introduction to petrophysics - uni-miskolc.hu
TRANSCRIPT
Introduction to
petrophysics
edited by P. Vass
for Petroleum Geoengineer MSc Students
Lecture 2
In general, rocks are composite materials produced by natural
processes. The primary source of these materials is the earth's
interior.
They are unchanging apparently, but the geological events going
together with transport of energy and material print their effects on
the rocks. The dynamism of these processes has been changing
both in time and space since the formation of the Earth.
Getting knowledge of the ancient events and environments which
contributed to the present arrangements and appearance of rocks
is based on the detailed investigation of rock properties.
The study of many different properties of rocks also provides
information to the assessment of potential mineral and petroleum
resources.
The properties of rocks can be studied by means of different tools
or apparatus and at different levels (macroscopic, microscopic).
Properties of rocks
A possible way of grouping the rock properties tries to emphasize
the hierarchical dependencies of properties.
By that classification the following groups can be distinguished
(Wayne M. Ahr 2008, Geology of carbonate reservoirs)
Primary (or fundamental) properties
The properties describing the rock texture, fabric, grain type,
mineralogical composition and sedimentary structures can be
listed here.
The rock properties of other groups are fundamentally dependent
on them.
The textural properties characterize the constituents of rocks
(grains, crystals) according to their size, shape, sorting (distribution
of different size fractions) and arrangement (packing).
Properties of rocks
The fabric properties of rocks include among the others the spatial
alignment, the orientation and the geometric configuration of the
constituents (depositional fabric, diagenetic fabric, biogenic fabric
and their combinations).
Secondary (or dependent or derived) properties
The petrophysical properties of rocks belong to this group:
porosity, permeability, saturation, wettability, capillary pressure.
They are dependent on the primary properties.
Third order or tertiary properties
The physical properties whose measurements are mainly
connected to the different geophysical methods form this group
(e.g. bulk density, electrical resistivity, acoustic wave velocity,
natural radioactivity, magnetic susceptibility).
They are dependent on both the primary and the secondary
properties of rocks.
Properties of rocks
The most important reservoir parameters whose values we
want to estimate from the evaluation of well logs with as
small uncertainties as possible are the following:
• porosity,
• water saturation,
• permeability,
• reservoir thickness.
The relationships which connect the measured (tertiary) rock
properties and the secondary rock properties are based on a
suitably constructed petrophysical model.
Reservoir parameters
A widely used model of reservoir rocks is divided into three
main components:
• rock matrix (the solid rock framework),
• clay or shale (because of their unfavourable effects on the
reservoir parameters they are generally separated),
• the pore space filled with fluid(s).
The ratios and properties of these components basically
influence the petrophysical properties of reservoir rocks.
Their effects also determine the well log responses of the
different methods (that is the shape of well log curves).
Petrophysical model of reservoir rocks
Petrophysical model of reservoir rocks
Pore space filled with
fluidsRock matrix
Volume of
rock matrix
Volume of
water
Volume of
hydrocarbons
Clay or
shale
Volume
of clay or
shale
Petrophysical model of clean reservoir rocks
Pore space filled
with fluidsRock matrix
Volume of
rock matrix
Volume of
water
Volume of
hydrocarbon
Bulk volume of the reservoir rock
For a petrophysicist, the rock matrix includes all the solid
constituents of a rock except the clay or shale component.
This conception of the rock matrix significantly differs from
that used in geology.
For a geologist, the matrix (or groundmass) of a rock may be
defined as follows:
“the smaller or finer-grained, continuous material enclosing,
or filling the interstices between the larger grains or particles
of a sediment or sedimentary rock; the natural material in
which a sedimentary particle is embedded” (Glossary of
Geology, A.G.I., 1977).
Rock matrix
So, the petrophysical term, rock matrix, covers a wider range
of solid components. It includes the grains, the “sedimentary”
matrix (allogenic), and the cement (authigenic minerals).
In the simplest case, the rock matrix is composed of a single
mineral (e.g. calcite or quartz).
In general, the rock matrix may contain a mixture of different
minerals: (e.g. quartz, feldspar and mica grains with calcite
cement).
Material balance equation of the rock model:
Vrock = Vma + Vpore + Vcl (or Vsh) for the total rock volume
1 = vma + vpore + vcl (or vsh) for the unit volume of rock
V: volume [m3],
v: volume fraction (dimensionless)
For a rock matrix of complex mineralogy:
vma = vma,1 + vma,2 + … + vma,n
Rock matrix
The ability of well logging methods to resolve the lithology is limited. It
means that only the mineral composition has significant effect on the
measured logs from the group of primary rock properties. Thus, textural
and fabric properties cannot be derived generally from the well logs.
The fundamental rock types which may be distinguished by means of well
logging methods are the following:
• sandstone - SiO2 (made up of mostly silica) : silt, conglomerate and
chert also belong to this group independently of the texture and fabric,
• limestone - CaCO3 (made up of mostly calcium carbonate): chalk also
belongs to this group,
• dolomite - CaCO3 + MgCO3 : due to its different physical properties, it
can be separated from the limestone by means of well log curves.
• evaporates (e.g. sylvine KCl, halite NaCl, anhydrite CaSO4, gypsum
CaSO4)
• clay and shale (fine grained siliciclastic sedimentary rocks with
significant amount of clay mineral content).
The more detailed identification within the groups is not always possible.
Rock types
Sandstone group (SiO2)
quartzite
(metamorphic)
sandstone and sand
siltstone and silt
conglomerate and gravel
chert
Limestone group (CaCO3)
chalk
limestone
(in a wide variety)
marble (metamorphic)
Dolomite CaCO3 + MgCO3
dolomitic marble
(metamorphic)
dolomite crystals
dolostone
dolomite breccia
Evaporites
gypsum (CaSO4 + crystalline water)
halite (rock salt, NaCl)
anhydrite CaSO4
sylvine (or sylvite KCl)
Clay is an unconsolidated very fine-grained sediment, whose
clay mineral content is higher than 50 %.
As a result of the diagenesis, clays change into claystone.
Subsurface clays and claystones usually have a high water
content.
Most of the water is bound to the clay minerals (clay bound
water CBW), so the movable water content of these rocks is
very low or none. Accordingly, clay and claystone beds are
typically impermeable and play the role of seal in petroleum
systems.
Clay minerals with their bound water (wet clay) can be
regarded as electrical conductors because of the cations
weakly connected to their surface (clay minerals have much
higher specific surface area than other minerals and their
surface area usually negatively charged).
Clay and claystone
Shale is a fine-grained, indurated sedimentary rock with
laminated structure. It is a frequent member of sedimentary
sequences.
Similarly to claystone it belongs to the group of mudrocks.
It normally contains at least 50% silt with, typically, 35% clay or
fine-grained mica and 15% other chemical or authigenic
minerals.
Due to its high clay mineral content, shale beds and laminae
usually form an impermeable barrier (seal) for the fluids
migrating through porous and permeable rocks.
Some shales (called black shales) formed in reducing marine
environments (low oxygen environments) have significant
organic carbon content (it may contain up to 20 % organic
carbon).
Black shales are regarded as source rocks in petroleum geology.
Shale
Shale and clay
claystone
shale black shale with pyrite concretions
surface clay
Clay and shale may also be present in reservoir rocks
(mainly in sandstones) as a contamination (e.g. fine laminae,
aggregates, pore-filling particles).
A sandstone contaminated with shale or clay is called shaly
sandstone or clayey sandstone.
A reservoir rock formation is clean when it does not contain
appreciable amount of clay or shale.
Clay and shale content has significant effect on the well log
readings of most logging methods, and typically reduces the
effective porosity and permeability of reservoir rocks.
Shale and clay
Porosity
the ratio of the volume of pore space within a rock to the total bulk
volume of the rock.
It can be expressed as either a decimal fraction or a percentage.
The collective void space is referred to as pore volume, so the total
porosity () is calculated as follows:
𝜙 = 𝜙𝑡 =𝑉𝑝
𝑉𝑡∙ 100 %
where Vp is the pore volume and Vt is the total rock volume.
In practice, different types of the porosity were defined. The most
commonly used two of them are the total porosity and the effective
porosity.
The theoretical interval of the porosity ranges from 0 – 1 (0 –
100%), but it is practically less than 0.5 (50 %) in rocks.
Porosity
Effective porosity
means the ratio of the interconnected pore space to the total bulk
volume in theory
𝜙𝑒𝑓𝑓 =𝑉𝑝,𝑒𝑓𝑓
𝑉𝑡∙ 100 %
where Vp,eff is the effective pore space and Vt is the total rock
volume. The relation between the total and effective porosity:
0 ≤ 𝜙𝑒𝑓𝑓 ≤ 𝜙𝑡
Problem with the theoretical definition
Not the total volume of interconnected pores contributes to the
fluid flow under pressure drop.
Its interpretation is highly dependent of the speciality (core
analysis, production engineering, petrophysics)
Other types of the porosity such as secondary, water-filled, vuggy,
and fracture porosity are also used in the characterization and
description of rocks.
Porosity
Baker Hughes Inc., Introduction to Wireline Log Analysis
Illustration of the effective, noneffective,
and total porosity
Porosity types in a complex model of the
reservoir rocks
Pore classification
Megaporosity: >256 mm (caverns)
Macroporosity: 1mm – 256 mm (small, medium and large
vugs)
Mesoporosity: 1 (or 2) m – 1 mm
Microporosity: < 1 (or 2) m
The so-called Darcy flow (laminar flow) holds true within
mesopores.
For micropores, the interfacial forces (surface tension)
impede the fluid flow.
For larger pores, the stream of fluid is more complex
because of the increasing heterogeneity.
Some of the porosity types differentiated (1)
Connected porosity: the ratio of connected pore volume to total
rock volume.
Effective porosity: actually the same as the connected porosity.
Primary porosity: the porosity of a rock coming from the original
deposition.
Secondary porosity: the volume fraction of pore space coming
from post-depositional processes (e.g. dissolution, dolomitization
and fracturing).
Microporosity: the volume fraction of small pores filled with
capillary bound water. (Pore tunnels having less diameter than 1
(or 2) m are regarded as small pores)
Intergranular porosity: the volume fraction of voids among the rock
grains.
Intragranular porosity: the volume fraction of voids within the rock
grains.
Dissolution porosity: a type of secondary porosity which is formed
by the dissolution of some rock minerals.
Some of the porosity types differentiated (2)
Fracture porosity: a type of secondary porosity which is formed by the
fractures in a rock.
Intercrystal porosity: the volume fraction of very small pores among
the crystals.
Vuggy porosity: a type of secondary porosity derived from the vugs
mainly in carbonate rocks. (Vugs are small to medium-sized cavities
inside rock bodies which may be partially filled with secondary
minerals. Vugs are mostly formed by tectonic activity, collapse of rock
bodies, erosion and dissolution)
Moldic porosity: a type of dissolution porosity which appears mainly in
carbonate rocks. The pore spaces preserve the original shapes of
dissolved minerals, fossils and other constituents.
Fenestral porosity: the porosity connected to the presence of
fenestrae (irregular cavities found in muddy intertidal to supratidal
carbonate sediments).
Rocks with fenestral porosity usually do not form good reservoir
rocks, because the cavities are usually isolated.
Typical ranges of total porosity in rocks
We must not forget that the effective porosity may be far less than
the total porosity in some cases (e.g. shales, clays, chalk etc.).
Main factors affecting the porosity
The amount of porosity is principally influenced by the
following factors:
• the shape of rock grains,
• the arrangement of rock grains (grain packing),
• the grain size and grain shape distributions,
• and the amount of cementing material.
In reality, porosity is rarely greater than 40%.
The highest values of the porosity occur in surface sands,
which are neither compacted nor consolidated.
Influence of the arrangement of grains on the
porosity
Baker Hughes Inc., Introduction to Wireline Log Analysis
Types of regular grain packing arrangements
with their total porosity
Paul Glover: Petrophysics MSc Course Notes
Exercise
Prove the correctness of total porosity values for the following
grain packing arrangements: cubic, hexagonal, rhombohedral
orthorhombic and tetragonal.
The volume of a spherical grain with a radius r : 𝑉𝑠𝑝ℎ𝑒𝑟𝑒 =4
3𝑟3𝜋
Influence of the grain size distribution on the
porosity
A wider range of grain size distribution reduces the porosity (not
well sorted grains), because the smaller grains are able to
partially fill the spaces among the larger grains.
In addition, non-spherical grains can fit better.
Influence of the cementation on the porosity
Baker Hughes Inc., Introduction to Wireline Log Analysis
In granular systems, the porosity normally changes between10%
and 35%, and the complete interval ranges from 3% to 40%.
Increasing cementation results in the reduction of porosity.
According to the type of rock matrix, hydrocarbon reservoirs
can be classified as follows:
• siliciclastic reservoirs, the rock matrix is mosty silica, the
primary (intergranular) porosity is typical,
• carbonate reservoirs, the rock matrix is made up of
limestone and/or dolomite, both primary and secondary
porosity can be present in the rock,
• igneous and metamorphic reservoirs, the porosity is
connected to the natural fractures (secondary porosity).
Lithological types of reservoirs
The sources of photosSandstone group:https://museumvictoria.com.au/melbournemuseum/discoverycentre/dynamic-
earth/overview/sedimentary-environment/turning-sediments-into-rock/
http://fionasrockproject.weebly.com/sedimentary-rocks.html
https://www.dwa.gov.za/groundwater/Groundwater_Dictionary/index.html?siltstone.htm
https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/limestone.html
https://www.blinn.edu/STEM/Geology/faculty/Meta_Web_Page/pages/Quartzite.htm
Limestone group:https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/limestone.html
http://www.vietnamlime.com/dolomite.html
https://www.dwa.gov.za/groundwater/Groundwater_Dictionary/index.html?siltstone.htm
Dolomite group:http://geology.com/minerals/dolomite.shtml
http://luirig.altervista.org/pics/display.php?pos=255556
Evaporites:https://www.flickr.com/photos/jsjgeology/8514005044
http://earthphysicsteaching.homestead.com/Anhydrite_Display.html
https://a2ua.com/gypsum.html
https://en.wikipedia.org/wiki/Sylvite
Shale and clayhttp://www.sandatlas.org/shale/
https://www.flickr.com/photos/jsjgeology/18859496909
http://growcreatively.blogspot.hu/2015_09_01_archive.html
http://paloaopalmining.weebly.com/what-rocks-are-opals-found-in.html