GIAO TRINH CHAPTER 2

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 “c ave rno us po ro s 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
be
reservoirs
are
sandstones
and
carbonates.




POROSITY
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.



Almost all reservoirs
have porosities in a
range of five to thirty
percent with the
majority falling between
ten and twenty percent.

Figure 2:
The frequency of oil and gas reservoirs plotted against porosity.


CLASSIFIED POROSITY
There are three main types of porosity
(based on Hydraulic properties):
+ 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


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.
 Secondary porosity

 Fenestral
 Intercrystalline
 Solution (moldic and vuggy)
 Fracture




Moldic porosity


Primary Po ro s ity
• Primary porosity is divisible into two types:
inte rgranular or inte rparticle poros ity,
which occurs between the grains of a
sediment ( Figure 1) and intragranular or
intraparticle poros ity,


Inte rgranular poros ity

Intragranular poros ity


S e c o ndary Po ro s ity
Secondary porosity is porosity formed within
a reservoir after deposition. The major types
of secondary porosity are:
• Fenestral;
• Intercrystalline;

• Solution (moldic and vuggy);
• Fracture.


Fe ne s tral poros ity 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 poros ity