Core analysis & wellsite core acquisition, handling and transportation

Course Title:
Coring & Core Analysis and Reservoir
Geology
Dr. Arzu Javadova
2018
Training for Grenergy company_2018

1st Day
1. Core Analysis Data: The Foundation of Formation Evaluation,
Coring and core analysis objectives
Core Analysis Data Uncertainty
Core analysis an overview
2. Wellsite Core Acquisition, Handling and Transportation
Wellsite core handling procedures and preservation objectives
Coring Systems
Conventional Coring Operations
Coring Fluids
Core Damage and Core Fluid/Petrophysical Property Alteration
Wellsite Handling
An electronic guideline of Core sample preparation, Routine core analysis , Preparation for Special Core Analysis
and Core Laboratory Processing and Screening

1. Core Analysis Data: The Foundation
of Formation Evaluation,
Core analysis an overview

The foundation of formation evaluation
• Estimation of the volume of hydrocarbons initially in place in a
reservoir.
• Understanding of the physics of the reservoir-fluid system so that the
ultimate recovery of hydrocarbons is maximised in the most economic
matter
• Both static and dynamic reservoir models draw on a variety of data
sources including regional geology, seismic, sedimentological modelling,
drilling data, wireline and logging/measurement while drilling data,
fluid pressures and rock and fluid property data

The foundation of formation evaluation
• Core is normally the only part of the (relatively) undisturbed reservoir formation we
can actually see, touch and feel at the surface. For example, the volume of stock tank
oil initially in place (OIIP) in a reservoir can be determined from
Determination of the gross rock volume (GRV) and gross factor (G) in the net/gross
ratio (N/G) is the primary responsibility of geophysicists and geologists.
The reservoir engineer is responsible for oil formation volume factor (Bo) from
pressure, volume temperature (PVT) experiments.
The petrophysicist is responsible for net (N), porosity (f) and water saturation (Sw)
where data input relies principally on logs. Reservoir net thickness is normally defined
by a permeability cut-off, and high-resolution permeability data are only possible from
core. Porosity interpretation (e.g. from density logs) should be verified by, or calibrated
against, stressed core porosities

The typical core analysis tests which are
offered by commercial core analysis
vendors and used as data input in
petrophysical static models are
summarised in Table 1.1.
Recovery factors may be determined on
purely technical criteria, but, more
probably, on economic or
environmental terms.
where Mrw and Mro are the relative mobilities of water and oil,
respectively. The parameters kro and kro are the relative
permeabilities to oil and water, and mw and m are water and
oil viscosities, respectively.

The typical core analysis tests which
are offered by commercial core
analysis vendors and used as data
input in dynamic reservoir models are
summarised in Table 1.2

Retrieved data
Geological evaluations include:
– Lithology.
– Depositional environments..
– Absolute age dating and chronological sequence establishment. Using fossil records.
– Regional scale correlation. Using fossils, geochemical proxies, and sedimentological properties.
– Diagenesis. The processes that have affected the rocks after deposition. They have a major role on
reservoir properties in many cases.
– Fracture analysis. These studies have some limitations on cores but still valuable information can
be retrieved.
– Pore typing. Using petrographical studies.
– Geochemistry. Both organic and inorganic geochemistry help in more accurate and effective
analysis and interpretation of the source and reservoir rocks.
– Geological rock typing. The basis of the heterogeneity reduction in the reservoir.

Retrieved Data
Reservoir engineering evaluation:
– Porosity determination
– Permeability measurement
– Reservoir rock typing and hydraulic flow unit
determination
– Oil–water or gas–water contacts
– Fluid saturation
– Acoustic velocity
– Gamma radiation
– Calibration of wire line logs using engineering
data retrieved from the cores (such as porosity,
acoustic velocity, or gamma radiation)
– Grain density
– Electrical properties
– Wettability
– Relative permeability
– Capillary pressure
– Pore volume compressibility
Geomechanical properties:
– Compressive strength
– Young’s modulus
– Poisson ratio
– Hardness
Information and
data obtained
using core
analysis

General information. What's coring?
• Coring is the method of providing rock samples from subsurface formations, by which
the core analysis results can potentially provide reliable information about their rock
and fluid properties
• In the petroleum industry, the main goal of coring is to practically identify formations
with a commercial scale of oil and gas content
• Coring had originally faced a lot of challenges including cost, technical problems
causing core damage due to coring causing invalidity of cores, cores with inadequate
geological data, etc.
• For the purpose of increasing the amount of measurable geological data from the
core and the amount of measured data while coring, respectively, oriented coring
and Logging-While-Coring (LWC) systems have been developed

It is performed in order to provide qualitative and quantitative geological and
mechanical data required for reservoir characterization and decision-making.
In addition to the reservoir engineering objectives, core samples with sufficient
diameter and length provide invaluable data about geological bedding,
formation dip and strike, stratigraphy, mineralogy, fractures, etc
The purpose of getting cores is to obtained rock samples of sufficient size for :
1) making reservoir analysis test to determined porosity, permeability ,residual
oil and water saturation (primary purpose of coring).
2) To establish the physical character of formation boundary.
3) To furnish paleontological data for age and facies correlation
4) To determined the structural attitude of the beds

Data retrieved from core analysis and their
role in reservoir characterization (Courtesy of
M. Naderi)
The first and main question before starting a coring job is
about the importance of the cores for the reservoir
evaluation. Is this really necessary?
In most cases the answer is yes.
Coring and core analysis are not expensive compared to
the overall budget of well drilling and completion.
Nevertheless, cores have vital information
for reservoir evaluations and assessments. A team of
geologists, petrophysicists, reservoir engineers, drillers,
and production personnel begin the core planning by
listing the objectives of the job

General advantages and
disadvantages of coring as an
exploration techniques.

• Logs alone cannot characterize the reservoir if knowledge of the rock is
absent so subsequent modelling must rely on un-calibrated and
unverified log-derived correlations .
• The predictable consequence of not having core analysis data is
greater uncertainty
With proper planning and management of the coring and analysis
processes, core data should be and can be the ground truth for
formation evaluation.

Reasons and Consequences
1. Poor inter-laboratory data comparability due to the lack of standardization
of test procedures and the sensitivity of core data to different test methods
2. The lack of thought given to the programme test design by the
commissioning end users including appropriateness of specified core tests;
the reliability of the data and their applicability; and the lack of
understanding of the practical difficulties faced by core analysis laboratories
and the technical and commercial constraints they must work under.
3. Historically inadequate reporting standards
4. Strong market competition which has required the commercial core analysis
vendors to produce data more reliably, for less money and with faster
turnaround times
5. Too often core analysis programmes are ill considered, badly designed and
poorly supervised, and the results are only crudely integrated with other
well and reservoir data. The results, in terms of data acquired, are often
unrepresentative or contradictory.

Coring, Core Handling and Core Processing
• Obtaining high-quality, undamaged core is an essential for representative and
reliable core analysis data
• Sample Preparation- For most core analysis tests, the core samples must be initially
cleaned and dried to remove oil and water, as well as evaporated salts, mud filtrate
and wettability contaminants
• In provided electron version of guideline there are:
• Details of the principal core/plug cleaning and drying methods and equipment, and
typical cleaning solvents
• Routine—or basic or conventional core analysis which principally involves
measurements of fluid saturations on core, and porosity and absolute permeability
measurements using single-phase fluids normally at ambient conditions on dry
cores (see attached an electron guideline)
• The methods and procedures to prepare and characterize test fluids used in SCAL
(oil, water and gas), and how to select representative test samples and
representative stress conditions (see attached an electron guideline)

Routine core analysis (RCA),
• There is no strict discrimination between routine core analysis (RCA), which is
often referred to as basic or conventional core analysis (CCA), and special core
analysis (SCAL). One lab’s RCA capabilities might reflect another lab’s SCAL
capabilities.
• A typical RCA programme involves the following measurements on plugs and
core samples:
• Fluid saturations
• Porosity l Air (nitrogen) and Klinkenberg permeability
• Probe (or profile) permeability
• The test plugs used for RCA are typically 100 or 1.500 diameter, with larger
samples being preferred for saturation and porosity measurements, in particular.
• Fluid saturations, porosity and permeability can be measured on full diameter or
whole core samples

Special Core Analysis
• These include:
• porosity at stress;
• formation resistivity factor;
• resistivity index;
• cation exchange capacity tests (wet chemistry cation exchange capacity and
multiple salinity Qv);
• drainage and imbibition capillary pressure tests using mercury injection (low
and high pressure), porous plate and centrifuge methods;
• contact angle, Amott and United States Bureau of Mines (USBM) wettability
tests;
• water–oil, gas–oil and gas–water and water–gas relative permeability tests
using unsteady-state, steady-state and centrifuge techniques;
• nuclear magnetic resonance tests to calibrate log responses.

Geomechanics Tests
• The tests most commonly used to determine fundamental rock
mechanics parameters are described including:
• unconfined compressive strength,
• thick wall cylinder,
• tensile strength,
• triaxial tests,
• pore volume compressibility,
• elastic moduli,
• particle size analyses.

Quality Control Procedures and Diagnostics
• Laboratory SCAL reports must include a detailed description of the
work performed, the equipment and procedures used and details of
the methods used by the lab in analyzing the data
• Laboratory data reporting requirements for both experimental and
interpreted data that can be used to check or verify the laboratory
results, and to provide an alternative interpretation of the data.
• Understanding the history of test samples can be crucial in evaluating
data quality—especially in formations of sensitive to stress cycling
and rock/fluid incompatibilities—yet sample history records from
standard report presentations can be challenging

Core plug history chart presents
an example of a customizable
single-page SCAL sample history
sheet that charts the history of
each sample throughout the
sample preparation and testing
sequence and provides sample
photographs before and after
testing.

Example Core Analysis Programmes.
Templates for typical RCA

Templates for typical RCA
if measurements are
repeated

Core Analysis Focal Points
• Amongst the key focal point responsibilities are as follows:
i. Design and costing of the test programmes, with the assistance of
the laboratory;
ii. Preparing cost justifications to management;
iii. Coordination with drilling and wellsite engineers and laboratory
staff to review core drilling, core recovery and wellsite handling,
storage and transportation procedures;
iv. Design and specification of the test and reporting procedures to
be adopted in the scope of work including deliverables, milestones
and project reporting;
v. Reviewing contractor performance against initially set goals,
objectives and deliverables;
vi. Analysis and checking of the contractors’ data as soon as possible
after they are received;
vii. Preparing a final report on the core analysis study, which
reconciles core results with other well and reservoir data and
provides appropriately interpreted and reliable core analysis data
that can be used for static and dynamic models.
Fig. 1.1.
Core analysis focal point in
core analysis management
The client focal point is the
liaison between the client’s
different subsurface
disciplines and the laboratory,
as indicated in Fig. 1.1.

An example of a geological core analysis program for 100 m of cores. The number of samples and time duration are
flexible based on available core length, laboratory potential, and number of personnel
Program Design Considerations
The key questions and factors that should be addressed prior
to embarking on acquiring core and designing a test program
are as follows:
1. Are there areas of concern or anomalies or suspicious data
in the existing core database that need to be resolved
2. Are special precautions or measures required to cut,
recover, handle and transport core?
3. How do we prepare the core for analysis?
4. What core analysis tests do we actually need?
5. Is the contractor interpretation correct and can operators
improve it?
With proper planning and management of the coring and analysis processes, core data should be and can be the ground
truth for formation evaluation.

2. Wellsite Core Acquisition, Handling and
Transportation
Coring Systems
Coring Fluids
Core Damage and Core Fluid/Petrophysical Property Alteration
Wellsite Handling

• The objectives of a core handling program are as follows:
a. Obtain rock material that is representative of the formation.
b. Minimize physical alteration of the rock material during core handling and
storage.
• The major problems confronting those handling and preserving
reservoir rocks for core analysis are as follows:
a. Selection of a nonreactive preservation material and a method to prevent
ﬂuid loss or the adsorption of contaminants.
b. Application of appropriate core handling and preservation methods based
upon rock type, degree of consolidation, and ﬂuid type

Data Sheet
• A suitable data sheet should be provided for and completed by the wellsite engineer or geologist, to
supply as complete a record as possible of the conditions of coring
• The following is a list of desirable information:
a. Well identification, API well number, elevation, vendor names and contacts, as well as
telephone/telefax numbers and addresses.
b. Drilling fluid type, contents, and measured data.
c. Core type and equipment used.
d. The formation(s) cored, with the top and bottom driller’s depth.
e. Designation of critical coring information and any pertinent coring notes, i.e., total coring/trip
time, difficulties, and recovery.
f. Formation water salinity and production fluid data.
g. Preservation guidelines. Exposure time.
h. Analysis requested.
i. Coring log and drilling records.
j. A core description.
k. Well logs and mud logs (if available).

What is Wireline-Retrievable Cores?
• In conventional coring operations, after the core sample has been
taken, the drill string is pulling out of hole and the rotary coring bit,
barrel and catcher are removed. The drill bit is reattached, and
drilling can commence again.
• Wireline-retrievable cores offer significant cost and time advantages
in that the core is cut using a conventional coring bottom-hole
assembly (BHA), but the inner barrel is retrieved on wireline.
Consequently, the driller can core or drill ahead without the need to
trip the BHA
• The core barrel assembly is forced down the inside of the drill pipe
using drilling mud pressure. When the core barrel assembly reaches
the lower end of the drill stem, a locking device holds the barrel in
place
• However wireline systems may require non-standard drill pipe, and
rapid retrieval can lead to gas expansion fracturing of the core which
negates the cost advantages as core quality is poor and the
recovered core may be unanalyzable.
FIGURE 2.12 Example wireline
retrievable core bit. Used with
permission from NOV

What is Wireline Continuous Coring?
• This coring method is composed of two modes of drilling and coring which can be
easily switched to each other using slick line/wireline. In coring mode, the inner tube
assembly is lowered into the outer tube assembly by slick line. Then, mud circulation
is started in order to hydraulically latch the inner tube assembly to the outer tube
assembly so that the cutting of the core can commence. After core-drilling was
accomplished or in case coring was terminated, the rock sample is brought up to the
surface through the outer tube assembly via slick line. Then, (in the drilling mode)
the inner drilling assembly including drill bit insert/plug at its bottom end is run in
the hole again via slick line in order to proceed with drilling.
• The main disadvantage of wireline continuous coring is the rather small size of core
retrieved (maximum 3 or 3½ in. in best case). This is because the cores have to be
tripped to the surface through the drill pipe. Using the wireline, for each run, shorter
length of core samples (e.g., 10–30 ft) can be retrieved. Therefore, the wireline
method is not optimal for long coring from a single formation, but rather the
conventional method is preferred.
Schematic of wireline
continuous coring assembly

FIGURE 2.1 Example
rotary coring bit for
full-diameter cores.
Conventional Full-Diameter Coring Systems
• Full-diameter coring systems are the most common source of core
for analysis. Drilling is halted at the designated coring point and the
drill string is pulled out of hole (POOH).
• The drill bit is removed and a rotary coring bit is attached in its
place. The rotary coring bit consists of solid metal with diamonds or
tungsten for cutting, but unlike a drill bit, a rotary coring bit has a
hollow center.
• The length of conventional core barrels can range from around
30 to over 400 ft. The core diameter depends on the hole size, as
indicated in Table 2.1. FIGURE 2.2
Example
(polycrystalline
diamond compact
or PDC core bit.
Image courtesy of
Baker Hughes.Training for Grenergy company_2018

• When drilling with conventional core bits
(e.g. Fig. 2.3), filtrate invasion occurs ahead
of the bit (referred to as the filtrate bank), in
the throat of the bit and in the core barrel
during coring and on exposure to a static
column of mud in the hole on core recovery
• Low-invasion core bits are often used to
minimize filtrate invasion (Fig. 2.4). These
bits are designed to cut as fast as possible
without breaking the formation apart as a
faster penetration rate reduces the mud
exposure time.

FIGURE 2.6 Lower barrel
assembly. Courtesy of
Halliburton
The core barrel is made up of an inner and outer barrel
separated by ball bearings

FIGURE 2.7 Spring catcher
(left) and full-closure
catcher (right). Courtesy of
ALS Oil and Gas.
• The core catcher is located within the core barrel and serves the primary purpose of
stopping the core falling out of the core barrel during tripping out of hole or rotating off
bottom
• Before disposable core liners were introduced, core was extruded from the barrel directly
into pre-marked wooden or cardboard boxes

• The three principal types of disposable core liner tubes that are
principally used nowadays are:
1.fibreglass/plastic 2. aluminum solid liners 3. aluminum clam shell liners
• Fiberglass liners are principally used for coring unconsolidated
formations
• Aluminum liners are used for the majority of coring operations
Fiberglass
inner liner.
Courtesy of
Halliburton
FIGURE 2.9 Solid (rigid)
aluminum liners. Courtesy of
Halliburton.
FIGURE 2.10 Aluminum
“LaserCut” liner. Image
courtesy of Baker Hughes
FIGURE 2.11
Aluminum
“LaserCut” liner
opened to expose
core. Image
courtesy of Baker
Hughes.
Selecting the appropriate liner for the conventional
coring application depends on a number of factors
including the degree of formation consolidation, the
reservoir fluids and reservoir pressure and temperature

What's conventional coring? Schematic conventional
coring assembly
A core bit example
As the core bit is connected to the outer tube assembly and drill pipes,
conventional core-drilling is accomplished using a core bit/head with
essentially the same principle as a drilling bit. However, the core bit cuts
cylindrical rock and thus possesses smaller bearings and cutters than a
drilling bit. The core barrel consists of an inner tube, wherein the sample
enters and the outer tube (together with the overlying drill collar and jar)
performs as the Bottom hole Assembly (BHA) during coring.
It is a method of rotary coring by which via a conventional trip, the inner
tube/barrel containing the core sample is retrieved along with the outer
tube assembly to the surface.
the most prominent advantage of conventional coring method over other methods is the possibility to
cut a large size rock sample. This can be up to even greater than 5-in. diameter. Using conventional
coring, the length of the core that can be cut in one run is significantly larger than the wireline
method.
The main disadvantage of conventional coring is that the inner tube and the sample inside can be only
retrieved through a conventional drill string trip to the surface.
This makes conventional coring time-consuming and thus costly.Training for Grenergy company_2018

Detail explanation of coring system

Gel Coring Systems
• Gel coring systems were developed principally as an
alternative to operator intensive wellsite core
preservation
• The gel is distributed around the core by a core-
activated floating piston valve after the core begins to
enter the core head. As the core is cut and enters the
inner tube, it displaces all but a substantial coating of gel
which forms a water-impermeable latex-like film on the
surface of the core.
• The gel coring process helps provide the core analyst
with what is claimed to be unaltered reservoir rock
sample, with formation fluid saturations that are largely
undisturbed.

Liquid and Gas Retention Coring Systems
• When a core is brought towards the surface and the hydrostatic mud
pressure reduces, the hydrocarbon fluid will expand and, in an oil
reservoir, gas will be liberated when the oil is brought below the
bubble point pressure.
• Gas liberation or expansion provides a force which will cause
displacement of both the native fluids and the invaded mud filtrate
from the core into the drilling mud system.
• The use of specially designed liquid and gas retention core barrels
provides a means to prevent loss of oil from the core on hydrocarbon
expansion on core recovery by maintaining reservoir pressure
throughout the tripping operation or by collecting the displaced fluids

Sponge Coring
FIGURE 2.14 Sponge core barrel.
Courtesy of Halliburton.
• It involves the use of a polyurethane sponge
liner in the annular space between the inside
of the liner and the core .
• The sponge is chemically conditioned to be
oil wet, so that it adsorbs oil that bleeds from
the core and holds it opposite the formation
from which it bled
• Sponge core analysis utilises whole cores
typically 3.5 in. diameter selected to be 12–
24 in. long
• Oil saturation (So) is calculated by summing
the volume of oil collected in the sponge
adjacent to the core (Vo sponge) and the oil
extracted from the core (Vo core), divided by
the cleaned core pore volume (Vp): FIGURE 2.15 Example of sponge core
oil saturation determination. After
Funk et al. (2004).

Liquid Capture Systems
• These systems have been developed to capture liquids
expelled from the core in core recovery, but allow gas
pressure to vent during the trip to surface
• The liquid trapper inner tube system is pre-saturated
with a selected fluid before it is run in hole. This barrel
saturating fluid is selected depending on the
information targeted from the core.
• Cells, shown in Fig. 2.16, are sealed around the core, thanks to specifically designed seal joints
that hold liquids yet still allow core gases to discharge into the mud system through a
modified inner head. Within the closed cells, oil and water from the core are trapped in the
annulus between inner and outer cell wall.
• On recovery, the core is laid down normally using core cradles. The liquid trapper cells are cut
into 3 ft. or 1 m sections, sealed and stored vertically in special core containers. The natural
separation of oil and water continues during storage and transport. In the lab, the fluids
surrounding the core are collected.

Pressure-Retaining Systems
FIGURE 2.17
Pressure core barrel
schematic.
• Pressure barrel coring systems are designed to maintain the reservoir pressure at
surface so preventing gas, oil and water displacement from the core on
depressurization
• Capturing and maintaining the core at or near bottom-hole pressure prevent gas
expansion and fluid loss. Once the core is cut, the core chamber is sealed at
hydrostatic pressure
• When pulling the barrel out of the hole, a valve mechanism incorporated in the
system will close the chamber to maintain bottom-hole pressure or, in some
applications (for example, gas compositional analysis), allows gas to vent until a
predefined hydrostatic pressure has been reached.
• Gas volumes can be determined by determining the volume of evolved gas using a
gas meter. For oil cores, if the gas–oil ratio of the oil is known at surface temperature
from PVT tests, then the volume of oil originally in the core can be determined from
the volume of gas evolved
• Pressure coring allows cores to be vented at controlled depletion rates on a batch
basis as the pressure coring drilling operation continues.

Oriented Coring
• Directionally (oriented) referenced core enables the
determination of the dip and strike of inclined strata or
fractures, stress field direction and directional permeability
• As the core is cut, it is pushed through a shoe (located behind
the core head) which has three scribe blades pointing inwards.
As the core moves through this shoe, the blades cut grooves on
the surface of the core Fig.2.18
• By orienting a survey instrument to one of these blades the
direction of the groove can be determined, referenced to
magnetic north.
• If oriented features are important yet the core is un-oriented,
then palaeomagnetic reorientation or matching formation
micro-image log features with the core can provide viable
alternatives
FIGURE 2.18
Illustration of
scribe lines cut
into oriented core.
FIGURE 2.19
Electronic survey
instruments are
referenced with
the primary line
scribed into the
oriented core.Training for Grenergy company_2018

Side wall coring

Sidewall Cores
• There are two main types of sidewall cores:
percussion/explosive and rotary.
• Percussion sidewall coring systems are essentially an
adaptation of a wireline-conveyed perforation gun.
Instead of firing perforation charges, the tool is designed
to shoot a series of hollow, chisel-edged ‘bullets’ which
are loaded with explosive charges
• The bullets are loaded into the tool which is run in on
wireline to the interval of interest. The gun is fired and
the explosive charge shoots the bullets into the
formation.
• The prime application of percussion sidewalls is therefore
restricted to obtaining samples for lithological
description, grain size and palynology and palaeontology,
although grain density measurements and particle size
analysis measurements should also be possible
FIGURE 2.20
Percussion
sidewall core
tool
schematic.

Sidewall Cores
• Rotary sidewall tools have now almost completely
replaced percussion sidewalls.
• The tool is still run in on wireline, but instead of shooting
hollow cylinders, it has a series of rotary coring bits which
cut a core plug from the formation at the borehole wall.
• The plugs are typically 0.92 in. diameter by 1–1.5 in. long.
The tool is lowered to the interval of
interest, and the core plugging bit is
extracted from the tool and pressed
against the borehole wall.
Either mud is circulated through the
tool which causes the core bit to
rotate and take the sample or an
electric motor is used which makes
them difficult to test in a standard
core holder for routine and special
core analysis, and which can impact
on certain porosity measurements.

• Recently, Baker Hughes (‘MaxCOR™’) has developed sidewall
coring systems (Fig. 2.23) that take much larger plugs—
typically 1.5 in. diameter by up to 3 in. long, so plug
dimensions and volumes are more comparable with typical
SCAL plugs. Other examples include Schlumberger’s ‘XL Rock’
system.
• There area several examples where rock strength measured
on sidewall cores is between 25% and 50% of the strength
measured on standard plugs taken from conventional full-
diameter core, and where sidewall plugs have anomalously
high grain densities as a result of infiltration of mud solids
weighting agents into the sample FIGURE 2.23 Baker Hughes
MaxCOR large-diameter
rotary sidewall plugs
Sidewall Cores

Sidewall Coring
Particularly in exploratory wells, the
formation intervals wherein obtaining cores
are desired may be unidentified and be
already penetrated by the drilling bit. This is
termed as missed coring points/interval. In
this case, following drilling, wireline logging
is first conducted
In order to verify economic or production potentials of formation, sidewall coring method is used as the only
compensating alternative to core the interval on the sides of the wellbore. Otherwise, some potentially productive
hydrocarbon-bearing formations may be bypassed. Sidewall coring is less costly than conventional coring and capable of
coring multiple zones
The main disadvantages of sidewall coring are that the size of the recovered cores is small (normally maximum 1½ in.
diameter and 3½ in. length); they have undergone considerable mud invasion and formation damage during coring and
retrieval; and they typically lack some features interesting from reservoir engineering and geological points of view
particularly in fractured or heterogeneous reservoirs
(a) Percussion sidewall coring and (b) rotary sidewall coring
with the circular diamond core bit (source: Schlumberger
coring services (2013)

Table 2 Drilling related advantages of wireline
continuous coring to other methods
Table 3 Geological and reservoir advantages of
wireline coring compared with side-wall coring
Table 1 A typical; comparison of time-related cost for continuous
and conventional coring of a critical formation
As shown in Table 2, the wireline method is strongly recommended for coring in
deep formations, for long core sections, for multiple zones, in exploration wells,
when core points are unknown, in formations with high jamming probability, when
logging is required following coring, and in out-of-gauge wellbores. In terms of
costs, Sidewall coring is considered as a rival of wireline continuous method
because it is run via wireline as well. However, the wireline continuous coring has
several geological and reservoir advantages to this method as listed in Table 3.

Some background information from drilling

• Mud loggers monitor the drilling operation
and intemperate the geology of the well by
examination of drill cuttings.
• The portable laboratory use.
• Mud logger often becomes a “control
tower” for the rig with the company Man
and geologist spending much of their time
there.
• The company man, tool pusher and some
other service personnel are on call at all
hours. The drilling crew and mud loggers
usually working 20 hours shifts called
“tours” (pronounced “towers”)

Functions of drilling fluid:
• Cool and lubricate the bit and drill string
• Clean the bottom of the hole
• Remove cuttings from the hole
• Resist formation fluid pressures
• Support the wall of the borehole.
Most drilling fluids are liquid.

Coring fluids
Mud Types
• Generally, if the core is to be taken to determine residual fluid
saturations, then water based drilling mud (WBM) is used for residual
oil saturation determination and oil based mud (OBM) for residual
water saturation. If sensitive clays or shales are expected, then OBMs,
synthetic oil-based muds (SBMs) or inhibited WBMs are used.
• The extent of any filtrate invasion depends on the core bit used, mud
type and mud rheology, the coring overbalance pressure, formation
wettability and formation absolute and relative permeability
• Invasion can be minimized by selecting an appropriate low-invasion core
bit and optimizing the coring parameters, especially mud weight.
• As far as possible, addition of surfactants to the mud should be
minimized (or eliminated) to prevent wettability alteration

Electric logs can be run in open or cased hole. Most logs run during
drilling evaluate open hole only.
Cased hole logs require special tools and require special tools and are
mostly run during the production phase of the well.
Fluid inversion of the formation by the drilling mud or mud filtrate ( the
fluid phase of the mud) strongly affects log results. The longer the time
between drilling and logging, the greater the chance of significant mud
invasion. Therefore operators have e- logs runs as soon as possible
sometimes immediately after entering the potential reservoir.
Most Electric logging tolls operate on 1 or 3 basic principles:
Formation resistivity; Spontaneous potential; dual induction (SFL) or
dual laterlog (MSFL) ; dipmeter (HDT);sonic transit time; borehole
compensated sonic (BHC); cement bond logs (CBL)indicate whether
cement fills the annular space; radioactivity; gamma ray (GR); formation
density compensated (FDC); neutron (CNL or SNP);

MWD tool log hole deviation and direction, mud
temperature, formation resistivity and natural GR
emissions. Among other possibilities operator use
MWD tools for :
Precise deviation control ; real time formation
logging and electric logging in horizontal or high
angle wellbores.
An MWD tool usually consists of a sensor package,
an electronics package and transmission system.
The while assembly rests inside a special non-
magnetic drill collar. The non-magnetic drill collar is
necessary to avoid interference with the directional
sensor.

MWD tools transmit data to surface using either or 2
principles.
1. Mud pulse data transmission
2. Electromagnetic data transmission
In certain cases MWD tools give higher quality logs
than wireline tools, because the slower logging rate
provides better resolution.

The repeat
formation
tester (RFT)

Drill steam tester DST
DST forms 2 basic classes.
1. Open hole tests, with no protective casing in place
2. cased hole tests, through perforations in the casings or
from a short section of open hole below the casing shoe.
Service equipment includes:
Test tree; surface safety valves (SSV); sensing equipment;
chock manifold; heater or steam exchanger; separator;
gauge tank or surge tank; oil manifold and transfer pump;
flare stack or burner boom
DST assembly can be simple or complex as required by the
test program. The main component of the test string are:
A tester valve assembly; one, two or three packers,
downhole gauges.

• The wellsite operations’ personnel involved in core recovery and handling
must ensure that the best quality core is received by the testing laboratory
• In any case, each coring job and reservoir should be carefully examined prior
to the design of a wellsite coring, handling and preservation programme. The
typical sequence of events at wellsite is:
1. Coring
2. POOH (pulling out of hole)
3. Remove from barrel and lay out liners
4. ‘Way-up’ and depth marking
5. Wellsite screening/gamma-ray logging or sampling
6. Wellsite sampling and sample selection (if required)
7. Core preservation/stabilisation (if required)
8. Liner division into suitable lengths (if required)
9. Core transportation to laboratory

Health, Safety and Environmental Considerations
• Health, safety and environmental (HSE) protection during coring operations should
be managed according to the current standards and procedures of the operating
company and coring contractors, and according to local regulatory legislation
• The operator should establish arrangements to ensure that contractors and service
providers are properly selected, managed and monitored according to their HSE
standards. All core handling and processing activities must be reviewed to make sure
that it does not conflict with other activities
• The area between the drill pipe and the drill collar racking area must be kept clear of
obstructions. Drill collars should be racked back using a racking arm. The elevator lift
sub should be hoisted with an air winch to the elevators and latched into the
elevators with the lift sub standing on the deck.
• The presence and concentration of hydrogen sulphide (H2S) must be established.
Any work related to core handling (retrieving, laying out, measuring, marking,
gamma-ray logging, cutting and packing) must only be carried out after safe
clearance is given.

Coring Team
Drilling and coring engineers, wellsite
geologists and the core analysis laboratory
personnel should hold a formal pre-coring
wellsite meeting with rig crew and key
personnel to highlight the importance of
good coring and core handling and to
promote teamwork
Ideally, the core analysis company must be
involved in any pre-coring activities, and at
wellsite as the core is recovered, as they
have a vested interest in getting good quality
core back to the laboratory

Managing Coring Risks
• Prior to running in with the core barrel, the bottom of hole should be clean,
and the depths defined and agreed between drilling superintendent and coring
contractor
• Ideally, the first core run should commence with as near as possible to a full 90
ft. (28 m) on the stand so a connection to complete the coring run need not be
made. This reduces the risk of shortening the core run. Picking up the top drive
every 1000–1500 ft. and pumping through the core barrel reduce the risk of
ingesting cuttings debris into the inner barrel.
• Jamming or blocking the inner barrel is one of the more common problems of
coring
• Inner core barrels should be fitted with shoes, catchers and inner tube
stabilisers off the critical path of the well prior to pick up of the assembly
• Circulation times should be limited in order to reduce the risk of core loss
unless hole conditions dictate heavy circulation is needed

Core damage and core fluid/petrophysical property alternation
During core journey from the reservoir to the wellsite or laboratory, the
stress acting on the cored formation is released rapidly, the pore pressure
depletes and the temperature reduces.
These changes result in:
• alteration of the spatial and volumetric distribution of fluids within the
reservoir pore space;
• alteration of the rock’s pore texture and mineralogical properties;
• alteration of the rock’s native wettability

Some details
SKIP!

Exercises

1. What is the objective of every coring operation in order to gather
information that leads to more efﬁcient oil or gas production?

Answer:
Some speciﬁc tasks might include the:
a. Geologic objectives:
1. Lithologic information: (a) Rock type. (b) Depositional environment. (c) Pore type. (d)
Mineralogy/geochemistry.
2. Geologic maps.
3. Fracture orientation.
b. Petrophysical and reservoir engineering:
1. Permeability information: (a) Permeability/porosity correlation. (b) Relative permeability.
2. Capillary pressure data.
3. Data for reﬁning log calculations: (a) Electrical properties. (b) Grain density. (c) Core gamma
log. (d) Mineralogy and cation exchange capacity
4. Enhanced oil recovery studies.
5. Reserves estimate: (a) Porosity. (b) Fluid saturations.
c. Drilling and completions:
1. Fluid/formation compatibility studies.
2. 2. Grain size data for gravel pack design.
3. 3. Rock mechanics data

2. What is conventional coring system?

Answer: Schematic conventional
coring assembly
A core bit example
As the core bit is connected to the outer tube assembly and drill pipes,
conventional core-drilling is accomplished using a core bit/head with
essentially the same principle as a drilling bit. However, the core bit cuts
cylindrical rock and thus possesses smaller bearings and cutters than a
drilling bit. The core barrel consists of an inner tube, wherein the sample
enters and the outer tube (together with the overlying drill collar and jar)
performs as the Bottomhole Assembly (BHA) during coring.
It is a method of rotary coring by which via a conventional trip, the inner
tube/barrel containing the core sample is retrieved along with the outer
tube assembly to the surface.
the most prominent advantage of conventional coring method over other methods is the possibility to
cut a large size rock sample. This can be up to even greater than 5-in. diameter. Using conventional
coring, the length of the core that can be cut in one run is significantly larger than the wireline
method.
The main disadvantage of conventional coring is that the inner tube and the sample inside can be only
retrieved through a conventional drill string trip to the surface.
This makes conventional coring time-consuming and thus costly.Training for Grenergy company_2018

3. What is Wireline-Retrievable Core Barrel?

Answer:
• Wireline-retrievable coring tools are operationally similar to
conventional coring systems except they are designed for the inner
core barrel to be pulled to the surface by a wireline. This speeds the
coring operation by eliminating the need to trip the entire drill string
for each core. A new section of inner core barrel is pumped down the
drill string and latched into place for additional coring, or a drill plug is
pumped down to facilitate drilling ahead.

4. What is Wireline sidewall coring?

Answer:
• Wireline sidewall coring systems were developed to obtain core
samples from a wellbore after it has been drilled and logged, and
before casing is run. These tools may be positioned in zones of
interest using data from gamma or spontaneous potential logs as
guides. The samples provide small pieces of formation material,
suitable for geologic and engineering studies

5. Which type of sidewall core exist? And what
is different?

Answer:
• There are two main types of sidewall cores: percussion and rotary.
• Percussion sidewall coring systems are essentially an adaptation of a wireline-
conveyed perforation gun. Most wireline sidewall cores are obtained by
percussion sidewall coring systems. The advantages of percussion sidewall
coring are speed, low cost, and the ability to sample zones of interest after
open hole logs have been run. The disadvantage is that the bullet usually alters
the formation, shattering harder rock or compressing softer sediments
• The rotary or drilled sidewall coring tool was developed to recover wireline
sidewall core samples without the shattering impact of the percussion system.
Suitable for hard-to-friable rock, the rotary sidewall coring tool uses a
diamond-tipped drill to cut individual samples. An advantage of the rotary
sidewall coring system is that it produces samples of hard rock suitable for
quantitative core analysis. Disadvantages are that it is more expensive than
percussion sidewall coring in terms of rig time costs, and sample recovery
tends to be low in unconsolidated formations.

6. What is
Wireline Continuous Coring ?

Wireline Continuous Coring
• This coring method is composed of two modes of drilling and coring which can be
easily switched to each other using slick line/wireline. In coring mode, the inner tube
assembly is lowered into the outer tube assembly by slick line. Then, mud circulation
is started in order to hydraulically latch the inner tube assembly to the outer tube
assembly so that the cutting of the core can commence. After core-drilling was
accomplished or in case coring was terminated, the rock sample is brought up to the
surface through the outer tube assembly via slick line. Then, (in the drilling mode)
the inner drilling assembly including drill bit insert/plug at its bottom end is run in
the hole again via slick line in order to proceed with drilling.
• The main disadvantage of wireline continuous coring is the rather small size of core
retrieved (maximum 3 or 3½ in. in best case). This is because the cores have to be
tripped to the surface through the drill pipe. Using the wireline, for each run, shorter
length of core samples (e.g., 10–30 ft) can be retrieved. Therefore, the wireline
method is not optimal for long coring from a single formation, but rather the
conventional method is preferred.
Schematic of wireline
continuous coring assembly

7. Which other special coring systems are
exist?

Answer:
• Pressure-retaining core barrels (sometime it has name Liquid and Gas Retention Coring Systems) are designed to retrieve cores
maintained at reservoir pressure conditions. Pressure-retained core barrels are available in two sizes: 6inch (152.4-millimeter) and
8-inch (203.2-millimeter) outside diameter that cut cores 2.50- and 3.75-inch (63.5- and 95.3millimeter) outside diameter,
respectively
• The sponge-lined coring system was developed to improve the accuracy of core-based oil saturation data. A sponge coring system
does not trap reservoir gases, instead it traps oil expelled as the core is brought to the surface. The saturation information is very
useful when evaluating enhanced oil recovery projects. A sponge coring system has the advantage of being less expensive to operate
than a pressure-retained coring system, while providing an opportunity to improve the accuracy of the core based oil saturation
data.
• Full-closure coring systems (or gel coating system)were developed to improve the recovery of unconsolidated formations. These
systems use core barrel liners or disposable inner core barrels, and a special core catching system to retrieve the troublesome rocks.
Full-closure coring technology allows the inner core barrel to slip gently over soft core with a minimum of disturbance, and then seal
the core within the core barrel
• The rubber-sleeve coring system was the ﬁrst system developed to improve the chances for recovering unconsolidated sands,
conglomerates, and hard fractured formations. The rubber-sleeve barrel is unique in that the top of the inner barrel does not move
relative to the core during coring. The outer barrel is drilled down around a column of rock that is progressively encased in a rubber
sleeve. The rubber sleeve is smaller than the diameter of the core; it stretches tightly around the core, wrapping it securely and
protecting it from the scouring action of the drilling ﬂuid
• Oriented coring refers to the combination of cutting a core which is marked with a narrow groove along its length and providing
directional survey data referenced to this groove
• Liquid Capture Systems have been developed to capture liquids expelled from the core in core recovery, but allow gas pressure to
vent during the trip to surface. The Liquid Trapper system for example, consists of a specifically designed liner assembly which,
through an inflatable seal system, ‘traps’ oil and/or water escaping from the core

8. What is oriented coring?

Answer:
• Oriented cores are used to orient fractures, stress ﬁelds, and
permeability trends. Exploration, production, and drilling operations
use the information to explore for fractured reservoirs, design
waterﬂoods, and plan horizontal wells

What is Core Analysis Data Uncertainty
Reasons and Consequences?

Answer:
Experience shows that, 70% of core analysis data are unfit for purpose due to their
unreliability, inapplicability or inappropriateness. This has resulted from a combination
of the following factors:
1. Poor inter-laboratory data comparability due to the lack of standardisation of test
procedures and the sensitivity of core data to different test methods
2. The lack of thought given to the programme test design by the commissioning end
users including appropriateness of specified core tests; the reliability of the data
and their applicability; and the lack of understanding of the practical difficulties
faced by core analysis laboratories and the technical and commercial constraints
they must work under.
3. Historically inadequate reporting standards
4. Strong market competition which has required the commercial core analysis
vendors to produce data more reliably, for less money and with faster turnaround
times
5. Too often core analysis programmes are ill considered, badly designed and poorly
supervised, and the results are only crudely integrated with other well and
reservoir data. The results, in terms of data acquired, are often unrepresentative or
contradictory. Training for Grenergy company_2018

Core analysis & wellsite core acquisition, handling and transportation

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