ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 2 docx

POROSITY cont.° There are three main types of porosity: + Interconnected porosity has multiple pore throat passages to connect neighboring pore.. Primary Porosity• Primary porosity is di

CHAPTER 02

RESERVOIR

CONDITION FOR AN ACCUMULATION OF

OIL AND GAS

° A mature source rock

° A reservoir rock

° A migration route (betw Source & Res.)

° An impermeable seal

° A trap

° 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

Exp: RESERVOIR

OF CUU LONG AND SOUTHERN CONSON BASINS

Figure 1

° Porosity is the percentage of volume of voids

to the total volume of rock It has the symbol

: 0 ≤  ≤ 1

° 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

POROSITY (cont.)

° There are three main types of porosity:

+ Interconnected porosity has multiple pore throat passages to connect neighboring pore

+ Connected porosity has only one pore throat passages connecting with another pore space.+ Isolated porosity has no connection between pore

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

Interconnected porosity

Connected porosity

Isolated porosity

Primary Porosity

• Primary porosity is divisible into two types:

intergranular or interparticle porosity, which

occurs between the grains of a sediment (

Figure 1 ) and intragranular or intraparticle

porosity,

Intergranular porosity

intragranular porosity

Secondary Porosity

Secondary porosity is porosity formed within a reservoir after deposition The major types of secondary porosity are:

• Fenestral;

• Intercrystalline;

• Solution (moldic and vuggy);

• Fracture

Fenestral 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

Crystalline dolomite reservoir: reservoirs are usually

composed of secondary dolomite formed by

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

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

Several types of secondary porosity can be caused by

solution

• In Figure 3 , fractures may develop from tectonic

forces associated with folding and faulting

Figure 3

• They may also develop from overburden

unloading and weathering immediately 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

• One must be able to distinguish between fracture

porosity and porosity which occurs within the rock itself Very often fractures are an important part of storage capacity, and sometimes only oil or gas from the fracture pore space itself is actually produced

• Fracture porosity can result in high production rates during initial testing of a well, followed by a rapid decline in production thereafter When a rock has

been fractured, the fractures do not necessarily

remain open They may be infilled by later

cementation by silica, calcite or dolomite

The fractures may be infilled by later cementation by

silica, calcite or dolomite

STUDY FOR BASEMENT POROSITY

° Basement construction simulation

° Determining values:

 Vuggy (range, dimension)

 Fracture (range, dimension)

 Determine collection capacity

PRIMARY FACTORS CONTROLLING POROSITY

RANGE OF POROSITY VALUES QUALITATIVE EVALUATION OF

° Permeability is the property of a medium of 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

Where:

• Q: rate of flow

° K: Permeability

° (P1-P2): Pressure drop across

° A: Cross-section area of sample

° : Viscosity of fluid

° L: Length of the sample

Due to flow rate depends on the Ratio of K to , so in term of commercial rates: Gas….

L

A P

P

k

*

* ) 2 1

PERMEABILITY (cont.)

(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 Absolute permeability/Effective permeability

RANGE OF PERMEABILITY VALUES QUALITATIVE EVALUATION OF

Grain Size

• Porosity is independent of grain size

Permeability, 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

Figure 6:

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

Rock 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

DIAGENESIS

SANDSTONE RESERVOIR

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

Preservation of porosities below the Top of the

Super-Normal Pressure zone

SANDSTONE CEMENTATION

A sketch of a thin section of a sandstone reservoir rock

from the Brent field in the North Sea

• Figure 8: 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 8

Sandstone Secondary Porosity

• 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 feldspar 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 9

20-30

Carbonat Rock Types

EFFECT DIAGENESIS

ON CARBONATE RESERVOIR

• At time of deposition initial porosities are as high as 50 percent (a)

• If burial takes place very quickly without early

diagenesis, porosity may be reduced, largely by

compaction as the shells and grains are squashed (b)

• Residual porosity may then be in filled by a sparite

cement (c)

• In some environments early diagenesis takes place with

a rim cement of sparry calcite crystals (d),

• sometimes accompanied by solution of the original cells

or grains giving rise to bimoldic porosity (e)

• If hydrocarbons invade the reservoir, further porosity

loss by cementation is prevented and the rim cement

gives the rock sufficient resistance to compaction (f).

• At any time in its history, even if all porosity has been destroyed by compaction and cementation, secondary

solution porosity can form (g)

•This can be either fabric-selective moldic porosity or

vuggy porosity, which cross-cuts the original grains and

fabric of the rock This later secondary porosity can also

be invaded by hydrocarbons preventing any further

cementation of the secondary pores (h)

• If petroleum invasion does not occur, the secondary

pores may be infilled with a sparry calcite cement (i)

Thus, it can be seen that the diagenetic pathways of

carbonates are extremely complex and that carbonate

reservoirs are very difficult to develop Porosity

distribution may be unrelated to the original depositional facies.

Two types of

secondary solution pores: moldic and vuggy, as shown in the previous diagram

Atypical Reservoirs Rocks

• About 90 percent of the world's discovered petroleum occurs in sandstone and carbonate reservoirs in about equal proportions

• 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 shales, granites and other igneous and metamorphic rocks 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 is shown in Figure 11 , a cross-section

through the Augila field of Libya (Williams, 1972)

Figure 11

CAUSE OF FRACTURING

IN LAYERED ROCKS (cont.)

° Relief lithostatic pressure

Fracture porosity in a brittle limestone formation caused by

folding (left) and faulting (right)

Figure 12

ROCK TYPES PROVIDING FRACTURED RESERVOIR

° Limestone and dolomites

° Chalks and marls

° Diatomites, cherts, siliceous shales

° Bituminous and siliceous shales

° Siltstone

° Igneous rock

° Basement rock in buried uplifts, overlapped by source sediments

Figure 14

Reservoir Continuity

• Most oil fields do not occur in single sheet-shaped

reservoirs of great lateral continuity with uniform

porosity and permeability distributions

• Most oil accumulations occur in heterogeneous

reservoirs with permeability barriers because of shale breaks or local cemented zones

Figure 15 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 16 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 17 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

Areal Continuity

The following is based on the scheme proposed by Potter (1962)

The dendroid variety has length: width ratios which are generally greater than

3 to -1 This is typically encountered in fluvial and deltaic sands which trend perpendicular to the paleo-shoreline The depositional environment of this type

of sand body is illustrated in Figure 18

Figure 18

The ribbon or shoestring sands are characteristically produced by marine barrier bar sands and usually trend parallel to the

paleoshoreline The depositional environment of this type of sand body is illustrated in Figure 19

Figure 19

Cross-Sectional Continuity

Reservoir continuity in cross-section is an important consideration in determining reservoir quality (Harris and Hewitt, 1977) Figure 20 ,

Figure 20

Figure 21 a , a series of channels has coalesced Oil entrapment in this case would be stratigraphic

and Figure 21b : oil entrapment can only be stratigraphic

Fig 21: Different degrees of vertical continuity.

If the sand body with lateral continuity, shown in Figure 21a , were deformed structurally, oil entrapment would become structural rather than stratigraphic

Figure 22

Case History: Intisar Field, Libya

Figure 23 is an isopach map of one of the Intisar

(formerly Idris) reef fields located in the Sirte basin of Libya

This field is a stratigraphic trap contained within a reef

or carbonate buildup In the map, notice the simple sub circular geometry of the reservoir The thickness of the reservoir increases from zero to about 1,200 feet in an approximate distance of only 2.5 miles

Figure 23

Figure 24: geological cross-section showing the various lithological facies of the reef

Figure 25: Petrophysical cross-section showing the distribution of zones of different porosity

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:

 Solution gas drive

 Gas cap drive

 Water drive

 Gravity Drainage

 Combination drive

DISSOLVED GAS DRIVE RESERVOIR

and then drops.

GAS CAP DRIVE RESERVOIR

1 Reservoir pressure Falls slowly and continuously

2 Surface gas-oil ratio Rises continuously in up

-structure wells

3 Water production Absent or negligible

4 Well behavior Long flowing life

WATER DRIVE RESERVOIR

1 Reservoir pressure Remains high

2 Surface gas-oil ratio Remains low

3 Water production Starts early and increases

appreciable amount

4 Well behavior Flow until water

production gets excessive

5 Expected oil recovery 35 to 70 percent

GRAVITY DRAINAGE DRIVE RESERVOIR

1 Reservoir pressure Remains in medium rates

2 Surface gas-oil ratio Stable

3 Water production Negligible

4 Well behavior Requires pumping at

early stage

5 Expected oil recovery 15 to 20 percent