Day 2 d coring & core analysis and reservoir geology

Course Title:
Coring & Core Analysis and Reservoir
Geology
Dr. Arzu Javadova
2018
Training for Grenergy

3. Core Laboratory Processing and Screening
• Core Receipt and Cutting
• CT Scanning
• Gamma Ray Logging
• Removal from Liners
• Core Viewing and Sample Selection
• Sample Preservation
• Core Plugging
• Core Slabbing
• Core Resination
• Core Photography and Imaging
• Weak or Unconsolidated Core Processing
• Special Handling Considerations for
Difficult Rock Types
4. Core Sample Preparation
Cleaning
Core Drying
Quality Control Issues, Checks and
Diagnostics
Clays and Clay Damage Mechanisms
Core Conditioning for Porosity
Measurements
Special Considerations in Core
Preparation
5. Petrography and Mineralogy
Thin Section Staining
Sedimentary features
Pore types
Quantitative Carbonate Petrography
Facies and facies coding classiﬁcation
Rock types
2d Day

3. Core Laboratory Processing and Screening
• Core Receipt and Cutting
• CT Scanning
• Gamma Ray Logging
• Removal from Liners
• Core Viewing and Sample Selection
• Sample Preservation
• Core Plugging
• Core Slabbing
• Core Resination
• Core Photography and Imaging
• Weak or Unconsolidated Core Processing
• Special Handling Considerations for Difficult Rock Types

Core receipt and cutting. Particular
attention should be paid to:
1.Well number.
2.Core catcher’s box log and information
written on, or attached to, the individual
core liner sections.
3. The top and bottom depths of the core
(driller’s report) and those given on the top
and bottom liners of the received core.
4. Total number of liner sections as
indicated on the shipper’s documents and
the client’s despatch fax/email, and the
number received.

CT Scanning. Linear X-ray and CT scanning of
core samples can be applied to scan the core
normally in (or sometimes removed from) its
liner, prior to plugging.
The core is scanned by a highly collimated X-ray
source
The spatial resolution in a CT image is
determined principally by the size and number
of detector elements, the size of the X-ray focal
spot, the source– object–detector distances and
the size of the pixel array used to reconstruct the
image. Conventional medical CT instruments
provide resolutions of the order of 1–2 mm

CT scanning with the core
in its liner is used to orient
the liner with respect to
bedding planes, to
evaluate damage and to
select representative plug
locations avoiding
heterogeneous, invaded or
fractured intervals
Advanced digital imaging
and image capture
techniques allow CT
images to be run as a
continuous moving image
along the core (Fig. 3.5).

Gamma Ray logging. As most cores are now recovered in
fibreglass and aluminium liners, and as these are relatively
‘invisible’ to gamma radiation, the core gamma ray log is
normally run with the core intact in the liners.
Typical equipment (Fig. 3.6) comprises:
• A time or speed controlled conveyor belt for moving core.
• A tunnel containing a lead-shielded gamma ray detector is
designed such that the core can be moved at a fixed speed
underneath the detector.
• A computer data acquisition system that can display and
plot total and spectral gamma counts versus depth
An example output from a spectral gamma log run is shown in Fig. 3.7.
Total gamma is shown as counts per second (cps), and potassium and
uranium/thorium contents in per cent and ppm, respectively.

Removal from liners. When laying
out the core, the following guidelines
are applicable:
• The bottom of the core should be
placed on the viewing table first
with each succeeding piece being
placed closer to the top.
• The proper sequence and
orientation of the core must be
respected.
• The total recovery can then be measured. If any errors are noted, the lab should contact the client so that some agreed
corrective action can be taken. l Any excess fluid or excessive mud cake should be wiped off the surface. The core
should never be washed with water
FIGURE
3.14 Core
orientation
markings

Core viewing and sample selection. The objectives of core viewing are to assess the quality of the core material and
to select plugging sites and preserved sample locations.
Final sample selection is based on a number of factors including:
• Petrophysical sampling requirements.
• Well logs from cored interval.
• Core gamma ray and CT scans.
• Core lithology and condition.
Sample preservation. Whole core sections can be temporarily preserved
by sealing the ends of the core liner sections.
Dry Preservation:
1.Barrier Foil Laminates ,
2.Hot Wax or Strippable Plastics
Wet Preservation. The most
common containers are glass or
PVC, but PVC allows diffusion of
oxygen and water
FIGURE 3.15 Heat-sealable
plastic/aluminium laminate
(ProtecCore TM).
FIGURE 3.16 Seal peel for
plugs (a) and whole core
(b).
FIGURE 3.17 Example
anaerobic jar for core
preservation

Core plugging. Most routine and special
core analysis tests are performed on plug
samples cut from the full diameter core.
Drill Press and Plugging Fluids. Core
plugs are taken using a diamond-tipped,
hollow cylindrical, rotary core bit mounted on
a drill press, similar to that shown in Fig. 3.20.
Typical plugging fluids are:
• Brine or synthetic formation water, which
is made up to the same composition as the
formation water
• Depolarised kerosene, base oil, or mineral
oil (e.g. Blandol) are used where brine–
rock incompatibility is expected, where the
formation brine composition is unknown
or where cores are cut with oil-based mud
and scheduled for DS or fresh-state
analyses.
• Compressed air is used where the invasion
of plugging fluid may change the plug
saturations.
• Liquid nitrogen or chilled air is often used
where the core is too weak to survive
conventional plugging due to erosion by
liquids.
FIGURE 3.20 Core plug
drilling press

Plug Orientation. For most core analysis,
formation damage or geomechanics test
applications, plugs are taken either in the
‘horizontal’ or in the ‘vertical’ direction with respect
to bedding planes on the core. Figure 3.21 shows
typical plug orientations that are cut from core—in
this case from a vertical well with relatively steeply
dipping beds.
Horizontal plugs can be taken parallel to bedding in
two directions: one where the long axis of the plug
is parallel to bedding plane (‘x’-direction) and the
other taken orthogonally, at 90° to the x-direction
(‘y’-direction).
Vertical or bedding plane normal plugs are typically
used for rock mechanics tests and routine core
analysis tests. They are taken perpendicular to the
maximum dip (bedding plane normal or
perpendicular), not parallel to the long axis of the
core. These sample the minimum permeability
direction

FIGURE 3.23 Schematic
guide for plug sampling
Though there are the occasional exceptions, most SCAL and
formation damage tests are carried out on horizontal (bedding
plane parallel) plugs.
SCAL plugs can also be
taken (and preserved)
adjacent and parallel
to RCA plug locations
on the unslabbed or
slabbed core, as
indicated in Fig. 3.24

• Rock Mechanics Test Plugs (V). Unconfined compressive strength (UCS),
single-stage (SST) and multi-stage triaxial, pore volume compressibility and acoustic
travel time tests are normally performed on vertical plugs cut perpendicular to the
bedding planes. The exception is plugs cut from lithologies such as shales which can
exhibit bedding plane strength anisotropy. In this case, a series of plugs are often cut
from 0° to 90° to bedding. Thick-wall cylinder (TWC) tests are normally performed
on horizontal (bedding plane parallel) plugs.
FIGURE 3.25
Standard 1.5”
diameter TWC test
sample
TWC tests are
performed on a 1” or
1.5” diameter hollow
cylinders, with a
standard outside to
inside diameter (OD:ID)
ratio of 3:1. The
example shown in Fig.
3.25 is for a 1.500
diameter plug sample.
FIGURE 3.28 UCS plug with irregular sides due to variable weight on bit when plugging.
FIGURE 3.27 Example of tensile
splitting of UCS test plug.

Core slabbing. Core slabbing exposes the sedimentological, lithological
and bedding features of the core and allows preparation of a clean surface for
core photography and for further geological examination
• Normally, once the routine core analyses are completed the lab, with
authorisation from the client, will perform a 2/3 to 1/3 cut (or similar)
longitudinally along the length of the core or core liner, as shown in Fig.
3.31.

FIGURE 3.32 Example of a
resinated biscuit slab

Core photography and imaging. Nowadays,
virtually all core images are captured digitally
rather than the large format photographic
plates that have been used historically. Not
only does this make it easier to store,
manipulate and interpret the image data,
high-resolution digital imaging means that it is
possible to view sedimentary or other features
in very fine detail.
Core is normally photographed under both
natural and ultraviolet (UV) light. UV imaging
should be carried out as soon as possible
following slabbing as light hydrocarbons can
often dissipate over relatively short times

360° Core Imaging. A recent
development in core imaging is the 360°
core digital scanning system (Fig. 3.36)
which enables high-resolution core
images to be taken around the entire full
core circumference of the core, or on
slabbed surfaces
Scans are normally taken over a core box
length across the interval of interest and
then they are added and fitted together,
creating larger aligned core intervals.
Figure 3.37 provides an example of a scan
which has been ‘flattened’ out to
simulate the kind of image from borehole
image logs

FIGURE 3.42 Liquid nitrogen plugging.

Core Preservation
In the case of unconsolidated material,
core will inevitably suffer a rapid
deterioration process with time, especially
if the core has been frozen and it is left to
thaw. Preservation will only help to
minimise the process but will not be able
to avoid it. Two options for long-term
storage are generally available:
• film, foil and wax;
• barrier laminates.

Special handling considerations for difficult rock types
Unconsolidated Core
• Unconsolidated or poorly consolidated core containing light oil or gas is susceptible
to significant fluid loss during handling at the surface
• Cored intervals should be limited in length to prevent possible collapsing under its
own weight, and liners must be cut into 1 m (3 ft.) sections. Specialised handling
protocols and liner stabilisation will almost certainly be required at wellsite to
minimise disturbance to the rock texture
• Heavy oil introduces additional challenges particularly due to the low mobility of gas
which can result in very slow drainage. This can, in turn, cause the core to expand,
filling the annulus between the core and the liner

Carbonates
• Generally, most carbonates are competent and do not often require
special precautions
• Resin, foam or gypsum used for stabilisation will enter and may fill larger
vugs and natural fractures
Low Permeability Rock
• Evaporation of fluids, a problem with all core materials, is a particular
difficulty in low permeability and low porosity core where the percentage
change in saturation may be much greater for the same volume of fluid
evaporated

Coal
• Coal cores are normally taken for coal-bed methane or in situ combustion projects
• Careful handling is essential and coal can be very brittle and shatter when the core
liner is flexed, dropped or struck with a sharp blow
• Pressure-retained core technology, that does not require an estimate of lost gas,
offers a more accurate means to determine the total in situ gas content of a coalbed.
The volume of gas evolved from the coal core in the pressure barrel is measured as a
function of time, temperature, and pressure
• Coal core must be handled with care at the wellsite because of its heterogeneous
nature, and because the methane found in small pores within the core is under
pressure
Diatomites are generally high-porosity, low-permeability rocks composed of opaline-
quartz phases with varying amounts of detrital material. Diatomites are cored with
disposable inner barrels or liners . . Freezing of diatomite is not recommended

Shaly Sands
• Shales are a mixture of organic and inorganic components with and have high
kerogen and clay contents, low porosity and ultra-low permeability
• One of the biggest problems in shales is that a large number of partings or
splitting are induced by stress unloading and relaxation during coring and core
recovery
• It is recommended that fissile shale cores be handled in the following manner:
a. Avoid excessive handling or movement of the core.
b. Remove any excess water.
c. Preserve immediately to stop desiccation.
d. Masking tape or fiberglass packaging tape may be wrapped around core
segments perpendicular to the fissility planes to reduce further splitting.
Alternatively, heat-shrinkable plastics can be used.

Recommendations.
The handling-induced core damage generally occur due to:
1. Rotating the inner tube (normal pin-box connection) during unscrewing the connections.
2. Cutting the core by splitters or saws.
3. Laying-down the inner tube joints containing the core from the vertical position (the rig floor) to the
horizontal (the pipe-decks).
Systematic and careful planning and execution of handling practices including good communication
among all the persons involved can guarantee prevention of damage to the core during its transportation
Use non-rotating inner tube stabilizers (NRITS) between the inner tube joints to minimize the rotation of
the steel inner tube joints (while opening their threads), which can cause severe damage at the threaded
connection points.
– Use hydraulically-powered core splitters/guillotine, instead of mechanical ones, for cutting the
core. This prevents damage to the core sample.
– Use a disposable inner liner (preferentially aluminum3) inside the steel inner tube to highly
maintain the core quality during handling, particularly in unconsolidated formations . Using easily-openable
liners such as half-moon or 2/3-moon aluminum liners or laser/plasma-cut liners is strongly recommended
particularly in unconsolidated formations.
– Use modern H-beam lay-down frames (LDF with latches, instead of ropes) for safer lay-down of
the core with less bending

Recommendations.
• As soon as the inner tubes containing the cores reach the pipe-decks
on the ground, quickly start cleaning, marking, and surface gamma-
ray spectroscopy, before cutting the inner tube joints into one-meter
sections.
• – Using air-powered saw is recommended for cutting the inner tubes
into sections while the core is kept in the cradle and is continuously
run into the saw location by rollers.
• – Avoid washing the core with water, just preserve it before loading.
• – Core cubes are recommended for loading rather than wooden
boxes, before shipment to the central core analysis lab.

4. Core Sample Preparation
• Cleaning
• Core Drying
• Quality Control Issues, Checks and Diagnostics
• Clays and Clay Damage Mechanisms
• Core Conditioning for Porosity Measurements
• Special Considerations in Core Preparation

Clean and dry plug samples methods
• API RP40 (1988) lists the following methods for cleaning core and plug
samples:
1. Flushing by centrifuge
2. Gas-driven solvent extraction
3. Liquefied gas extraction (pressurised Soxhlet extraction)
4. Distillation–extraction (Soxhlet extraction)
5. Solvent flush cleaning by direct pressure
• API RP40 (1988) lists the following methods for drying core and plug samples:
1. Conventional oven 2. Vacuum oven 3. Humidity oven.
These methods are still in common use today and other specialised drying technologies, such as critical
point drying (CPD), have been developed for sensitive cores. Selection of the most appropriate method
depends on the lithology, clay content, potential wettability contamination of the core, as well as the
objectives of the analysis for both RCA and SCAL.

Cleaning
Solvents. Core plugs need to be cleaned and dried to:
1. remove native oil, mud filtrates, evaporated salts and connate water prior to
porosity and permeability analysis and single-phase SCAL experiments (e.g.
formation factor and brine permeability);
2. condition or render the core water wet prior to certain two-phase SCAL
experiments (primary drainage capillary pressure) and prior to wettability
conditioning (e.g. restoration).
Effective cleaning of oilfield core can only reliably be achieved using at least two
solvents:
(a) a non-polar solvent to dissolve the lighter ends of the oil and evaporate water in
high-temperature extraction and
(b) (b) a polar solvent to dissolve the heavier ends of the oil; to miscibly solvate water
in lower temperature flush cleaning methods; and to leach/dissolve any
precipitated salts from the core remaining after evaporation or removal of
formation brine.
A variety of solvents are therefore available depending upon the application.

Common Cleaning Methods
FIGURE 4.1 Schematic
of Soxhlet apparatus
FIGURE 4.2 Typical hot Soxhlet
apparatus set-up in the core analysis
laboratory.
1. Soxhlet extraction (hot and cool),
2. constant or total immersion (submerged) Soxhlet extraction
3. solvent flush cleaning (mild, miscible cleaning).
The principal components are:
• a round bottomed flask or extraction pot (2) filled with
solvent
• a stirrer bar or anti-bumping granules (1)
• a side arm (3)
• a soxhlet thimble containing the core samples (5)
• a siphon arm inlet (6) and outlet (7)
• a reduction adaptor (8)
• a condensor unit which can be circulated with chilled
water via ports 9 and 10
High-boiling point extraction solvents such as toluene and
methanol, it is referred to as hot Soxhlet extraction

• Cool Soxhlet Extractor. This variation on the standard hot Soxhlet
extraction method increases the solvent residence time in the
apparatus after condensation. As the solvent is cooler, it is less
effective and it takes significantly longer to clean cores than in a hot
Soxhlet apparatus
• Total (or constant) immersion cleaning combines the non-damaging
benefits of cool static cleaning and the efficiency of hot Soxhlet
cleaning. The system design means that the cores are continually
submerged under solvent, throughout the cleaning process and are
not exposed to air during the reflux cycle in the standard solvent. As
the cleaning temperature is lower than standard Soxhlet and the
interfacial forces are minimised, delicate clay structures can be
preserved.
FIGURE 4.3 Total
(constant)
immersion
Soxhlet
equipment
schematic
FIGURE 4.4 Typical
constant immersion
cleaning assembly.

Flush (Flow) Cleaning . Two variations of flow-through cleaning are
commonly used: one utilising miscible solvents (mild, miscible cleaning) and
the other immiscible solvents.
Flush cleaning uses similar solvents to the Soxhlet extraction technique.
However, to prevent interface movements through the core, a miscible
sequence of cleaning solvents is often required, especially if the core
contains delicate clays or the core is to be critically point dried.
Miscible cleaning implies that the solvent used is miscible with the
preceding and succeeding solvents in the sequence. Typical flush clean
sequences for cores containing oil include:
• toluene – isopropanol - methanol;
• toluene - chloroform/methanol azeotrope - methanol.
For gas cores drilled with water-based muds only methanol or ethanol
cleaning may be required as both solvents are miscible with water
FIGURE 4.5 Schematic of solvent
flushing coreholder
FIGURE 4.6 Flow-
through core cleaner

Core drying
FIGURE 4.7 Vacuum ovenFIGURE 4.8 Humidity chamber details.
Two drying methods: 1. critical point drying- CPD; 2. flow-through
(lenient air) drying.
Conventional (and Vacuum) Oven Drying. Vacuum is often used to
speed up the drying process. Conventional oven drying used in
combination with hot Soxhlet extraction is considered to be a harsh
preparation method as it can result in clay destruction due to
dehydration of water held within or on the clay mineral structures.
Fig.4.7
Humidity Oven Drying (HOD).HOD is used where excessive temperature and anhydrous conditions associated with
conventional oven drying can cause damage to clays.
1. The direct injection environmental test chamber (Fig. 4.8).
2. A much cheaper solution is to use a dessicator vessel inside a conventional oven
FIGURE 4.9 Dessicator used
for humidity-conditioned
drying over saturated salt
solution

Critical Point Drying (CPD)
• CPD is a well-established method of dehydrating biological tissue prior to
examination in the scanning electron microscope (SEM).
FIGURE 4.10 CO2 phase diagram
• The CPD technique prevents the development and advancement of a gas/
fluid or fluid/fluid interface within the rock by raising the fluid within its
pores above its critical point.
• Figure 4.10 shows the phase diagram for CO2. Consider the phase envelope
separating the liquid from the gas phase. At pressures and temperatures
above this line, CO2 exists as a liquid. At pressures and temperatures below
this line, it exists as a gas
• CO2 remains the most common medium for the CPD procedure as the
critical pressure and temperature (32 °C and 1072 psi) can be easily
achieved in practice in the lab.
• Fig.4.11 shows a typical CPD apparatus developed principally to prepare
biological specimens for SEM analysis though specialist systems have been
developed for core drying.
FIGURE 4.11 Example CPD apparatusTraining for Grenergy

Quality control issues, checks and diagnostics
1. SEM and XRD analysis should be considered to identify the presence of sensitive minerals and delicate
clays in the core so that the cleaning and the subsequent drying processes can be selected to minimise
potential damage due to water evaporations.
2.In some cases, a cleaning pre-study should be undertaken, depending on the test objectives
3. During cleaning, the lab must check the non-polar solvent effluent for solvent discolouration and oil
fluorescence in core (presence of oil), and polar solvent effluent for presence of chlorides (silver nitrate
solution test) to ensure that the cleaning process has been completed.
4. During drying, plugs should be dried until a constant weight is achieved: +/- 0.01 g over three or four
successive measurements over 2–3 days.
5. Grain density is a powerful QC tool. Anomalously, low grain density values could result from insufficient
cleaning or drying
6. It is worth checking with the laboratory which type of humidity oven they use: direct injection or
saturated salt solutions, and if the latter, the salt solution used or proposed.
7. In critical point drying (CPD), the transition liquid should fill 50% of the specimen pressure chamber.
This will ensure that specimens are not uncovered during initial flushing stages and in addition, this
should enable critical constants of temperature, pressure and density to be achieved relatively
simultaneously without excessive pressure or evaporation conditions occurring

Whole core analysis measurements
• Only a limited number of commercial laboratories have the experience
and resources to perform RCA measurements on whole cores up to 400
in diameter and up to 1200 long, especially at elevated stresses. Even
fewer commercial laboratories have the capabilities to perform anything
other than basic SCAL measurements
• All whole core samples should be photographed and CT scanned to
evaluate heterogeneity
• A whole core sample may not be cut exactly normal or parallel to the
rock’s bedding planes as this depends on the well deviation and
formation dip
• For fluid saturation Dean–Stark measurements are possible in whole
core samples provided the lab have the necessary large capacity
apparatus
• The most common porosity measurement method involves
determination of helium expansion pore volume (Vp) into a whole core
coreholder at stress, and ambient condition calliper bulk volume (Vb).
• Whole core analysis uses a similar system to steady-state core plug tests,
but the coreholder is designed to allow flow both across the sample
(radial transverse flow) and along the axis of the core (linear flow).
Figure 5.44 provides a schematic of a typical whole core system.

The parameters of interest are different for siliciclastic and carbonates. Generally, the
following parameters are recorded.
• Missed layers. In a routine plugging, plugs are prepared every 30 cm
• Color of the core. There are two primary factors for any color including hue and the quality of
lightness (light and dark colors).
• Sedimentary structures such as laminations, cross-beddings, graded beddings, soft-sediment
deformations, ﬂaser and lenticular beddings, mud cracks, and load casts.
• Compaction-related features such as stylolites and solution seams
• Beddings and stratal surfaces. Such surfaces are the result of changes in environmental
conditions, such as depositional energy
• Intraclasts and rip-up clasts. They are mostly derived from the lower bed and indicate a
reactivation of the environmental energy
• Visual porosities
• Trace fossils
• Fractures
• Fining or coarsening upward sequences
• Oil staining

Fracture Presentation
• The first step in fracture study of a
reservoir is image log interpretations,
if there are any, but the first step on
the cores is whole core CT-scanning
• Fracture properties are also
transferred to a fracture log and the
final result is illustrated beside the
other rock properties (such as
porosity, permeability, and lithology).
• The depths of photos taken from the
fractures are also recorded on the
sheet. The fracture frequency per
meter represents the fracture
intensity in each meter of the cores

5. Petrography and mineralogy
Thin Section Staining
Sedimentary features
Pore types
Facies and facies coding classiﬁcation
Rock types

Abstract
• Macroscopic core studies investigate large-scale core properties that are not
detectable by any other methods.
• A macroscopic description of the core is the final step in geological observations
and reveals the remaining rock properties between the plugs.
• Building both conceptual and numerical reservoir models without involving
macroscopic core properties leads to vital mistakes in reservoir characterization.
• Routinely, there are two stages of macroscopic core description.
• The first stage is after core cleaning for the first evaluation of the rocks, helping microscopic
considerations and selecting primary samples for various analyses.
• The second stage is at the end of geological studies. Cores are slabbed prior to description.
The fresh uncontaminated surface of the rock is available at this time. Core-scale rock
properties are recorded on standard sheets that were prepared prior to description.
• The recorded parameters are illustrated on a core description log with
appropriate scale. The results are integrated with microscopic geological studies
as well as routine core analysis data to achieve the best interpretations and build
an accurate reservoir model.

Abstract
• Microscopic observations are one of the main sources of information for geological
studies.
• Routine microscopic studies of a core sample include petrographical analysis to
understand facies properties and diagenetic processes, paleontological studies for
absolute age dating, X-ray diffraction for mineral identification (especially clays), scanning
electron microscopy equipped with energy dispersive spectroscopy for pore and pore
throat determination, mineral identification, and elemental analysis.
• The static reservoir properties of a sample depend completely on primary (facies) or
secondary (diagenesis) characteristics of the rocks.
• The rock mineralogy, constituents, sedimentary environment, microscopic porosities,
cements, compaction features, and many other parameters are gained by study of a rock
sample under a polarizing microscope.
• Such a framework is integrated with other microscopic and macroscopic geological data
and provides the reservoir zonation scheme and understanding of reservoir geometry.
• Fine-size minerals, especially clays, are identified by the X-ray method. They play a vital
role in reservoir properties and future drilling in the field. Pore types and pore throats
determine the fluid flow properties and major rock types of the reservoir. They have a
major effect on reservoir heterogeneity.

Thin Section Preparation. The slide must be frosted before attaching the sample. The rock
sample is about 0.03 mm thick and light passes through the rock and glasses. Every mineral
has its special properties in normal or polarized lights. All visible parameters from the rock
are recorded

Thin Section Staining. There are
different techniques for staining
various minerals. The most widely
used methods for carbonates are a
dilute hydrochloric acid containing
alizarin red S (ARS), potassium
ferricyanide, or a mixture of both
Carbonate staining is used for
distinguishing calcite from dolomite
or aragonite. Potassium ferricyanide
(PF) is used for distinguishing
ferroan calcite and dolomite
Hydrochloric acid is used in both
methods and thus the sample will
react with the solution. The ﬁnal
result is thinner than the original
rock sample. Both ARS and FP are
useful in carbonate staining and a
mixture containing these two
materials is recommended (Fig. 3.2).

Sedimentary environments. The facies and sedimentary environments are
some of the most useful data in sequence stratigraphy of a reservoir.
The amount of porosity and pore types. The porosity value is determined
with visual estimation, point counting, or image analysis using various
software. The results of porosity values derived from the point counting are
comparable to laboratory tests. Commonly, visual estimates of porosity from
thin sections are lower than the routine core analysis (RCAL) tests.
Pore types. Pore typing is also an appropriate method for rock typing and
grouping the samples to reduce reservoir heterogeneity

• Various types of macropores, pores that
are visible under the light microscope, are
determined at this stage. The results are
comparable with laboratory measurements
such as mercury injection capillary pressure
(MICP) or nuclear magnetic resonance
(NMR) data
• Fractures. The most important point is that
fracture study based on thin sections or even
plugs is not an accurate method
• Cementation. Type and frequency of cements
have major effects on reservoir properties
• Compaction features. Stylolites and solution
seams represent chemical compaction
• Dolomites and the dolomitization process.
These change the porosity and permeability,
their relationships, rock density, pore throat
size distribution, and wettability of the
reservoir

Many parameters are the same in carbonate and
siliciclastic petrography
• General information and depth are exactly the same.
• Lithology is composed of three main components
including quartz, feldspars, and rock fragments .
• The size of grains in a siliciclastic rock characterizes
the energy of the transporting media and the
depositional environment
• Siliciclastic reservoirs are often sandstones. A
conglomerate or muddy reservoir is not common.
Sieve analysis is the best method for size
measurement in loose unconsolidated cores. Loose
sands of the Ahwaz Sandstone unit of the Asmari
Formation, the ﬁrst discovered carbonate reservoir
of the world, is a good example of such reservoirs

• Minor components are seen in low amounts and are not used
for rock classiﬁcation including but not limited to glauconite,
bioclast debris, carbonate particles, siderite, and organic
materials.
• Textural parameters include sorting, roundness, and maturity
• Wadell (1932) proposed using the ratio of the average radius of
curvature of the particle corners to the radius of the maximum
inscribed circle as the roundness (Fig. 3.10a).
where r is the radii of curvature of the grain corners, N is the
number of corners, and R is the radius of the largest inscribed circle
within the particle.
• Various methods have been devised for calculating grain
sphericity

Facies and facies coding classification of sandstones is based on
Pettijohn (1975) in most cases (Fig. 3.11).
Sandstones are classified based on the proportion of matrix and the
grain composition
• Other features such as laminations, mud cracks, brecciation, and the
frequency of opaque minerals are recorded. It is worth mentioning
that this part is strongly dependent on project objectives.
• The sedimentary environment process is the same as the
carbonates.
• Porosity types and amount, including the main types of porosity in
sandstones which are intergranular, dissolution, and fracture
porosities.
• The fracture process is the same as the carbonates. Sandstones are
generally less fractured than carbonates.
• Cementation where the main cements are ferron, carbonate, and
silica deposited as rim, syntaxial, and pore filling between the grains.
• Compaction is the same as the carbonates.
• Sample selection, laboratory porosity, permeability and grain
density, rock types, and comments are also the same as carbonates

Fracture analyses.
• Detection of fractures and vugs from core
can be done qualitatively by examination
• Basic parameters obtained from an
evaluation of fractures and the
relationships between those parameters
are shown in Fig. 3-18. Some core analysis
techniques used for naturally-fractured
reservoirs are listed in Table 3-II.
• Fracture orientation measured by a
goniometer in conjunction with core
orientation data obtained by coring with a
special orientation tool, a goniometer
defines dip and strike of observed
fractures,

Heterogeneities
• Pore space in carbonate rocks is highly variable and porosity can exist as
microporosity, intergranular porosity, vuggy porosity, fracture porosity, or a
combination of all four. These characteristics impose unique procedures
when acquiring and analyzing carbonate reservoir cores.
• Core retrieval from carbonate reservoirs requires special considerations
• The heterogeneous nature of carbonates often requires whole core rather
than plug analysis. Whole core analysis is typically more costly and requires
more time. The CT scan can help in early quantification ofheterogeneities,
even while the core might still be in a container. In vuggy and fractured
formations, it is still very helpful to drill and test plugs from the matrix part
to define its properties, such as capillarity. These properties are important
because they control initial water content and, hence, matrix hydrocarbon
saturation.
• Carbonates containing multiple pore systems present challenges to obtaining
representative capillary pressure data.

Multimineral composition
• Carbonate reservoirs can contain complex mineral composition. It is not unusual to
have evaporites, e.g., halite (salt), anhydrite, or gypsum (if shallow), layered with or
in the pores of the carbonate rocks. Limestone and dolomite mixtures are possible.
• Dolomites can contain uranium salts, left in the rock during dolomitization, which
make the traditionally low gamma ray rock radioactive
• Gypsum presents a difficulty in obtaining accurate core analysis. Gypsum de-waters
at temperatures just above 100~ (38~ The presence and de-watering of gypsum can
be recognized by examination of core analysis saturation results. The sum of water
saturation and oil saturation will total more than 100%, a physical impossibility.
• Core analysis of gypsum-bearing rock needs to be done below de-watering
temperature. Halite (salt) can add complexity to core analysis. Pore-filling salt can
dissolve during coring if the drilling mud is not kept at saturated salinity (at the
downhole temperature). In this case, porosity that is not present in situ could be
created by the salt dissolving out of the pores. Pore-filling or nodule salt can be a
problem during Dean-Stark saturation analysis.

Clay Types
Montmorillonite (smectite) group minerals are composed of two silica tetrahedral sheets with a central alumina
octahedral sheet. Some 80% of exchange cations (commonly Na+ or Ca2+) occur between the silicate layers, with the
remainder associated with the external surfaces of the particle
Illite (mica) group minerals comprise a layer composed of two silica tetrahedral sheets with a central octahedral sheet
FIGURE 4.15 SEM
photomicrograph of
pore-filling smectite in
shaly sand
FIGURE 4.16 SEM
photomicrograph of fibrous illite
bridging across pore
Kaolinite minerals are composed of a single silica tetrahedral sheet and an alumina
octahedral sheet combined in a unit such that the tips of the silica tetrahedrons and one
of the layers of the octahedral sheet form a common layer
Chlorite group minerals consist of alternate mica- and brucite-like layers.
FIGURE 4.17 SEM
photomicrograph of kaolinite
booklets loosely adhering to
quartz grains (discrete habit).
FIGURE 4.18 SEM
photomicrograph
of pore-lining
chlorite
Major clay group are:

What is Rock type?
• The term “rock type” is used to describe the major distinguishing feature(s)
of core material
• Consolidated rocks are hardened as a result of cementation. They need no
special treatment at the wellsite. Common consolidated rocks include
limestone, dolomite, sandstone, and chert
• Unconsolidated rocks have little or no cement and are essentially compacted
sediments
• Unconsolidated Rock—Light Oil and Gas. It is critical to preserve
unconsolidated cores containing light oil in an efﬁcient and expedient
manner. Any unnecessary movement of the core should be avoided. If
freezing is used to stabilize unconsolidated materials, the core should not be
transported before it is fully frozen, as partial freezing can cause structural
damage to the core
•

What is Rock type?
• Unconsolidated Rock—Heavy Oil. The greatest difﬁculty in handling
unconsolidated rocks that contain viscous heavy oil is prevention or
minimization of delayed core expansion
• Improving core quality in unconsolidated heavy oil sandstones requires the
following considerations:
a. Provide mechanical restraint to expansion.
b. Provide a means to allow gas drainage.
c. Provide mechanical strength to the core
• Freezing of unconsolidated heavy-oil core may be necessary, although in
general, freezing is not well understood
• Core handling during transportation and storage for unconsolidated
materials containing heavy oil must maintain the mechanical restraint and
low temperature
• Carbonate; Evaporites; Fractured rocks; Rocks reach in Clay minerals; Shale;
Low permability rocks; Coal; Diatomite;

Geological Rock Typing. A perfect rock type has the same geological, petrophysical, and
reservoir properties
• Grain-dominated samples may have more porosity or loose sandstones contain a
considerable amount of intergranular empty spaces.
• Ooid grainstone with disconnected moldic porosities has high porosity with low
permeability. This is the same for a mud-dominated carbonate sample with a
considerable amount of microporosity, but both of them are not routinely present in a
reservoir. In the same way, an ooid grainstone with high interparticle porosity has the
same behavior with a dolomitized packstone from the ﬂuid storage and ﬂow point of
view. This time, both of them can be present in a reservoir and they form one rock type
• The depositional characteristics and diagenetic processes of one reservoir are limited
• For example, in a carbonate ramp environment, facies and environments have low
diversity. In most cases, one or two parts of the ramp (inner, middle, and outer) are
present

The defined rock types are as follows.
• GRT1 is ooid/bioclast packstone to grainstone with
fabric-destructive dolomitization or a crystalline
carbonate. Both facies and lithology are different but
the final product of facies and diagenesis
combination is one rock type.
• GRT2 has the same facies as GRT1 with fabric-
retentive dissolution. It has high porosity but they are
not connected with each other.
• GRT3 has the same facies again deposited in near-
shore environments and thus is highly anhydritic
cemented. It has no considerable porosity or
permeability and so is classified as a nonreservoir
rock type.
• GRT4 changes from mudstone to bioclast
wackestone with microporosity. It has high porosity
with very low permeability

Summary
• Rock typing is the process of assigning reservoir properties to geological facies.
• The properties of interest are fluid storage and flow and thus it is possible that
two facies are grouped in one rock type or one facies is divided into two rock
types.
• There are three main categories for this process including geology, reservoir
(static properties), and petrophysics.
• Geological rock types are defined by integrating facies and diagenesis characteristics of the
samples in a porosity–permeability framework.
• Reservoir methods use porosity, permeability, pore throat size, and their relationships for
dividing the samples into various rock types.
• The final result is an ideal unit that contains the same geological, reservoir, and wire line
characteristics.
• As wire line data are available from almost all wells and reservoir intervals, this
ideal unit could be distributed in 3D space, even with limited core data. These
units are perfect for both static and dynamic modeling.
• On a regional scale, the concept of flow unit is used to divide the reservoir into
compartments with the same reservoir quality.
• All methods try to reduce the reservoir heterogeneity to understand reservoir
behavior on microscopic and macroscopic scales.

Summary continue
• Petrographic studies include such methodologies as optical mineralogy, thin-
section analysis, scanning electron microscopy, image analysis, X-ray
diffraction, Fourier transform infrared spectroscopy, calcimetry, dating and
fracture analyses
• Thin sections are thin slides made of rock, commonly, after the pores have
been impregnated with an epoxy that solidifies. They furnish samples for
visual study of textural properties, for mineral identification and abundance,
and the processes that have affected the rock, e.g., depositional and
diagenetic activity
• In a scanning electron microscope (SEM), a rock sample that has been
previously coated with a thin film of conductive material is bombarded with
electrons in a high vacuum, resulting in a secondary electron emission picture
for sample study (the SEM) and a characteristic set of X-rays (EDX spectrum).
• Core images come in several sizes: scanning electron microscope (very small),
thin section (small), and core photographs (regular). Core images can be used
for visual analysis of the rock, or can be used to acquire quantitative data for
characterization of petrophysical and geological rock properties

Exercises

Which factors includes the final sample
selection ?

Core viewing and sample selection. The objectives of core viewing are to assess the quality of the core material and to
select plugging sites and preserved sample locations.
Final sample selection is based on a number of factors including:
• Petrophysical sampling requirements.
• Well logs from cored interval.
• Core gamma ray and CT scans.
• Core lithology and condition.
Sample preservation. Whole core sections can be temporarily preserved
by sealing the ends of the core liner sections.
Dry Preservation:
1.Barrier Foil Laminates ,
2.Hot Wax or Strippable Plastics
Wet Preservation. The most
common containers are glass or
PVC, but PVC allows diffusion of
oxygen and water
FIGURE 3.15 Heat-sealable
plastic/aluminium laminate
(ProtecCore TM).
FIGURE 3.16 Seal peel for
plugs (a) and whole core
(b).
FIGURE 3.17 Example
anaerobic jar for core
preservation

Which are typical plugging fluids ?

Core plugging. Most routine and special
core analysis tests are performed on plug
samples cut from the full diameter core.
Drill Press and Plugging Fluids. Core plugs
are taken using a diamond-tipped, hollow
cylindrical, rotary core bit mounted on a
drill press, similar to that shown in Fig. 3.20.
Typical plugging fluids are:
• Brine or synthetic formation water, which
is made up to the same composition as
the formation water
• Depolarised kerosene, base oil, or
mineral oil (e.g. Blandol) are used where
brine–rock incompatibility is expected,
where the formation brine composition is
unknown or where cores are cut with oil-
based mud and scheduled for DS or
fresh-state analyses.
• Compressed air is used where the
invasion of plugging fluid may change the
plug saturations.
• Liquid nitrogen or chilled air is often used
where the core is too weak to survive
conventional plugging due to erosion by
liquids.
FIGURE 3.20 Core plug
drilling press

What is exposing the Core slabbing process?

Please deﬁne rock types

Which methods are known for clean and dry
plug samples?

What does mean the Critical Point Drying (CPD)?

Which clay types do you know?

Day 2 d coring & core analysis and reservoir geology

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