutrients can be injected into the formation along with the microbes. Other
suitable microbes include methane producing (methanogen) and CO2 producing
microbes (anaerobic fermenters). These microbes will have fast enough rates of
growth and gas production using low cost carbon sources other than oil, in the
severe high salinity and hydrocarbon containing conditions of the post-CHOPS
wells. Types of low cost carbon sources to serve as nutrients for these
microbes
include: molasses, sugar plant waste streams, malting wastes, sugar, manure,
and residual hydrocarbons in the reservoir. Other additives could include:
nutrients containing nitrogen and phosphorous such as NaNO3, KNO3, NH4NO3,
K2PO4, NH4CI, vitamins such as folic acid, ascorbic acid and riboflavin, and
trace
elements.
The selected exogenous microbes can be cultivated from exogenous samples.
Alternatively, the exogenous microbes can be cultivated from naturally
occurring
microbes such as methanogens and anaerobic fermenters, which can be isolated
from samples taken at the well or other sites where microbes tolerant to high
salt
and to hydrocarbons are prevalent and appropriate nutrients to grow them are
determined.
Referring to Figure 3, an injectant delivery system is fluidly coupled to a
post-
CHOPS well 6 and serves to inject an injectant comprising a fluid mixture of
nutrients and exogenous microbes in an aqueous fluid base into the well 6. The
injectant delivery system 2 comprises an injectant tank 4 for containing the
injectant, a fluid conduit 5 in fluid communication with the tank 4 and a
wellhead
of the post-CHOPS well 6, a pump coupled 8 to the conduit 5, and a control
valve
9 coupled to the conduit 5 near the wellhead. The post-CHOPS well(s) 6 extend
downhole and are in fluid communication with a subsurface reservoir 3
containing the wormholes.
The injectant is prepared by mixing the selected microbial organisms and
suitable nutrients with an aqueous fluid base in a mixing tank 4 to form a
microbe
/ nutrient fluid mixture. A suitable concentration of exogenous microbes and
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nutrients in the injectant is determined by selecting a sufficient amount of
exogenous microbes that would generate commercially viable amounts of biogas
in the reservoir formation; one definition of a commercially viable amount of
biogas is enough biogas to significantly repressurize a drainage area of the
well
6 to a target pressure within a selected period of time. For example, a
selected
amount of exogenous microbes can be selected to generate enough biogas to
re-pressurize a drainage area of the well 6 to about the initial reservoir
pressure
during primary CHOPS production within six months. Alternatively, a
commercially viable amount of biogas can be defined by the volume of biogas
produced within a certain period of time, and can for example be about
2,000,000
m3 of biogas produced within six months of shut-in.
Once the amount of exogenous microbes has been selected, a suitable amount
of nutrients can be selected, which is the amount that needs to be injected
into
the well 6 to enhance that rate of gas generation by the exogenous microbes to
produce the expected amount of biogas within the selected period of time.
Referring again to Figure 2, the injectant is injected into the post-CHOPS
well 6
at a selected injection pressure and a selected injection temperature (step
14). In
most cases, it is expected that the selected injection temperature will be at
or
about the same as the reservoir temperature (about 15 C in Lloydminster area).
The selected injection pressure should be greater than the reservoir pressure
and be sufficient to cause the injectant to flow easily into the reservoir 3
and
preferentially fill the wormholes but not cause the reservoir formation to
fracture,
i.e. be below the formation fracturing pressure of the reservoir. Injection
continues until there is enough injectant in the reservoir 3 to generate a
target
amount of biogas within the selected period of time.
After the selected amount of injectant has been injected into the well 6, the
well 6
is shut in for the selected period of time, to allow biogas to be generated
within
the wormholes (step 16). Conventional means for shutting in wells (not shown)
can be used as is known in the art. The selected period of time, i.e. the shut
in
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period, can be selected by the operator at its preference, and can be for
example
between six months and one year. As noted above, once the shut-in period has
been selected, the amount of injectant can be calculated that is required to
produce enough biogas to reach the target pressure within the selected shut-in
period or produce enough biogas within the shut-in period. The target pressure
in
this embodiment is substantially the same as the initial reservoir pressure
during
primary CHOPS production; however, the MEOR process can work at pressures
below the initial reservoir pressure, although this would present less than
ideal
conditions. During the shut in period, the reservoir pressure is monitored and
when the pressure is observed to be close to the target pressure, the shut-in
period is ended.
After the shut-in period has been completed the well 6 is then put on
production
and oil and gas is produced (step 18). The well 6 is produced in the same
manner as a conventional CHOPS well with progressive cavity pumps during
primary CHOPS production.
According to a second embodiment and referring to Figure 4, the post-CHOPS
MEOR process utilizes indigenous gas-producing microbes, i.e. microbes
already present in the reservoir 3. In this embodiment, a determination is
made
as to whether any indigenous gas generating microbial consortia (hereinafter
referred to as "indigenous microbes") are present in the reservoir 3 (step
20). If
the formation contains suitable species of indigenous microbes and in
sufficient
concentrations, then one or more nutrients suitable to these indigenous
microbes
are selected (step 22) and mixed with an aqueous fluid base in the mixing tank
4
to form the injectant (step 24). This injectant is injected by the injectant
delivery
system 2 into the reservoir 3 (step 26) to promote gas production of the
indigenous microbes. Once sufficient injectant has been injected, the well is
shut
in for a selected period. Once the shut in period has been completed, the well
is
produced (Step 28).
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To determine the amount of nutrient to be injected into the well 6, the
formation
water in the reservoir 3 is analyzed and then a determination is made of the
total
amount of nutrient that needs to injected at a well head to enhance the rate
of
gas generation by the indigenous microbes so as to provide for commercially
viable amounts of biogas in the reservoir formation. In one embodiment,
commercially viable amounts would include enough biogas generation to
significantly repressurize a drainage area of the well within six months. In
this
embodiment, the amount of biogas generated during this time frame is expected
to be 2,000,000 m3.
The nutrients should be added to the reservoir 3 in a manner that does not
significantly alter the bulk salinity and make-up of the formation water while
at the
same time allowing rapid dispersion of the nutrients into as much of the
formation
water as possible.
The maximum concentration of nutrient in the well should be determined such
that the nutrient concentration is non-lethal to the gas generating microbial
consortia but enhances gas generation. In determining this maximum
concentration, consideration should be given to the amount of nutrients
already
in the reservoir such that the total nutrient concentration does not exceed
the
lethal concentration.
If a suitable species of indigenous microbial consortia is present in the
reservoir
but in insufficient quantities, then according to an alternative embodiment, a
sample of these indigenous microbes can be extracted from the reservoir and
cultivated in a facility. Once a sufficient amount has been cultivated, the
cultivated microbes are mixed with a suitable nutrient and fluid base in the
mixing
tank to form an injectant. The injectant is then injected into the reservoir
in a
manner according to the first embodiment.
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