Note: Descriptions are shown in the official language in which they were submitted.
CA 02782313 2012-07-06
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METHODS FOR INCREASING METHANOGENESIS IN SUBSURFACE
RESERVOIRS
BACKGROUND OF THE INVENTION
[0001] Subsurface oil reservoirs are promising and important sources for the
generation and
collection of increasing amounts of hydrocarbons as an energy source. In
particular, since at
best about 40% of oil is recoverable from oil reservoirs, methods for the
recovery of the
remaining oil in the reservoirs either as oil or as methane are of great
interest. Microbes are
responsible for biodegrading conventional crude oil to intermediates that are
converted by
other microbes (methanogens) into methane leaving behind heavy oil, which can
be then
further biodegraded. The processes of heavy oil formation and methane gas
generation
(methanogenesis) occur over geological time scales of millions of years.
Microbial
conversion of oil to methane involves a variety of multi-step pathways and a
consortium of
microorganisms working together in concert. Pathways include fermentation of
oil to
acetate, CO2 and hydrogen, as well as microbial oxidation of acetate to CO2
and hydrogen
and subsequent microbial reduction of CO2 to methane with hydrogen. Other
pathways
include direct fermentation of acetate to methane. Each of these biochemical
transformations
are carried out by specific types of microbes and often take place in the face
of competing
reactions such as conversion of methane and the acetate and hydrogen
intermediates to other
products such as H2S, H2O, and CO2.
[0002] It has been recognized that the naturally slow methanogenic
biodegradation process
can be accelerated to enhance methane production in the reservoir. For
example, as
described in WO 2005/115649 entitled "Process for Stimulating Production of
Methane from
Petroleum in Subterranean Formations", techniques are described for injecting
one or more
agents into a reservoir in which methanogenic microbial consortia are present
to modify the
reservoir environment to promote in situ microbial degradation of petroleum,
promote
microbial generation of methane, and to demote in situ microbial degradation
of methane.
[0003] While that reference describes generic procedures for enhanced methane
production,
there is still a need for improved methods for stimulating methane production
to maximize
the potential of subsurface oil reservoirs as sources of hydrocarbons. Adding
the proper
amounts and types of agents and nutrients to optimally stimulate methane
production from
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the particular petroleum components found in an oil reservoir remains a
considerable
challenge in view of the variety of microbes and pathways involved, including
microbes that
may be present in the reservoir but that do not participate or are deleterious
to methane
production. For example, stimulation and the growth of non-methanogens may out-
compete
methanogens for common intermediates essential to methane production or drive
microbial
transformations to products (e.g. CO2, H2S, etc.) other than methane.
BRIEF SUMMARY OF THE INVENTION
[0004] This invention is based, in part, on the discovery that significant
enhancement in
methane generation from oil reservoirs can be achieved. In one aspect, very
high
concentrations of nutrients shown to be effective for microbial conversion of
oil to methane
are added as a bolus into the reservoir thereby enhancing the total amount of
nutrients
available to the microbes in a manner such that methane generation is
maximized while at the
same time ensuring that the concentration of nutrients in the reservoir is non-
lethal to the
microbes.
[0005] In a preferred embodiment, the methane production levels in the
reservoir are
increased significantly such that methane production in the reservoir is
exponentially
increased over endogenous rates. Under optimal conditions, methane production
can be
increased to a level of at least 25 MCF and preferably at least 50 MCF per
day.
[0006] In view of the above, in one of its embodiments, this invention is
directed to a
method for enhancing methanogenesis in an oil reservoir comprising
methanogenic microbial
consortia and foundation water which method comprises adding a solution
comprising
methanogenic nutrients which comprise at least nitrogen and phosphate ions
wherein the amount of said nutrients in the solution is sufficient to enhance
the rate of
methanogenesis in said reservoir at its final dilution without being lethal to
said
methanogenic microbial consortia, and
further wherein the added concentration of said nutrients in said solution
facilitates
dispersion of the nutrients from the said solution into the foundation water.
[0007] The solution of nutrients is preferably an aqueous solution and can
occur in a single
injection or in an iterative process where the aqueous solution is divided
into several
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injections wherein each injection contains a known concentration of nutrients.
In one
preferred embodiment, after each injection, the concentration of nutrients in
the foundation
water is determined and adjustments are made to each additional injection to
achieve the
desired concentration of nutrients in the foundation water.
[0008] In a preferred embodiment, the amount of water used to inject the
nutrients is
minimized such that the injected water does not significantly alter the
composition of the
foundation water but for the nutrients added. That is to say that the salinity
of the foundation
water will not vary by more than 1% and preferably not more than 0.1% after
injection as
compared to prior to injection except for changes in concentration of the
nutrients added.
Examples of characteristics which are not significantly altered include
salinity, pH,
temperature, non-nutrient ion concentrations, etc.
[0009] When higher concentrations of nutrients in the water are injected, the
volume of
reservoir penetration by nutrient at concentration levels sufficient to
provoke the desired rate
methanogenesis acceleration is increased. As a corollary, nutrient
concentration diminishes
the further from the point of nutrient injection which, in turn, lowers the
amount of nutrient
available to the methanogenic consortia.
[0010] This invention is predicated in part on the discovery that increasing
concentrations to
levels previously believed to be deleterious to methanogenesis actually is
beneficial as it
increases the distance penetration by the injection water/ nutrient mix before
the quantity of
nutrient available is too low to provoke the desired rate of methanogenesis
acceleration. As a
consequence the quantity of methane produced from an injection point is
increased. This, of
course, is an important benefit for provoking sufficient hydrocarbon
production to make the
process commercially viable. In one embodiment, the concentration of ammonium
ions
(NH4) added is up to the saturation concentration and the concentration of
phosphate ions
(H2PO4-) added is up to the saturation concentration. One could also use less
than the
saturated concentration at one or more well heads. The concentration of
ammonium and
phosphate ions is selected so as to achieve the ideal concentration to provoke
the desired rate
of methanogenesis acceleration.
[0011] In one preferred embodiment, the concentration of ammonium ions added
to the
reservoir is sufficient that at least a portion of the foundation water has a
concentration of
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ammonium ions of about 5.4 g/L (typically ammonium chloride) and a
concentration of
phosphate ions (H2PO4") of about 1.376 g/L (typically a potassium or sodium
salt).
[0012] Surprisingly, it has been shown that the methanogens are hardy microbes
and
tolerate concentrations of nutrients heretofore thought to be toxic. This
finding allows for a
significant increase in the amount of nutrients which can be added to the
reservoir.
[0013] The term "saturation concentration" refers to either the concentration
of the nutrient
itself or as a complex with a sequestering agent as is well known in the art.
[0014] In evaluating the increase in methanogenesis, one can maintain the
reservoir under
conditions wherein methane concentration produced at the well head can be
measured.
Preferably, the amount of methane produced is at least 0.7 MCF per day. If the
pressure
increase is insufficient, additional nutrients can be added as necessary.
Alternatively, the
reservoir can remain closed and the methane produced can increase the pressure
in the
reservoir in either a localized or a reservoir wide manner.
[0015] In another embodiment, there is provided a method for increasing the
rate of
methanogenesis in a petroleum reservoir comprising methanogenic microbial
consortia and
foundation water which method comprises:
a) injecting through a well head a solution of stimulants comprising
ammonium
and phosphate ions to the reservoir in an amount such that their concentration
in the reservoir
is above the critical concentration to effect enhanced methanogenesis but
below a lethal
dosing to the methanogenic microbial consortia; and
b) maintaining said reservoir under conditions such that the rate of
methanogenesis is increased,
wherein the concentration of the ammonium ions injected into the reservoir is
up to
about saturation concentration and the concentration of phosphate ions
injected into the
reservoir is up to about saturation concentration.
[0016] In another embodiment, there is provided a method for increasing the
rate of
methanogenesis in a petroleum reservoir comprising methanogenic microbial
consortia and
foundation water which method comprises:
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a) injecting through a well head a solution of stimulants
comprising ammonium
and phosphate ions to the reservoir in an amount such that their concentration
in the reservoir
is above the critical concentration to effect enhanced methanogenesis but
below a lethal
dosing to the methanogenic microbial consortia; and
b) maintaining said reservoir under conditions such that the rate of
methanogenesis is increased,
wherein the concentration of the ammonium ions injected into the reservoir is
from
about 1 g/L to its saturation concentration and the concentration of phosphate
ions injected
into the reservoir is from about 0.4 g/L to its saturation concentration.
[0017] Preferably, the amount of nutrient enriched solution added to the
reservoir is such
that the salinity of the reservoir does not change by more than 1% and more
preferably by no
more than 0.1% once equilibrium is established. In another preferred
embodiment, the
temperature of the nutrient enriched solution is maintained at approximately
the temperature
of the foundation water in the reservoir to which it is being added. In yet
another preferred
embodiment, the amount of nutrient added is from about 1 g/L to its saturation
concentration
or about 3 g/L to its saturation concentration of ammonium ions and from about
0.4 g/L to its
saturation concentration or about 1.5 g/L to its saturation concentration of
phosphate ions. In
one preferred embodiment, the concentration of nutrients added provides for a
maximal zone
(volume) in the foundation water of an ammonium ion concentration of about
5.400 g/L of a
phosphate ion concentration of about 1.376 g/L. Such can be done by a single
addition or
multiple additions at single or multiple well heads.
[0018] In one embodiment, the temperature of the solution of stimulants is
maintained at
approximately the temperature of the foundation water in the reservoir to
which it is being
added.
[0019] Preferably, the solution is an aqueous solution.
[0020] In an optional embodiment, a second solution can be injected into the
reservoir
through the well head to enhance methanogenesis. Such a second solution
comprises an
inhibitor or a mixture of inhibitors wherein the amount of the inhibitors in
the second
solution is sufficient to maintain a rate of methanogenesis in said reservoir
in conjunction
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with the added nutrients and wherein the amount of inhibitors added to said
reservoir water is
non-lethal to said microbes.
[0021] In one embodiment, the inhibitors are included in an aqueous solution
and, in
another embodiment, the inhibitors are combined with the solution of
stimulants. In another
embodiment, the inhibitors are included in a separate aqueous solution from
that of the
stimulants.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 shows the various microbial mediated pathways that convert
hydrocarbons to
methane via an acetate intermediate. Also shown are pathways that degrade
methane and
acetate.
DETAILED DESCRIPTION OF THE INVENTION
[0023] In this specification and in the claims that follow, reference will be
made to a
number of terms that shall be defined to have the following meanings unless
specified
otherwise:
Definitions
[0024] As used in the specification and claims, the singular form "a", "an"
and "the" include
plural references unless the context clearly dictates otherwise.
[0025] As used herein, the term "comprising" is intended to mean that the
compositions and
methods include the recited elements, but not excluding others.
[0026] As used herein, the term "about" when used in association with a
measurement, or
used to modify a value, a unit, a constant, or a range of values, refers to
variations of +/- 3%.
It is to be understood, although not always explicitly stated that all
numerical designations
are preceded by the term "about". Accordingly all numerical designations,
e.g., pH,
temperature, time, concentration, and molecular weight, including ranges, are
approximations
which are varied (+) or (-) by increments of 3%. It also is to be understood,
although not
always explicitly stated, that the reagents described herein are merely
exemplary and that
equivalents of such are known in the art.
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[0027] The term "ppm" or "parts per million" as used herein refers to the mass
ratio of
solutes to water multiplied by one million. 1 ppm is equivalent to 1 mg/L.
[0028] The terms "methanogenic microorganisms" or "methanogenic microbes"
refer to
those microbes or combination of microbes that produce methane from oil in
hydrocarbon
reservoirs. Such microbes are anaerobic and, accordingly, exogenous oxygen is
contra-
indicated during any injection. Methanogenic microbes are well known in the
art and
include, by way of example only, Methanocalculus spp., Methanogenium spp.,
Methanoculleus spp., members of the Methanosarcinales (all methanogenic
archea), and
associated syntrophic organisms providing acetate and hydrogen for the
methanogens these
including Syntrophus spp., Smithella spp. Marinobacter spp., Syntrophobacter
spp.,
Syntrophomonas spp. (all syntrophic bacterial partners that may convert
hydrocarbons to
substrates for methanogenic archaea) and the like.
[0029] The term "non-lethal" means that after addition of the stimulant
solution and
optionally the inhibitor solution, a viable population of methanogenic
microbes remain in the
foundation water. Even if some amount of methanogenic microbes may die, as
long as there
is a viable population of methanogenic microbes in the foundation water, the
level of
stimulant solution added is considered "non-lethal."
[0030] The term "nutrient" or "methanogenic nutrient" (sometimes referred to
herein as
"stimulant") refers to a component or mixture of components such as gases,
inorganic or
organic ions including anions, cations and combinations thereof (salts) which
stimulate the
activity and/or facilitate growth of one or more methanogenic microbes. To
facilitate growth,
the nutrients can supply one or more key nutritional components to one or more
of the
microbes comprising the consortium of methanogenic microbes.
[0031] The nutrient can be either an endogenous nutrient already present in
the foundation
water or an exogenous nutrient ¨ one which is not present in the foundation
water.
[0032] In one embodiment, the nutrient is an inorganic salt and more
preferably is an
inorganic salt selected from one or more of NH4C1, KH2PO4, FeSO4.7H20,
MnC12.4H20,
CoC12.6H20, NiC12.6H20, CuC12.2H20, ZnSO4.7H20, Na2Mo04. H20, Na2Se03.5H20,
Na2W04.2H20, POx compounds were x is 2, 3 or 4, Na3PO4, K3PO4, KH2PO4, K2HPO4,
NaH2PO4, Na2HPO4, H3PO4, H3P03, H3P02, C1-C20 alkyl phosphate compounds, (C1-
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CA 02782313 2012-07-06
C20)3trialkyl phosphate such as triethyl phosphate), tripoly phosphates,
condensed forms of
phosphoric acid, including tripolyphosphoric acid, pyrophosphoric acid, salts
of condensed
phosphoric acids, e.g., potassium or sodium tripolyphosphate, and the like.
Both hydrated
and anhydrous forms may be used.
[0033] Components heretofore considered as nutrients have been found to
deleterious to
methanogenesis and in a preferred embodiment are excluded from the nutrient
composition.
Such components include sulfate, nitrate, nitrite, and oxygen.
[0034] The term "inhibitor" refers to a component or mixture of components
such as
inorganic or organic compounds including anions, cations and combinations
thereof (salts)
which inhibit one or more microbial reactions which either degrade methane
and/or inhibit
one or more reactions which divert the petroleum components in the reservoir
into products
other than methane ("competing reactions"). Such inhibitors can be components
that
interfere with one or more of these competing reactions or which are
selectively toxic to non-
methanogenic microbes. Preferably, such inhibitors are one or more components
that
interfere with such competing reactions. In a preferred embodiment, the
inhibitor is an
inorganic salt and more preferably is a molybdate salt such as sodium
molybdate (Na2M004),
and hydrates thereof which are inhibitors of sulfate reducers, and sodium
chlorate (NaC103)
for inhibiting nitrate reducers.
[0035] The inhibitor can be either an endogenous inhibitor ¨ one which is
already present in
the foundation water or an exogenous inhibitor ¨ one which is not present in
the foundation
water.
[0036] The term "non-nutrient" refers to components which are not nutrients or
inhibitors.
Such non-nutrients include sodium chloride and other salts which affect the
salinity of the
water in the reservoir. In general, the microbes are adapted to the salinity
of the foundation
water. The injection strategy seeks to maintain essentially the same gross
salinity of the
foundation water after injection as was present prior to injection.
[0037] The term "petroleum components suitable for methanogenesis" refers to
liquid,
gaseous or solid hydrocarbons (hydrocarbon only) or related petroleum non-
hydrocarbons
(those containing hydrogen and carbon plus one or more heteroatoms such as
sulphur,
nitrogen or oxygen) all of which are the major biodegradable components of
oil. Preferred
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oils are those rich in n-alkanes in reservoirs in the carbon number range 3 to
30 where natural
biodegradation is occurring. Typically, n-alkanes represent up to a maximum
around 10
weight percent of the petroleum components and typically petroleum/oils
suitable for
methanogenesis will have from 1-5% n-alkanes present. Especially preferred
oils will also
contain an extended suite of homologous alkylbenzenes and alkyltoluenes. Oil
viscosity can
range from very low values (from 5 or 10 centipoise (cP) at 20 C) to values as
high as 7000
cP at reservoir conditions. Low values generally mean more reactive oils but
higher values
favor gas over oil production. While n-alkane rich oils are preferred the
inventors have
shown that in many reservoirs oils without n-alkanes or alkylbenzenes
acceleration of natural
methanogenesis is possible as the microorganisms have adapted to consumption
of less
desirable reactants.
[0038] The term "foundation water" refers to the water endogenously present in
the
reservoir and includes the cations, anions, soluble organics, and other
components as well as
its temperature, pH, salinity, etc.
[0039] The term "MCF" means one thousand (1,000) cubic feet.
[0040] The term "incremental increase in methane per day" refers to the
increase in methane
production when the reservoir has been treated under conditions to stimulate
methanogenesis.
In one embodiment, this can be measured by the rate of methanogenesis (e.g.,
as determined
by the increase in pressure at a well head). In a preferred embodiment, this
increase is at
least 80% of the maximum rate of methane generated from the reservoir over a
60 day period
and preferably over a 120 day period.
[0041] The term "well head" refers generically to any well head in the
reservoir. Reference
to a single well head is not intended herein limiting. Rather, injection of
stimulants and/or
inhibitors can be conducted at one or more well heads and hydrocarbon
production measured
at the same or different well heads in the reservoir.
Methodology
[0042] This invention is predicated on the discovery that prior failed
attempts to reach
reasonable rates of methane production in a reservoir via enhancing in situ
methanogenesis
were based on the erroneous belief that very high concentrations of nutrients
would be toxic
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to the methanogens. This invention is further predicated on the discovery that
the injection of
a very high concentration of nutrients allows for dispersion across a wider
range of the
foundation water thereby allowing contact of sufficient quantity of nutrient
with a greater
number of methanogens.
[0043] In one embodiment, the nutrients are added to the reservoir without
prior assaying of
the reservoir, as most reservoirs retain a viable population of methanogens.
[0044] In another embodiment, an analysis of the reservoir is conducted prior
to addition of
the nutrients. Such an analysis comprises:
a) evaluating petroleum components in the reservoir, methanogenic microbial
consortia present, stimulants and/or inhibitors already present in the
reservoir, pressure and
temperature, and salinity of foundation water in the reservoir;
b) confirming the presence of at least one or more of the microbes that
comprise
a methanogenic consortium selected from the group consisting of members of the
Methanomicrobiales (Methanocalculus spp., Methanogenium spp., Methanoculleus
spp.), the
Methanosarcinales and anaerobic hydrocarbon fermenting bacteria such as
Syntrophus spp.,
Smithella spp., Syntrophobacter spp., Syntrophomonas spp., and Marinobacter
spp.;
c) evaluating the nature and amount of stimulants to be added based on the
evaluation in a) and b) above wherein the stimulants are selected from the
group consisting of
those that stimulate one or more members of the methanogenic microbial
consortium set
forth in b) and further wherein said stimulants are either endogenous and/or
exogenous
stimulants;
d) evaluating the nature and amount of optional inhibitors to be added
based on
the evaluation in a) and b) above wherein the inhibitors are selected from the
group
consisting of those that inhibit one or more non-methanogenic pathways and
further wherein
said inhibitors are either endogenous and/or exogenous inhibitors;
e) injecting a solution of stimulants comprising up to saturation
concentration of
ammonium ions and up to saturation concentration of phosphate ions into the
reservoir
through a well head such that after injection the total concentration in the
reservoir of
stimulants is above the critical concentration to effect enhanced
methanogenesis but below a
lethal dosing for the methanogenic microbial consortia set forth in b) above
wherein the
CA 02782313 2012-07-06
amount of solution employed facilitates dispersion of the nutrients from the
aqueous solution
into the foundation water;
f) optionally injecting a solution comprising inhibitors into the reservoir
through
the well head such that after injection the total concentration in the
reservoir of inhibitors is
above the critical concentration to effect inhibition of non-methane producing
competing
reactions but below a lethal dosing for the methanogenic microbes set forth in
b) above
wherein the amount of solution employed facilitates dispersion of the
nutrients into the
foundation water; and
g) maintaining said reservoir under conditions such that the rate of
methanogenesis is increased.
[0045] As is apparent, a thorough analysis of the reservoir, while not
necessary, allows for
the evaluation of a sufficient number of factors affecting methanogenesis to
be properly
addressed by e.g., adjusting the amount and type of nutrients, etc., and then
injecting such
nutrients into the reservoir under appropriate conditions, enhanced
methanogenesis at
commercially viable rates can be achieved.
[0046] Referring to Figure 1, methanogenic petroleum biodegradation in oil
reservoirs
proceeds primarily through syntrophic fermentation. Such biodegradation first
leads to
acetate and hydrogen as intermediates that are then utilized by various
microorganisms for
further biodegradation to methane. Microbes involved in the acetoclastic
methanogenesis
pathway convert acetate directly to methane (CH4) and carbon dioxide (CO2).
Microbes in
the syntrophic acetate oxidation pathway proceed to biodegrade acetate to
carbon dioxide and
hydrogen. The carbon dioxide is then reduced by microbes in the
hydrogenotrophic
methanogenesis pathway to methane. These microbes may co-exist with those that
are
adverse to methanogenesis such as sulfate reducing bacteria that metabolize
acetate and
hydrogen producing hydrogen sulfide (H25) water and CO2 and methanotrophs that
convert
methane to various compounds including water and CO2. The latter two are
examples of
"non-methane producing competing reactions" described above.
[0047] The present invention relates to methods for promoting microbial growth
and
activity that result in a substantial enhancement in the rate of production of
methane (CH4) in
a subsurface oil reservoir.
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[0048] In one preferred embodiment, such enhancement can be effected by
identifying the
petroleum components of a reservoir and determining that they are suitable for
methanogenesis, selection of nutrients shown to be effective for the
particular microbial
consortia in the oil reservoir and then enhancing and preferably maximizing
the total amount
of nutrients available to the microbes such that methane generation is
maximized while at the
same time employing a concentration of nutrients in the well that is non-
lethal to the
microbes.
[0049] A major factor in activating adequate subsurface organisms to produce
methane over
a large volume of subsurface reservoir is to deliver nutrient solutions, at a
critical nutrient
concentration (ConcA) to activate the microorganisms at a commercial rate, to
as large a
volume of reservoir as possible. The objective then becomes injecting the
minimum volume
of water containing nutrients at the maximum safe concentration to avoid
deactivating any
key organisms (ConcH). At high nutrient concentrations below the critical
concentration
(ConcH), diffusive transport of nutrients into the formation away from high
concentrations of
injected advected solutions is at a maximum driven by the large concentration
gradients and
means that maximum reservoir can be accessed with minimal injected water
volumes and
raise the largest volume of water to ConcA from the smallest volume of
injected water with a
nutrient concentration at injection of below ConcH. Fluid concentration
injected into the
well will therefore be between ConcA and ConcH but ideally at a computed
concentration
dependent on the known diffusivity of the reservoir medium.
[0050] As a starting point in such a preferred embodiment, a determination is
made of the
petroleum components and the endogenous microbes present in the reservoir.
Such factors
are critical as each reservoir or oil field contains a unique mixture of
petroleum components
and microbes. Accordingly, the type of petroleum components and microbes
present will
dictate the nutrients and/or inhibitors which are to be used for that
reservoir. Assays for
determining the microbes present are known in the art including laboratory
incubations of
reservoir samples, culturing and culture independent analysis. Likewise, the
mixture of
hydrocarbons in the reservoir can be determined by conventional analytical
means. In one
embodiment, a single sample of hydrocarbons is used to determine the
hydrocarbons present.
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In another, multiple samples are used to provide for a higher degree of
certainty regarding the
hydrocarbon components.
[0051] Nutrients and/or inhibitors for the microbes again can be determined
based on the
microbe type or by laboratory incubations under different conditions.
[0052] The selection of the appropriate nutrient(s) and/or inhibitor(s) as
well as the total
injected amount, rates of injection and injection points is then based on the
size of the
reservoir, the reservoir properties such as permeability and porosity and the
amount and type
of petroleum components present as well as the endogenous microbes and the
presence of
any non-nutrients. As to the size of the reservoir, determination of the field
size, the water
present, the concentration of nutrients and non-nutrients (e.g., salinity)
already in the
reservoir are well within the skill of the art.
[0053] The amount of nutrient (and/or inhibitors) added (injected) into the
reservoir is
conducted in an iterative process wherein a first injection of nutrients is
conducted and after
diffusion and equilibration, the concentration of nutrients is determined.
Second and
subsequent injections, if necessary, can be included until the desired
concentration of
nutrients is reached.
[0054] Optionally, the reservoir can be tested after injection and preferably
after
equilibration to confirm the concentration of each nutrient and/or inhibitor.
Additionally, the
reservoir can be retested periodically during methanogenesis to determine if
additional
nutrient and/inhibitor should be added.
[0055] In one embodiment, each of the injections is made through a well head
or a plurality
of well heads. Such well heads are conventional well heads having access to
the subsurface
reservoir. If recovery of methane is desired, then the well head(s) for
injection can be the
same well head(s) for such recovery. Alternatively, the well head(s) can be
used to measure
the increase in pressure within the reservoir which is reflective of the
amount of methane
production.
[0056] In another embodiment, the phosphate ion concentrations for use in
combination
with the ammonium ions are chosen such that the molar ratio of nitrogen to
phosphorus is
approximately 4:1. Higher concentrations of phosphorus may be employed
including
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nitrogen to phosphorus ratios of 3.5:1, 3:1, and 2.5:1. The appropriate
phosphorus
concentration relative to the nitrogen concentration may be dependent on the
particular
phosphate reagent that is used and on the nature of the subsurface reservoir.
In general the
type and amount of phosphate chosen will minimize any precipitation of solids
that is
typically seen at higher phosphate concentrations.
[0057] In one embodiment, the nitrogen is provided by NH4C1. Other sources of
nitrogen
include ammonium phosphate.
[0058] In one embodiment, the phosphate ion is provided by KH2PO4. Other
sources of
phosphate ions for use in the present methods include NaH2PO4, either in
anhydrous or
hydrous form. The phosphate ion (KH2PO4-) is preferably added in an amount of
from 0.2 to
about 1.376 g/L.
[0059] In one preferred embodiment, the injected stimulants comprise ammonium,
phosphate, nickel, and cobalt ions. It is understood, of course, that type and
amount of the
injected stimulants will be based on the presence and amount of stimulants
already present in
the reservoir.
[0060] In still other embodiments, one or more buffers are injected into the
reservoir.
Suitable buffers include carbonates (such as sodium carbonate, sodium
bicarbonate,
potassium carbonate, potassium bicarbonate) and alkali and alkaline earth
hydroxides (such
as Li0H, NaOH, KOH, Cs2OH, Mg(OH)2, and Ca(OH)2). In one aspect, the buffer is
NaHCO3. The amount of buffer to be added is dependent on the salinity and pH
of the
reservoir, which can vary from reservoir to reservoir.
[0061] In still other embodiments, one or more complexing agents are injected
into the
reservoir. Such agents include nitrilotriacetic acid. The complexing agents
can be used to
bind to the stimulants to prevent precipitation and aid in stabilizing the
stimulant mixture, can
act as inhibitors by binding to metals detrimental to methanogenesis, or act
as a source of
carbon or nitrogen for microbes to further facilitate biodegradation of oil
and
methanogenesis, or act as complexing agents to accelerate natural uptake of
key nutrient
elements from the host reservoir rock.
14
CA 02782313 2012-07-06
[0062] In yet other embodiments, one or more inhibitors that minimize
microbial activity
that slow or are detrimental to methanogenesis may be used. Inhibitors include
those that
inhibit the activity of iron-reducing, nitrate-reducing, or sulphate-reducing
bacteria. Specific
inhibitors include sodium molybdate (Na2Mo04) and hydrates of sodium molybdate
for
inhibiting sulfate reducers, and sodium chlorate (NaC103) for inhibiting
nitrate reducers.
Concentrations of sodium molybdate and sodium chlorate of about 20 mM are
contemplated
as suitable for use in this invention.
[0063] The stimulants (ammonium and phosphate ions, minerals, etc.)
inhibitors, buffers,
and other agents may be combined together in one or more aqueous formulations.
In one
aspect, the ammonium and phosphate ions are contained in a single formulation.
Salts such
as NaCl and CaC12 may also be included in the formulations. The formulations
may also be
sparged with N2 that will remove oxygen. In some aspects the water used in the
formulations
is the natural foundation water from the reservoir. In one embodiment, there
is provided an
aqueous solution of saturated ammonium and phosphate ions, optionally having a
salinity
between 0.1 to 1. In one preferred embodiment, the aqueous solution is
foundation water,
wherein the foundation water contains NaCL or CaCl2.
[0064] In one embodiment, the total salinity of the injected solutions being
added to the
reservoir will have a salinity similar to that of the reservoir. The salinity
of the solutions may
be adjusted by modifying the amount of added salts such as NaC1 and CaCl2 or
other major
ions present in the foundation water naturally.
[0065] Without being bound by theory, the formulation is believed to promote
growth of
microbes in the syntrophic fermentation, acetoclastic methanogenesis,
syntrophic acetate
oxidation, and hydrogenotrophic pathways shown in Fig. 1. Microbes involved in
methanogenic hydrocarbon degradation include methanogens from the Met
hanomicrobiales
(Methanocalculus spp., Methanogenium spp., Methanoculleus spp.),
Methanosarcinales and
anaerobic hydrocarbon fermenting bacteria such as Smithella spp., Syntrophus
spp.,
Syntrophobacter spp., Syntrophomonas spp., and Marinobacter spp..
[0066] In one embodiment of the methods, an optional step of identifying
reservoirs having
one or more of above features are provided.
CA 02782313 2012-07-06
[0067] Such an optional analysis of the reservoir's environment can provide
information that
can be used to determine suitable microbial growth stimulants or in situ
environmental
conditions for microbial activity. The analysis can include determining the
reservoir's
temperature and pressure, which can be obtained in any suitable manner. While
many
reservoirs contain biodegraded oils, not all reservoirs contain currently
active microbial
populations. In one implementation, the analysis is to identify a zone in a
reservoir that
includes relevant active organisms biodegrading reactive petroleum components
that can be
accelerated to recover economic levels of methane through petroleum
biodegradation.
[0068] To determine the environment in the reservoir, a geochemical analysis
can be made
of one or more fluids of the reservoir, such as foundation water and
petroleum, and/or one or
more solids of the reservoir, which analyses are familiar to those skilled in
the art. The fluid
analysis can include measurement of the state values (e.g., temperature and
pressure) as well
as a geochemical analysis of the foundation water, which can include assays
for major anions
and cations, pH, oxidation potential (Eh), chloride, bicarbonate, sulphate,
phosphate, nitrate,
ammonium ion, salinity, selenium, molybdenum, cobalt, copper, nickel, and
other trace
metals contents. The geochemical analysis can identify products that are known
to be
produced by indigenous microbial activity. For example, the presence of
methane, CO2,
RNA, DNA, and/or specific carboxylic acids can be indicative of microbial
activity.
Methane relatively depleted in the carbon 13 isotope is frequently found in
oilfields where
natural methanogenesis has occurred. In particular, anaerobic hydrocarbon
degradation
metabolites, such as alkyl and aryl substituted succinates or reduced
naphthoic acids, are
markers of systems in which the anaerobic degradation of hydrocarbons is
taking place. The
identification of such markers can be used in determining the presence of
active anaerobic
petroleum degrading microbial consortia. See, for example, International
Patent Application
PCT/CA2009/001069 which is incorporated herein by reference in its entirety.
[0069] Injection of methanogenic nutrients into the foundation water of a
petroleum
reservoir can be accomplished using a variety of methods. The nutrients may be
injected
from a vertical injection well or a horizontal injection well. A horizontal
injection well can be
beneficial for the commercial scaling of the process as a larger area of the
methanogen active
formation oil/water zone can be accessed with the nutrient containing water
injected for a
16
CA 02782313 2012-07-06
single injection well. The commercial impact of the process can also be
increased by
injecting the nutrients using hydraulic fracturing which can increase the
distance penetrated
by the nutrient at quantities sufficient to provoke the desired rate of
accelerated
methanogenesis.
can include injecting fluid into the zone through a first set of one or more
well heads, and
producing gas from the zone can include producing gas from the zone through a
second set of
one or more wells. Injecting fluid into the zone can be concurrent with
producing gas from
the zone or can cease while producing gas from the zone. Injecting fluid into
the zone can
[0071] While generating methane gas in situ, production parameters can be
monitored
including the pressure in the reservoir and composition of the generated gas.
Based on the
100721 Implementations of the invention can include one or more of the
following features.
Controlling injection into and/or production from the zone to enhance
generation of
biogenerated methane from the zone can include controlling one or more of the
following: an
[0073] Before producing hydrocarbons from the zone, an increase in reservoir
pressure in
17
CA 02782313 2012-07-06
monitored. Based on the reservoir pressure, the zone's gas saturation can be
determined.
Production of hydrocarbons (oil, methane, and the like) from the zone can be
commenced
when the zone's gas saturation reaches a threshold gas saturation.
[0074] The injectants can be added to the reservoir together or in separate
injection steps.
For example, a slug or bank of water carrying one injectant can be followed by
a second slug
or bank of water carrying a second injectant. Another example may include
alternately
injecting one water bank followed by a gas injection step. In some
implementations,
injectants operating as stimulants may be injected at one location to enhance
methanogenesis,
and injectants operating as inhibitors may be injected at a different
location, to prevent or
minimize detrimental processes, such as methane oxidation. Injection of gas
below a
degrading oil column may facilitate circulation of water and nutrients to the
microorganisms
present.
[0075] In some implementations, layered reservoir bioreactors can be used for
methane
production and to facilitate methane removal. In such a reservoir bioreactor,
the
biodegrading oil column and/or residual oil zones are vertically segmented and
the
environment can be controlled, for example, in the following manner: (a) a
lower zone of
degradation of oil or injected reactive organic substrates can be
environmentally modified to
produce abundant free gas (e.g., methane and/ or carbon dioxide); (b) an upper
zone of
degradation of oil or injected reactive organic substrates is environmentally
modified to
produce abundant free methane; and (c) free gas from the lower layer buoyantly
moves up
through the layered bioreactor and any free methane or methane in aqueous or
oil solution
partitions into the moving gas phase and is carried to a gas-rich zone for
production.
[0076] Microorganisms in subterranean reservoirs tend to be most active at
environmental
boundaries where they have access to water and the requisite hydrocarbons for
biodegradation, such as between fermentation zones and methanogenesis zones.
Therefore,
microorganism activity in a reservoir may be increased by increasing the area
where these
conditions prevail. One method for increasing the area with suitable
conditions is to modify
the water flood injection rates. Yet another method involves forming small-
scale
environmental interfaces by forming petroleum-water emulsions in the
reservoir, or by
changing the clay chemistry.
18
CA 02782313 2012-07-06
[0077] Preferably, increases in methanogenesis of at least 104, 105, 106 or
higher over
endogenous methane production can be achieved. For the sake of completeness,
endogenous
methane production refers to the amount of methane produced over a given
period of time
without any intervention in the reservoir which is low being typically on the
order of 10-4
kg/m2 of oil water contact area/ year.
[0078] Further descriptions of methods and systems for gas production from a
reservoir are
set forth in Canadian Patent Application No. 3,638,451 which is incorporated
herein by
reference in its entirety.
[0079] The following examples are provided to further illustrate certain
aspects of the
present invention and to aid those of skill in the art in practicing the
invention. These
examples are not meant to limit the scope of the invention.
EXAMPLES
[0080] The following three solutions were prepared and used in the methods of
the
invention.
Example 1
[0081] A 1L aqueous solution of nutrients for reservoir injection was prepared
using the
following compounds:
Nutrient g/liter H20
MgC12 0.363543
NH4C1 0.066819
KH2PO4 0.016991
NaHCO3 0.800235
FeSO4 0.001012
H3B04 2.65 X 10-5
MnC12 5.61 X 10-5
CoC12 0.000914
NiC12 0.000115
CuC12 1.39 X 10-6
ZnSO4 7.13 X 10-5
Na2M004 1.97 X 10-5
Na2Se03 4.38 X 10-6
Na2W04 6.28 X 10-5
NaC1 7
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CA 02782313 2012-07-06
Nutrient g/liter H20
CaCl2 0.12
NaH2PO4 0.175
Example 2
[0082] The following solution is prepared with reservoir water from a Western
Canadian
sandstone heavy oil reservoir:
Nutrient Per liter
H20
NH4C1 5.4g
KH2PO4 1.376g
Selenite-tungstate Solution 1 mL
Trace Element Solution 1 mL
[0083] Optionally, 2.5 mM Na2S can be added as a reducing agent/oxygen
scavenger if the
solution is stored over a prolonged period of time.
[0084] The Selenite-tungstate Solution is prepared as a 1 liter aqueous
solution using the
following compounds:
Nutrient per liter H20
NaOH 400 mg
Na2Se03 x 5 H20 6 mg
Na2W04 x 2 H20 8 mg
[0085] The Trace Element Solution is prepared as a 1 liter aqueous solution
containing the
following compounds:
Nutrient per liter H20
HC1 (25% = 7.7 M) 12.5 ml
FeSO4 x 7 H20 2100 mg
H3B04 30 mg
MnC12 x 4 H20 100 mg
CoC12 x 6 H20 190 mg
NiC12 x 6 H20 24 mg
CuC12 x 2 H20 2 mg
ZnSO4 x 7 H20 144 mg
Na2Mo04 x 7 H20 36 mg
CA 02782313 2012-07-06
The nutrients are added to a mixture of foundation water and heavy oil such
that the final
concentration of the ammonium ions is 5.4 g/L and the final concentration of
the phosphate
ions is 1.376 g/L. the mixture is incubated under conditions in which
methanogenesis is
significantly enhanced.
100861 All publications and patent applications mentioned in this
specification are herein
incorporated by reference to the same extent as if each individual publication
or patent
application was specifically and individually indicated to be incorporated by
reference.
100871 Modifications to the invention will be apparent to one of skill in the
art given this
disclosure. Such modifications and the resulting equivalents to the
embodiments and
examples described above are intended to be included within the scope of the
following
claims.
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