Department of Petroleum Engineering
Indian Institute of Technology (ISM), Dhanbad – 826004
III Semester B.Tech Petroleum Engineering 2020-21
Lecture 9 / 2020-21
(For internal Circulation only)
CORING METHODS
BY
Prof. (Dr.) A. K. Pathak, Professor
Department of Petroleum Engineering, Indian School of Mines, Dhanbad – 826004
E-mail: akhilendra56@iitism.ac.in
INTRODUCTION:
The objective in drilling an oil or gas well is to locate a hydrocarbon bearing structure, which
will produce oil and/ or gas in quantities sufficient to repay the cost of drilling and completing
the well and also to provide the nominal profit to the owner. Before the completion of the well
the actual and precise information regarding the lithology and fluid bearing characteristic of
the formation is desired. For this actual sampling of the reservoir rock is to be taken. Coring is
the term applied for the technique whereby relatively large samples (by compression with the
normal size of the drill cuttings) of reservoir materials are removed form their native state and
brought to the surface for physical examination. Normally the most important information
desired from coring operation are:
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Porosity or fluid carrying capacity of the reservoir rock.
Permeability which indicates whether the formation fluid will be able to flow at rates
fast enough to permit economical production of the hydrocarbon fluid.
Water saturation and
Hydrocarbon saturations & relative percentage of oil and gas.
Geological studies
Studies of fracture pattern in fractured formations.
Studies of formation in order to obtain better well completion.
Rotary Coring: In order to obtain a core with rotary drilling tools, provision must be made for
cutting the formation in the desired shape and retaining the core. The rotary coring bit is used
to cut the core, and a core barrel is used to retain the core after cut.
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Department of Petroleum Engineering
Indian Institute of Technology (ISM), Dhanbad – 826004
III Semester B.Tech Petroleum Engineering 2020-21
1. Punch Type Coring:
To obtain a core, drill pipe was not rotated but spudded The core material was retained in the
upper portion of the device. The coring bit used in this method was poor boy or Texas-type.
This bit was made in the fields by cutting the lower end of a section of the pipe, teeth which
would increase the penetration rate of the core bit. After sufficient length has been cut the
teeth of the core bit were turned by rapidly rotating the drill pipe with increased weight on the
bit and a reduced circulation rate. This prevented the core from being lost as it was brought
up to the surface.
2. Conventional Coring:
A core bit located on extreme end of the drill stem and a core barrel located immediately
above the core bit for retaining the core after cut. Since during core cutting, only the outer rim
of the formation is cut, leaving a maximum amount of the formation intact, the cutting and
bearing surface of a core bit are considerably smaller then the conventional drill bits. The core
bit should drill a large gauge hole as fast as possible with minimum wear on the cutting
surface. The coring bit should cut the optimum size core in such a manner that the maximum
amount of the core can be retained and brought to the surface with out any damage.
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Department of Petroleum Engineering
Indian Institute of Technology (ISM), Dhanbad – 826004
III Semester B.Tech Petroleum Engineering 2020-21
A core bit of drag type is suitable in soft formation core drilling, while the rolling cutter bits, are
for hard formations. The core barrel is used to retain the core after cut. The core barrel has
evolved from a very simple retaining space above the coring head or bit. A conventional core
barrel consists of an inner barrel, an outer barrel, a core retainer or catcher and a vent or
pressure relief valve for venting pressure inside the core barrel to the out side of the drill
stem. Drilling fluid circulates between inner core barrel and outer core barrel and not passed
through the inner core barrel. The advantage of the conventional coring is to obtain the large
diameter core (5 inch or more) in one operation. The major disadvantage of this method is the
tripping time for each operation and direct interruption in drilling process.
3. Wire Line Coring:
In order to overcome disadvantages of the conventional coring, method is developed for
obtaining the core, bringing to the surface, and proceeding the normal drilling, all without
removing the drilling tools from bottom. This is accomplished by inserting the proper
equipment in the lower part of the drill stem by means of wire line which can be run inside the
drill pipe. This wire line coring has decreased the cost of obtaining cores. To obtain a core
after the core bit is in place, the core barrel assembly is forced down the inside of the drill pipe
using drilling mud pressure. When the core barrel reaches the lower end of the drill stem, a
locking device holds the barrel in place.
. The core barrel assembly consists of a cutter head, core catcher, core barrel, vent or
pressure relief, locking device and a retrieving head. During coring operation, the circulating
fluid passes between the core barrel assembly and the drill collar. After the core has been cut,
the core barrel assembly, with its core is retrieved by a retrieving tool (an overshot run on
wire line), which is designed to get engaged with the upper end of the core barrel.
4. Diamond Core Drilling:
In order to increase the core recovery and penetration rate, use has been made of a diamond
faced coring bit. Diamond bits may be used in hard, dense formation where cost of the rolling
cutter bit is very high. Because of the longer life of diamond core bit, conventional cores of 90
ft. length can be obtained without any difficulty.
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Department of Petroleum Engineering
Indian Institute of Technology (ISM), Dhanbad – 826004
III Semester B.Tech Petroleum Engineering 2020-21
Wire line coring equipment have also been developed using diamond coring bits. Diamond
core bits are specially designed for the formations like large grained sandstones, sandy
shales, conglomerates, or fractured formations. The diamonds are placed in a mould with the
powered metal, and heat and pressure are applied until the mass becomes sintered.
5. Reverse Circulation Coring:
In an effort to increase core recovery and at the same time eliminate the necessity of
retrieving the core, either by removing the drill pie in conventional coring or by retrieving the
core barrel in wire line coring, the technique of reverse circulation coring has been developed.
The principal feature of this method is the change in the flow path of drilling fluid. The drilling
fluid is circulated down the outside of the drill pipe and returned back to the surface through
the interior. In this manner all the formation, which is being penetrated including cuttings, will
enter inside the drill pipe and can be collected at the surface. Originally attempts were made
to circulate the cores to the surface without use of the core barrel. But sue to the difficulties of
passing the core through the surface connections, the use of inner core barrel at the surface
is recommended. After 5 to 10 ft core has been cut the core barrel would be removed and the
core extracted.
6. Side Wall Coring:
Side wall coring is the supplementary tool. It can be used in zones where core recovery is
small. Side wall cores can be obtained at any time after the formation for which the core is
desired, has been penetrated. The side wall coring device is lowered in to the hole on a
logging cable, and a sample of the formation at desired depth is obtained. Three basic type of
side wall coring devices are in practice: (a) punch type side wall sampler used in conjunction
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Department of Petroleum Engineering
Indian Institute of Technology (ISM), Dhanbad – 826004
III Semester B.Tech Petroleum Engineering 2020-21
with the drill pipe, (b) rotary type side wall coring device used a rotating core assembly and (c)
percussion type side wall coring device.
7. Cable Tool Coring:
Cores obtained by cable tool method are more nearly approximate the natural state of cores
than those obtained by any other method, since by invading action of a high pressure column
of drilling fluid is not present. A cable tool core must be obtained by the churn action of the
tools, and the design of a good cable tool coring device must take these factors in to
consideration. A typical cable toll core device consists of two parts, an outer drilling barrel and
an inner core retaining tube. The inner core retaining tube rests on bottom on all time during
the coring operation. The outer barrel consists three parts, the drill barrel head, the drill barrel
and the drill barrel shoe. Which actually performs the cutting operation. The inner core tube
has a relief valve that permits water trapped in the core tube to escape as the core enters the
tube. In operation the cable tool core barrel is attached to the lower end of the drill stem. The
entire assembly is lowered to the bottom of the hole, which has previously been cleaned.
Churning action is begun, care being taken to ensure that the tools are not raised more than
three feet in order to prevent the core retaining tube from being lifted from bottom. Cores
obtained with this method may be as large as 2 11/16 inches in diameter and over 7 feet in
length.
8. Chip Coring:
It is the technique in which ordinary cable tools are used, and the large cuttings are saved for
core analysis. When chip cores are being obtained, the cuttings must be removed quite
frequently, normally once each foot. These large cutting chips must be cut during the last
stroke or two of the drill bit and have not been reduced by churning action. Thus the large
chip represent the last inch formation been drilled. Frequently chips as large as 1.0 cm in
diameter and 1.5 inch in length are recovered at the surface. The cuttings are removed by
sand pump. Satisfactory techniques have been developed for analysis of these cuttings for
permeability, porosity and fluid saturation.
PRESERVATION OF CORE:
After the cores have been removed from the core barrel, proper preservation is
essential for further evaluation. Immediately after removed from core barrel, the core is
normally is laid down on a flat surface for visual examination. A geologist or an engineer
makes the detailed description of the core, noting any section of the core not recovered.
Usually field tests will be made on part of the core. As these cores may be needed later for
further reservoir studies, the core should be sealed as soon as possible to preserve the
original fluid saturation. Cores may be preserved for further analysis by quick freezing,
coating in paraffin, sealing in plastic bags. The use of quick freezing is recommended only
when the cores will be analysed within a very short time.
Different Coring Tools:
Advanced Piston Corer (APC): used in soft zone and sediments.
Rotary Core Barrel (RCB): used in medium to hard crystalline sediments
Sonic Core Monitor (SCM): Used to monitor the contamination of core.
Extended Core Barrel (EXCB): used in firm sediments.
Advanced Diamond Core Barrel (ADCB): used in hard sedimentary or igneous
formations.
Motor Driven Core Barrel (MDCB): Used in inter bedded materials and hard
fractured rock.
Pressure Core Sampler (PCS): used in sediments maintaining in situ pressure
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Department of Petroleum Engineering
Indian Institute of Technology (ISM), Dhanbad – 826004
III Semester B.Tech Petroleum Engineering 2020-21
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Practice Sheet
What are objectives of Coring?
Name different methods of coring.
What is difference in drill string used for coring and for normal drilling?
What is Punch Coring and how it is done?
What is basic difference between drill bit used for coring and normal drilling?
What is core barrel and where it is used?
Name the components of drill string used for coring.
What is wireline coring and how it is done?
What is Diamond coring and how it is done?
What is sidewall coring and how it is done?
What is Reverse circulation coring and how it is done?
What is cable tool coring and how it is done?
What is chip coring and how it is done?
What is proper method to core handling and processing?
What is core conditioning?
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