GIAO TRINH CHAPTER 2

RESERVOIR DEFINITIONS A single continuous deposit of gas and/or oil in the pores of a reservoir rock..  The common rock types that have favorable combination of porosity and permeabili

CHAPTER 02

RESERVOIR

HCMUT-UA/2011

RESERVOIR DEFINITIONS

 A single continuous deposit of gas and/or oil in the pores of a reservoir rock A reservoir has a single pressure system and don’t communicate with other reservoirs.

 The portion of the trap that contains petroleum, including the reservoir rock, pores, and fluids

 A pond, lake or environment that is used store liquids

RESERVOIR TYPICAL TYPES

Main reservoir rock

The Reservoir Rock: Sandstone

An outcrop of pebbly sandstone (at base of cliff) overlain by red sandstone The Budleigh-Salterton pebble beds, of Triassic age A few kilometres to the east these beds dip into the subsurface, and form part of the oil reservoir at the Wytch Farm Field, which is Britain’s largest onshore oil field

The Reservoir Rock: Dolomite

• The Cairns Formation, of Devonian age, exposed near Canmore, in the Front ranges of the Rocky Mountains, just east of Banff, Alberta This is one of the more important reservoir units in the subsurface of Alberta

The Reservoir Rock: Dolomite

This is an example of an important reservoir rock type Fossil stromatoporoids have been hollowed out by the chemical

conversion of limestone to dolomite, creating pore spaces so

large that they are sometimes called “cave rnous poros ity”

Making reservoirs today: Limestones

• An exposure of modern

limestone in the Florida Keys

This limestone is only a few

hundred years old It shows the

structure of coral and other

organic remains Note the

numerous pore spaces

• Burial of this limestone would

probably lead to reduction in

porosity as a result of

cementation Good quality

reservoir rocks, such as the

dolomite shown in another

picture, are created by

dissolution of some of the rock

This usually occurs many

millions of years after the initial

formation and burial

Fundamental physical properties of a reservoir

RESERVOIR (cont.)

 There are two fundamental physical

properties that a good reservoir must have:

+ Porosity: sufficient void space contain

significant petroleum

+ Permeability: the ability of petroleum

to flow into, or out of these voids

 The common rock types that have favorable combination of porosity and permeability to

carbonates.

 Porosity is the percentage of volume of

voids to the total volume of rock It has the symbol Φ: 0 ≤ Φ ≤ 1 (or 0% ≤ Φ ≤ 100%)

 Effective porosity: the amount of internal space

or voids that is interconnected, and so able to transmit fluids

 Non-effective porosity: isolated pores and

pores volume occupied by adsorbed water

Figure 2:

The frequency of oil and gas reservoirs plotted against porosity

Almost all reservoirs

have porosities in a

range of five to thirty

percent with the

majority falling between

ten and twenty percent.

 There are three main types of porosity (based on Hydraulic properties):

throat passages to connect neighboring pore

passages connecting with another pore space

pore

 Interconnected and connected pore contribute effective porosity because hydrocarbon can move out from them

CLASSIFIED POROSITY

Interconnected porosity

Connected porosity

Isolated porosity

CLASSIFIED POROSITY (cont.)

Porosity can be also classified into two major types

according to their origin:

 Primary porosity

 Intergranular, or interparticle porosity with

occurs between grains of sediment.

 Intragranular, or intraparticle porosity which

actually occurs within the sediment grains themselves.

Moldic porosity

Primary Poro s ity

• Primary porosity is divisible into two types:

inte rgranular or interparticle porosity,

which occurs between the grains of a

sediment ( Figure 1) and intragranular or

intraparticle poros ity,

Inte rgranular poros ity Intragranular porosity

Se c ondary Poros ity

Secondary porosity is porosity formed within

a reservoir after deposition The major types

of secondary porosity are:

• Fenestral;

• Intercrystalline;

• Fracture

Fe ne s tral porosity is developed where there is a

gap in the rock framework larger than the normal grain-supported pore spaces

Fenestral porosity is characteristic of lagoonal

pelmicrites in which dehydration has caused

shrinkage and buckling of the laminae This type of porosity is less frequently encountered

Fe ne s tral porosity

Crystalline dolomite re s e rvoir: Reservoirs are

usually composed of secondary dolomite formed

by "dolomitization", the process whereby a existing calcium carbonate deposit is replaced

pre-by dolomite

Figure 1: A sketch of a thin section of a crystalline dolomite

Several types of secondary porosity can be caused by solution

Frac ture poro s ity

• Fractured reservoirs can occur in any brittle rock that breaks by fracturing rather than by plastic

deformation Thus, there are fractured reservoirs

in shales, hard-cemented quartzitic sandstones, limestones, dolomites and, of course, basement rocks such as granites and metamorphics

• In Figure 3 , fractures may de ve lo p fro m

te c to nic forc e s associated with folding and faulting

Figure 3

• They may also develop from overburden

under unconformities Shrinkage from

cooling of igneous rocks and dehydrating

of shales also causes fracturing

• Fractures are generally vertical to

subvertical with widths varying from paper thin to about 6 mm

STUDY FOR BASEMENT POROSITY

° Basement construction simulation

° Determining values:

 Vuggy (range, dimension)

 Fracture (range, dimension)

 Determine collection capacity

RANGE OF POROSITY VALUES QUALITATIVE EVALUATION

allowing fluids to pass through it without change in the structure of the medium or displacement of its parts

 Permeability is related to porosity but not always dependent upon its

 It is controlled by the size of the connecting passages (pore throats or capillaries) between pores

 It is measured in darcies or millidarcies

Figure 4

K

ϕ

Whe re :

• Q: Flow rate

• K: Permeability

• (P 1 -P 2 ): Pressure drop across

• A: Cross-section area of sample

• µ: Viscosity of fluid

• L: Length of the sample

Due to flow rate depends on the Ratio of K to µ , so in

term of commercial rates: Gas ????

L

A P

P

k

*

* )

2 1

CLASSIFIED PERMEABILITY

 Absolute permeability is a measure of the

ease (permeability) in which a single fluid can flow through the pores of the rock when it is 100% saturated with that fluid

 Effective permeability refer to the presence

of two fluids in a rock, and is the ability of the rock to transmit a fluid in the presence of another fluid when the two fluids are immiscible

 Relative permeability is ratio of Effective

permeability & Absolute permeability

RANGE OF PERMEABILITY VALUES QUALITATIVE EVALUATION OF

PRIMARY FACTORS CONTROLLING POROSITY &

Grain Size

• Porosity is independent of grain size

Pe rme ability, however, is very different All other things being equal, finer grain sizes of sediment mean lower permeabilities This is because the finer the grain size, the

narrower the throat passages between pore spaces and, therefore, the harder it is for

fluids to move through a rock Therefore,

permeability declines with decreasing grain size

Figure 5 : A sketch of a poorly-sorted sand and a

well-sorted sand

Grain Sorting

The effect of sorting on porosity and permeability: the better sorted the sand, the higher are both the porosity and permeability.

Roc k Fabric

Figure 7: A sketch of a typical bedded sandstone consisting of quartz grains elongated parallel to current direction and mica flakes and other particles aligned parallel to the bedding

EFFECT DIAGENESIS

ON SANDSTONE RESERVOIR

SANDSTONE BURIAL

 In general, sandstone lose porosity with burial at various rates according to several factors:

 The chemical composition of a sand is one

of controlling factors on its overall rates of porosity loss

The geothermal gradient, the higher the geothermal gradient, the greater the rate

of porosity reduction with depth

Overpressure can help to preserve porosity

at great depth

• Figure 12: a graph on which porosity is plotted

against permeability on a logarithmic scale,

showing the porosity: permeability distributions for illite-cemented sands and kaolin-cemented sands from some North Sea gas fields

• It should be noted that the porosity is mostly

between 5 to 25 percent, irrespective of the type of clay, but the permeabilities for kaolin-cemented

sands are far higher than the permeabilities of the illite-cemented sands

Figure 12

Sands tone Se c ondary Poros ity

• Secondary porosity generally involves the leaching of carbonate cements and grains, including calcite, dolomite, siderite and

shell debris It also involves the leaching of unstable detrital minerals, particularly

felds par. In this latter case, leached

porosity is generally associated with kaolin cementation, both replacing feldspar and occurring as an authigenic cement in its

own right

Summary: Diagenetic Pathways

Figure 13

20-30

EFFECT DIAGENESIS

ON CARBONATE RESERVOIR

Two types of

secondary solution pores: mo ldic and

vug gy, as shown in the previous

diagram

Dolomite s

A secondary dolomite,

showing that the

intercrystalline pores are

large and often

interconnected

Atypic al Re s e rvoirs Roc ks

• About >90 percent of the world's discovered

petroleum occurs in s ands tone and carbonate

reservoirs

• The remaining reserves occur in what can best

be described as atypical reservoirs Almost any

rock can serve as a reservoir, providing that it

has the two properties of porosity and

permeability

• Atypical reservoirs include s hales , granites and

Generally, porosity that occurs in these is due to fracturing

This field consists of an old basement high of weathered granite with onlapping sands and reefal carbonates

Production comes from the carbonates and sands, as well as the granite

One well, the #1 well on the cross-section, penetrated through the cap rock of the field into granite without penetrating either reefal or sand reservoir This well flowed at over 40,000 barrels

of oil per day from the granite

The porosity was a mixture of fracturing and solution, where

chemically-unstable feldspar grains were leached out to leave a granite wash largely made up of residual quartz grains

The Augila field of Libya

An atypical reservoir, a cross-section through the Augila

field of Libya (Williams, 1972)

Figure 15

The Bach Ho field of Viet Nam

Fig 16:

Fig 16: Schema of weathered activities when the structure is uplifted to the surface

Fig 17:

Paleotecto nic sections along

White

Tiger-Northern Eastern

Dragon

structures

Fig 18:

Fig 18: Geological longitudinal section along White Tiger-Northern Eastern Dragon structures

Re s e rvo ir Co ntinuity

• Most oil fields do not occur in single

sheet-shaped reservoirs of great lateral continuity with uniform porosity and permeability distributions

• Mos t oil accumulations occur in

heterogeneous reservoirs with permeability

barriers because of s hale breaks or local

cemented zones

Figure 22 is the reservoir engineer's dream: a blanket sand of uniform porosity and permeability distribution This occurs with

a single oil- water contact In this case for a well drilled at

location 1 or through the reservoir of any other location, gross pay equals net pay

Figure 23 is somewhat different: the sand is shaling out from right to left across the section, thus for a well drilled at location

2 the net pay of the reservoir is less than the gross pay There

is still one oil accumulation, or at least one major one, but

there is a small separate accumulation with its own oil: water contact in the lower left-hand part of the figure

Figure 24 shows another situation There is a series of

separate oil pools with their own oil: water contacts This is not a genuine anticlinal structural trap, but a series of

stratigraphic traps which pinch out towards the crest of the structure For each reservoir, net pay equals gross pay

Cro s s -S e c tio nal Co ntinuity

Reservoir continuity in cross-section is an important consideration in

Figure 27

Figure 28 a: a series of channels has coalesced Oil

entrapment in this case would be stratigraphic

and Figure 28b : oil entrapment can only be stratigraphic

Fig 28: Different degrees of vertical continuity.

If the sand body with lateral continuity, shown in Figure 28a , were deformed structurally, oil entrapment would become structural rather than stratigraphic

Figure 29

RESERVOIR ENERGY SOURCE

 Gas dissolved in oil

 Free gas under pressure

 Gas reservoir

 Oil reservoir wet/free gas cap

 Fluid pressure

 Hydrostatic – hydrodynamic

 Compressed water, gas, oil

 Elastically compressed rock

 Gravity

 Combination of the above

RESERVOIR DRIVE

 Reservoir drive is the natural energy in a reservoir that forces the fluids out of the rock and into the well

 Every oil field has at least one reservoir drive

 Type of reservoir drives in oil field include:

RESERVOIR DRIVE (Cont.)

Fo rme r:

 Solution gas drive.

 Gas cap drive.