Basics of Reservoir Engg

8/12/2019 Basics of Reservoir Engg

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Introduction to Reservoir Engineering

U. S. Prasad

C. P. Verma

pgradk

V


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Reservoir Engineering

Learning Objectives :

Basic concerns of Reservoir Engineering

Scope of Reservoir engineering

Basic concepts and operating variables

Tools of reservoir engineering


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DEFINITIONs OF RESERVOIR

ENGINEERING

The phase of engineering which dealswith the

transferoffluidsto, fromor throughthe

reservoirs

oil


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Objectives of reservoir

engineering

a) To enhance ( increase recovery

factor) and

b) To accelerate ( increase production

rate)

the oil recovery


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Types of reservoir energy

1. - Energy of compression of waterand rock within the reservoir

2. - Energy of compression of oil

within the reservoir3. - Energy of compression of gas

within the reservoir

4. - Energy of compression of waterthat are in adjacent or underlyingaquifers


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Types of reservoir energy

5. The gravitational energy that

causes oil and gas to segregate

within the reservoir6. The surface energy manifesting

itself in capillary pores


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Producing mechanismsbased on dominant reservoir energy being released

Common special

Depletion

drive

Gas cap

drive

imbibition compaction

Solution

Gas drive

Full Partial

Segregating

Nonsegregating

Water drive Combination drive

Formation

driveI.Edge

II.Bottom

I.Edge

II.Bottom

gravity


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Basic concerns of reservoir engineers

(i.e.Reservoir engineer has to

continuously answer:)1. To calculate the volume of the initial

hydrocarbon present in the

reservoir ?

2. How much of the initial fluids have

been recovered ?

3. How much is left ?


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Reservoir engineer has to

continuously answer:

4. How can we increase recovery

economically?

5. What data are needed to answer the

questions?


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Scope of Reservoir engineering:

Reservoir engineer has to understand

(1) the nature of reservoir fluids

(2) the nature of reservoir rocks and

(3)the nature and behavior of rock

fluid systems


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Scope of Reservoir engineering:

In dealing with rock fluid systems we

have

basic concepts and

operating variables.

Basic concepts are: multiphase fluidflow, capillary behavior and fluid

displacement are to name a few.


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Tools of Reservoir engineering:

Under the influence of these operating

variables, answering the previous

questions requires material balancecalculations and performance

evaluations i.e predictions and

interperations.


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Tools of Reservoir engineering:

Numerical simulators are in fact are

multidimensional, multiphase

dynamic material balanceprograms.

The classical MBE approach is well

worth as it provides valuable insight

into behavior of HC reservoirs


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Secondary drive Mechanism & EOR

Phases in Field

Development

Broadly three phases in thedevelopment of a field. The

phases are defined as;

Primary recovery phase

Secondary recovery phase

Tertiary recovery phase

Primary

Secondary

Tertiary

QO

Time


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Secondary Recovery Phase

Lack of sufficient natural drive needs supplementing the naturalreservoir energy by introducing some form of artificial drive, themost basic method being the injection of gas or water.

Waterflooding, called secondary recovery because the processyields a second batch of oil after a field is depleted by primaryproduction

The practice of Waterflooding apparently began accidentally asearly as 1890, when operators realised that water entering theproductive formation was stimulating production.


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Tertiary Recovery/EOR Phase

The tertiary recovery is also a supplementation of

natural reservoir energy; however it is defined as that

additional recovery over and above what could berecovered from primary and secondary recovery

methods.


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Different EOR processes are

EOR Processes

Thermal EOR

Processes

Chemical EOR

Processes

Miscible EOR

Processes

Immiscible EOR

Processes

Microbial EOR

Processes

In-situcombustion

Airinjectio

n

Steamflooding

Alkali-Surfactant-Polymer

Polymer

Hydrocarbonmiscible

CO2 miscible

N2 miscible

Flue gas

Hydrocarbonimmiscible

CO2immiscible

N2immiscible

Flue gas

Consortium

of Bacteria

used for

insitu

generation

of

suphonates

, CO2,etc.

for profile

modificatio

n


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Water Flooding

Waterflooding is the most widely used post-primaryrecovery methods practiced all over the world as it isinexpensive.

Waterflooding serves two purposes in maintaining thereservoir pressure which energises the system and indisplacing oil towards the production wells.


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Waterflooding


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While deciding suitability of a candidate

reservoir for Waterflooding following reservoir

characteristics should be considered;

Flood Pattern

Reservoir Heterogeneity

Mobility Ratio Recovery Efficiency


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Flood Pattern

The areal geometry of the

reservoir will influence the

location of wells and that willessentially decide the flooding

pattern (injection-production

well arrangements).

The commonly used floodpatterns are given in the

following figures;


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Reservoir Heterogeneity

Substantial reservoir heterogeneity is one of themajor problems for successful Waterflooding.

Variation in properties can be areal and vertical.

Heterogeneity of the reservoirs is attributed to thedepositional environment and subsequent events.

Permeability variation is considered to be one ofthe most important parameter which affects the

efficiency of water flooding.


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Recovery EfficiencyA simplistic model for estimating overall recovery involves

factoring the recovery efficiency into individual processefficiencies.

ER= EA* EV* ED* EM

Where;

ER= Overall recovery efficiency

EA= areal sweep efficiency

EV= Vertical sweep efficiency

ED= Displacement efficiencyEM= mobilization efficiency


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Areal Sweep Efficiency

It is defined as the fractional area of the field that isinvaded by an injected fluid. The major factorsdetermining areal sweep are fluid mobility, patterntype, areal heterogeneity, extent of fielddevelopment, and total volume of fluid injected

Vertical Sweep Efficiency

It is defined as the fraction of the vertical section

that is contacted by injected fluids and is primarilya function of the vertical heterogeneity and thedegree of vertical segregation


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Displacement Efficiency It is the fraction of the mobile oil in the swept zone that has been

displaced and is a function of the volume injected, the fluid viscosities

and the relative permeability curves of the rock

Mobilization Efficiency It is defined as the fraction of the oil in place at the start of a recovery

process that ultimately could be recovered by that process and is given

as

oi

oi

oforpoi

oi

M

BS

BSB

S

E

/


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Enhanced Oil Recovery (EOR) Processes

Enhanced oil recovery (EOR) processes include all

methods that use external sources of energy

and/or mater ials to recover oil that cannot be

produced, economical ly by conventional means.


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Currently Used EOR Processes

Thermal methods

steam stimulation,

steamflooding,

hot water drive,

in-situ combustion


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Currently Used EOR Processes

Chemical methods

Polymer, surfactant,

Caustic, and micellar/polymerflooding.

Miscible / Immiscible methods

Hydrocarbon gas CO2, nitrogen, flue gas


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Thermal (Steamflooding)


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Steamflooding Highlights

Description Steamflooding consists of injecting about

80% quality steam to displace oil.

Normal practice is to precede and

accompany the steam drive by a cyclic

steam stimulation of the producing wells(called huff and puff).


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Steamflooding Highlights

Mechanisms That Improve Recovery

Efficiency

Viscosity reduction / steam distillation.

Thermal expansion.

Supplies pressure to drive oil to the

producing well.


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Thermal (Steamflooding) Highlights

More Limitations

Steamflooding is not normally done in carbonatereservoirs.

Since about 1/3 of the additional oil recovered isconsumed to generate the required steam, the costper incremental barrel of oil is high.

A low percentage of water-sensitive clays isdesired for good injectivity.

Challenges

Adverse mobility ratio and channeling of steam.


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Polymer Flooding


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Polymer Flooding Highlights

Limitations

High oil viscosities require a higher polymerconcentration.

Results are normally better if the polymer flood is startedbefore the water-oil ratio becomes excessively high.

Clays increase polymer adsorption.

Some heterogeneity is acceptable, but avoid extensivefractures. If fractures are present, the crosslinked or gelled

polymer techniques may be applicable.


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Polymer Flooding Highlights

Challenges

Lower injectivity than with water can adverselyaffect oil production rates in the early stages of the

polymer flood. Acrylamide-type polymers loose viscosity due to

shear degradation, salinity and divalent ions.

Xanthan gum polymers cost more, are subject to

microbial degradation, and have a greater potentialfor wellbore plugging.


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Surfactant/Polymer Flooding


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Highlights

Description

Surfactant/polymer flooding consists of injecting aslug that contains water, surfactant, electrolyte(salt), usually a co-solvent (alcohol), followed by

polymer-thickened water.Mechanisms That Improve Recovery Efficiency

Interfacial tension reduction (improvesdisplacement sweep efficiency).

Mobility control (improves volumetric sweepefficiency).


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Highlights

Limitations An areal sweep of more than 50% for waterflood is

desired.

Relatively homogeneous formation.

High amounts of anhydrite, gypsum, or clays areundesirable.

Available systems provide optimum behavior within anarrow set of conditions.

With commercially available surfactants, formation water

chlorides should be


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Highlights

Challenges

Complex and expensive system.

Possibility of chromatographic separation of

chemicals.

High adsorption of surfactant.

Interactions between surfactant and polymer.

Degradation of chemicals at high temperature.

i ibl l di


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Miscible Gas Flooding

(CO2Injection)

i ibl G l di


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(CO2Injection) Highlights

Description

CO2flooding consists of injecting large quantitiesof CO2(15% or more hydrocarbon pore volumes)in the reservoir to form a miscible flood.

Mechanisms That Improve Recovery Efficiency

CO2extracts the light-to-intermediate componentsfrom the oil, and, if the pressure is high enough,develops miscibility to displace oil from the

reservoir. Viscosity reduction / oil swelling.

Mi ibl G Fl di


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Limitations

Very low Viscosity of CO2results in poor

mobility control. Availability of CO2

Surface Facilities

Mi ibl G Fl di


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Challenges

Early breakthrough of CO2causes problems.

Corrosion in producing wells.

The necessity of separating CO2from saleable

hydrocarbons. Repressuring of CO2for recycling.

A large requirement of CO2per incremental barrel

produced.


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MBE Terminology

N Initial reservoir oil volume, STB

Boi Initial oil formation volume factor, bbl/STB

Np Cumulative produced oil, STBBo Oil formation volume factor, bbl/STB

G Initial reservoir (free) gas (in gas cap), SCF

Bgi Initial gas formation volume factor, bbl/SCF


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MBE MBE Terminology

Rsoi Initial solution gas oil ratio, SCF/STB

Rp Cumulative produced gas oil ratio , SCF/STB

Rso Solution gas oil ratio, SCF/STB

G Initial reservoir (free) gas (in gas cap), SCF

Bg Gas formation volume factor, bbl/SCFW (Volume of) initial reservoir water


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MBE Terminology

Wp Cumulative produced water, STB

Bw Water formation volume factor, bbl/STB

We Water influx into the reservoir, bbl

WI Cumulative water injected, bbl

GI Cumulative gas injected, SCFBIg Injected gas formation volume factor, bbl/SCF


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MBE Terminology

Cw Water isothermal compressibility, psi-1

p Change in average reservoir pressure, psi

Swi Initial water saturation

Vp Initial pore volume, bbl

Cf Formation isothermal compressibility, psi-1


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MBE variables:

(1 )p wi oi oiV S NB mNB

Reservoir (oil zone plus gas cap) pore

volume relations:

(1 )

(1 )

oip

wi

NB m

V S

Assumes uniform Swiin oil and gas zones

C ibilit l ti f


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Compressibility relations for any

material

By definition:

1dvc

v dp

1p V

p Vi i

cdp dvv

1p Vi

p Vi

cdp dvv

lnii

Vc p p

V

exp i

i

Vc p p

V

1 ii

Vc p p

V i i iV V V c p p

Expanding the exponential term for small c values:


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Compressibility relations

Defining the positive V and p

values as follows:

iV V c p

iV V V ip p p

We obtain

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Basics of Reservoir Engg