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Reservoir-Engineering
Analysis of Microbial
Enhanced Oil Recovery
Reservoir-Engineering
Analysis of Microbial
Enhanced Oil Recovery
Generally, microbial enhanced oilrecovery (MEOR) recovers less
of
the remaining oil in place thanother chemical EOR processes,
bothin the laboratory and the field.Efforts to explain this
difference arelimited by the lack of quantitativemeasures of
microbial performance(e.g., reaction rates, stoichiometry,and
required product concentra-tions). However, it is possible
todemonstrate quantitative relation-ships between microbial
perform-ance, reservoir characteristics, andoperating conditions
(e.g., well
spacing, injection rates, and resid-ual-oil saturation).
IntroductionLow oil prices led to a marked shift infocus from
EOR to improved-oil-recovery processes that use well tech-nology
(i.e., completions, well stimu-lation, or water shutoff).
However,interest in developing MEOR methodspersisted, largely
because of the per-ceived potential to provide incremen-tal oil
production at low cost.
The full-length paper details a studythat adopted a
reservoir-engineeringperspective, focusing on issues such asscaling
up laboratory results, processdesign, and field implementation
andoperation. This approach provides aconsistent framework for
comparingMEOR with other EOR processes. Thestudy focuses on
enhanced recoveryprocesses that induce or promotemicrobial activity
that, in turn, gener-ates appropriate chemicals in thereservoir
that enhance recovery.
MEOR as a Chemical EORTable 1 shows the wide range
ofmicrobial-reaction products common-
ly cited as relevantto EOR. Except
for biomass, allthe microbialproducts corre-spond to familiesof
chemicals al-ready used or pro-posed for use inEOR processes.The
claimed ef-fects of biomasscorrespond toeffects achievableby use of
other
chemicals. Cur-rent MEOR proc-esses propose nofundamental
lynovel mechanismof oil recovery.Hence, MEORdiffers from otherEOR
processesonly in the way that chemicals areintroduced into the
reservoir (i.e., in-situ generation) and, therefore, shouldbe
evaluated on the same basis as otherEOR processes. Therefore, any
advan-tage of MEOR will result from beingmore efficient than other
EORprocesses. In-situ generation has apotential logistics
advantage, especial-ly if residual oil can be used as anin-situ
carbon source. This logisticsadvantage is the most important,
andpossibly the only, advantage overother processes. However,
in-situ gen-eration introduces a new set of techni-cal
difficulties.
In addition to challenges associatedwith in-situ chemical
generation,
MEOR must overcome similar techni-cal problems and difficulties
as otherEOR processes, particularly the place-ment and propagation
of recovery-enhancing chemicals. Previous studieson chemical EOR
underscore theimportance of chemical dissipationthrough dispersion
and diffusion, andconsumption or retention by means ofinteractions
with the rock and oil.Similarly, propagation cannot be takenfor
granted in microbial treatments;injection of chemicals generated
out-
side the reservoir recovered little or nooil, even though the
same chemicalsdid lead to oil recovery when generat-ed in the
reservoir. Reservoir hetero-geneity may severely reduce contact
ofthe chemical slug with the oil-contain-ing reservoir rock, and
fingering intothe mobilized oil bank must be sup-pressed. Also, the
presence of recovery-enhancing chemicals in the producedoil may
create processing problems.
An EOR process moves fluids froman injection to a production
well, andphysical and chemical interactionsoccur as those fluids
encounter differ-ent rock and fluid compositions.Theories that
account for this move-ment are established for steam, poly-
mer, surfactant, miscible gas, andalkali-surfactant-polymer
flooding.These theories account for the behav-ior of MEOR also,
insofar as MEORoperates by means of microbially gen-erated
chemicals. However, an exten-sion of existing theory is required
toaccount for the time required to gen-erate these species in-situ
and thefeedback loop that arises when gener-ated species (e.g.,
polymers) alter theflow field. The full-length paperdetails this
extension.
This article is a synopsis of paperSPE 63229,
Reservoir-EngineeringAnalysis of Microbial Enhanced OilRecovery,
bySteven L. Bryant, U. ofTexas, and Thomas P.
Lockhart,SPE,Enitecnologie, originally presented atthe 2000 SPE
Annual TechnicalConference and Exhibition, Dallas,14 October.
Product Effect
Acids Increased rock porosity and permeability.Production of CO2
by reaction withcarbonate minerals.
Biomass Selective and nonselective plugging.Emulsification
through adhesion to oil.Changing wettability of mineral
surfaces.Reduction of oil viscosity and pour point.Desulfurization
of oil.
Gases Reservoir repressurization.Oil swelling.Viscosity
reduction.Increased permeability by dissolving
carbonate rocks.
Solvents Dissolution of oil.
Surfactants Lowering interfacial tension.Emulsification.
Polymers Mobility control.Selective or nonselective
plugging.
TABLE 1MICROBIAL REACTION PRODUCTS AND
THEIR CLAIMED EFFECTS FOR EOR
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Base-Case ImplementationIn essence, MEOR introduces
reactionengineering into reservoir engineering,accompanied by
concepts such as theresidence time in the reaction zone,reaction
kinetics and selectivity, andlimiting reactants. The current
under-standing of microbial reactions is inad-equate for
quantifying these conceptsin a reservoir setting. However, it
is
possible to delineate some microbialperformance constraints
imposed byengineering considerations. Theirderivation assumes that
the most likelyimplementation of MEOR will entailinoculation of
injection wells withmicrobes, a suitable shut-in period
forincubation, then resumption of water-flooding with water
containing appro-priate nutrients.
The incubation period establishesbioreactors in the reservoir.
It isassumed that microbes remain station-ary once established.
This assumptionis convenient, but not crucial to theconstraints.
When waterfloodingresumes, the microbes within eachbioreactor
convert the injected nutri-ents and the carbon source into
chem-icals that move with the aqueousphase into the reservoir and
displacethe target oil.
This implementation implies contin-uous bioreactor operation,
rather thanthe batch operation commonly used inmicrobial well
stimulations. Typically,the bioreactor volume is a small frac-
tion of the reservoir volume; therefore,production of
recovery-enhancingchemicals during the incubation peri-od will make
a negligible contributionto the overall process. However,
pub-lished laboratory studies of MEORreport on batch microbial
reactions,highlighting one of the gaps hinderinga
reservoir-engineering assessment ofthe technologys potential.
Table 2 summarizes the designoptions for the base-case
implementa-tion. The carbon source may be ex situ
(included in the injected stream) orin situ (residual oil in the
bioreactor).The economic and logistic advantagesare greatest for an
in-situ carbonsource and the discussion will proceedon that
premise. Use of indigenousmicrobes (i.e., microbes native to
thereservoir) also circumvents some con-straints. Generally,
achieving thedesired reactions with such microbes
is more difficult. Some applicationsthat use indigenous microbes
appearto rely only on microbe growth, ratherthan on reaction
products.
Reaction-Engineering ConstraintsThe effective reactor size is
limited bythe distance to which exogenousmicrobes can be placed and
by the dis-tance dictated by nutrient solubility.This limit imposes
constraints on themicrobial performance and on slugvolume. The
constraints are nontrivialfor reasonable values of
reservoirparameters. Moreover, in-situ genera-tion of viscous
fluids appears to beintrinsically unreliable.
MEOR ExperienceTo date, laboratory work on MEORhas not
demonstrated recovery ofresidual oil from cores at levels
com-parable with those of other EORprocesses. There are
insufficient dataand inadequate mechanistic under-standing to
determine whether thislack of recovery is caused by unfavor-
able kinetics, insufficient concentra-tion of reaction products,
or low effi-cacy of microbial-reaction productsfor oil
mobilization. Most field appli-cations of microbial technology
arewell stimulations rather thanenhanced-recovery processes.
Theapplications are intended to increaseoil-production rates rather
than tomobilize incremental oil. Significantincremental oil
recovery by means ofincreased displacement efficiency hasnot been
demonstrated in the field.
Increasing volumetric efficiency bypromotion of biomass growth
is tech-nically simpler and appears to have abetter chance of
success. However,mechanistic explanations of laborato-ry and field
results are lacking.
ConclusionsThe use of microbes introduces reac-tion engineering
into reservoir engi-
neering, with associated conceptsincluding bioreactor volume,
nutrient-reaction kinetics and selectivity, andminimum required
level of conver-sion. These concepts enable quantita-tive
relationships between reservoircharacteristics, operating
conditions,and microbial performance. Unfor-tunately, the current
state of knowl-edge does not allow direct use of theseconstraints
to assess MEOR field-implementation feasibility.
Analysis with plausible values ofreservoir and microbial
parametersindicates an MEOR process that usesin-situ carbon must
overcome severeperformance constraints. Use of anex-situ carbon
source circumvents orrelaxes some of the technical con-straints,
but the logistical and costadvantages of an in-situ sourceare
lost.
Microbial gas production con-tributes to oil recovery. Analysis
showsthat it is unlikely that gases such asCO2 and CH4 could be
producedin situ in quantities needed for effec-
tive oil displacement.In-situ generation of viscosifying
agents is intrinsically unstable. Thus,robust mobility control
of MEOR islikely to require an ex-situ polymer,reducing the
potential logisticaladvantage of the process.
Please read the full-length paper foradditional detail,
illustrations, and ref-erences. The paper from which thesynopsis
has been taken has not beenpeer reviewed.
Design Feature Design Options Comments
Reactor Type Fixed Microbes extend a finite, constant distance
from wellbore.Growing Extent of microbes from wellbore increases as
biomass is created.Mobile Microbes suspended in aqueous phase and
propagate without retention.
Carbon source In situ Residual oil.Ex situ Added to injected
water; molasses is an example.
Microbe provenance Indigenous Bacteria native and hence specific
to individual reservoir; use for MEORmust be determined case by
case.
Exogenous Bacteria cultivated for use in MEOR; must adapt to
reservoir conditions.
TABLE 2KEY OPTIONS FOR MEOR-PROCESS IMPLEMENTATION
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