1. CHAPTER-5
CORING
Core analysis has become a long way from the days when reservoir productivity
was determined by blowing through a piece of cable tool produced core. Our
tools and methods for drilling and core analysis have changed but our interest is
not. The reservoir rock properties that determine hydrocarbon production, the
variation in this properties and how these properties effect ultimate recovery
are still of primary concern. Properly engineered core analysis provides a direct
measurement of these reservoir rock properties and is an essential step in
formation evaluation ,reservoir and production engineering. Fundamental core
analysis measurements are unchanged but advances provide the ability to test at
reservoir conditions and to acquire simultaneous measurements of reservoir
related properties.
DEFINITION AND THEORY
The objective in drilling an oil or gas well is to locate a hydrocarbon-bearing
structure which will produce oil and gas in quantities sufficient to repay the cost
of drilling and completing the well and also provide a nominal profit to the driller
and owner. During the drilling, therefore, more precise information may be
necessary concerning the lithologic and fluid-bearing characteristics of the
2. formation before a decision can be made to complete the well and spend any
additional thousands of dollars for completion equipment and services. One of
the most reliable sources of information on the lithologic and fluid-bearing
characteristics of a reservoir is an actual sample of the reservoir rock ,with the
fluid contained in it. CORING is the term applied to the technique whereby
relatively large samples (by comparison with the normal size of the drilling
cuttings) of reservoir material are removed from their native state and brought
to the surface for physical examination.
Normally the most important information desired from the coring operation
concerns:
1) Porosity, which is a measure of the fluid-carrying capacity of the
formation.
2) Permeability, which indicates whether the formation fluids will be able to
flow at rates fast enough to permit economical production of the
hydrocarbon fluids
3) Water saturation
4) Hydrocarbon saturations, including the relative percentages of oil and gas.
Cores are also obtained for a variety of other reasons, including geological
studies , studies of fracture patterns in fractured formations , studies of
formations in order to obtain better well completion and also some climate
related studies.
3. Rotary coring
Rotary coring was probably first introduced by the French Engineer Leschat in
1863; however it did not come in to general use in the oil industry until the early
1920’s. 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 it has been cut.
Rotary coring equipment has continued to keep abreast of technological
developments in the drilling industry. At the present time there are basically
three types of rotary coring equipment:
1) Conventional coring
2) Wire-line retrievable coring
3) Diamond coring
Conventional Coring
Conventional coring is a continued development of the original rotary coring
methods. Special equipment required in conventional coring includes a core bit,
which is located on the extreme lowerend of the drill stem, and a core barrel,
for retaining the core after it has been cut , which is located immediately above
the core bit.
4. The basic requirements for a good rotary core-cutting head are essentially
same as for a good drilling bit,because both are performing essentially the same
function. The problem of design a good coring bit is somewhat more
complicated, however, because only the outer rim of the formation is cut,
leaving a maximum amount of the formation intact. Therefore the cutting and
bearing surfaces of a core bit are considerably smaller than the same surfaces
on a drilling bit. In addition to the normal requirement that a bit should drill a
gauge hole as fast as possible with minimum wear on the cutting surfaces, the
coring bit must also satisfy the additional requirements of (1) cutting an
optimum size core in such a manner that (2) the maximum amount of the core
can be retained and brought to the surface for examination.
Since in coring operations it is highly important that the cutting surfaces of the
bit be kept clean and free of cuttings particular attention must be directed to
the placing of the nozzles through which the drilling fluid circulates.
As one of the major objectives in coring is to recover and bring to the
surface 100 percent of the cored formation, the development of suitable core-
retaining equipment is essential. The major item of such equipment is core
barrel. A conventional core barrel consists essentially of
1) an inner barrel
2) an outer barrel
3) a core catcher
5. 4) a vent or pressure-relief valve for venting pressure on the inside of the
core barrel to the outside of the drill stem.
Drilling fluid circulates between the inner and outer barrels but cannot
pass through the inner barrel, with the result that there is increased core
recovery and less flushing of the formation by the drilling fluid.
The advantages of conventional coring are that a large diameter core ,as
much as five inches or more, can be obtained in one operation. The principal
disadvantages of the conventional coring technique are that drill pipe must be
removed from the hole and the special equipment attached before coring
operations can begin; normal drilling operations cannot be resumed without
removing the drill pipe from the hole and replacing the coring equipment with
the normal drilling equipment; and the core cannot be recovered without
removing the entire drill string from the hole.
Wire-line Coring
In order to overcome the previously cited disadvantages of conventional
core drilling, methods have been developed for obtaining a core, bringing the
core to the surface and proceeding with normal drilling operations ,all without
removing the drilling tools from the hole. This is accomplished by inserting the
proper equipment in the lower part of the drill stem by means of a wire line
which can be run inside the drill pipe. This wire-line coring decreased the cost of
obtaining cores and thus many more cores may be obtained than would otherwise
6. be possible. As the average depth of wells continues to increase,the time and
money saved by not having to remove the drill pipe in order to obtain a core is
substantial. The only special equipment required on the lower end of the drill
stem is a core bit.
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 assembly 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 inside pressure relief, locking device and a
retrieving head. During coring operations, 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 by lowering through the drill pipe on a wire
line retrieving tool, or overshot, which is designed to engage the upper end of
the core barrel. As the overshot is lowered over the upper end of the assembly,
the locking devices are released, permitting removal of the entire assembly. As
much as fifteen feet of core can be obtained in one operation.
The use of wire-line coring technique is especially suitable for wildcat
drilling operations where coring depths are not known in advance, because a
program of alternate drilling and coring can be used without removing the drill
pipe to change bits.
7. Diamond Core Drilling
In order to increase both core recovery and penetration rate, use has
been made of a diamond-faced coring bit. Diamond bits may be used to
advantage in coring hard, dense formations where the cost of the coring with
roller cutter bits is high. Although the cost of a diamond core bit may be as
much as fifteen to twenty times the cost of a conventional core bit, the
reduction in the number of round trips and the increased penetration rate in
many cases make the diamond bit more economical.
Side-wall Coring
Side-wall coring is a supplementary coring tool. It can be used in zones where
core recovery by conventional or wire-line methods is, or in zones where the
latter cores were not obtained as drilling processed. Side-wall cores can be
obtained at any time after the formation from which a core is desired has been
penetrated.
The side-wall coring device is lowered in to the hole, usually on a logging cable
and a sample of the formation at the desired depth is obtained.
Coring should begin at light bit weight and low rotary speed; these may be
increased as soon as cutting action is established. Normally the applied bit
weights and table speeds should be held within the limits calculated, unless
specific experience in the area dictates otherwise. Circulating volumes for
8. conventional core bits approach those of regular bits of the same size. Diamond
bits require less fluid volume, and may actually be pumped and bounced
off bottom by excessive circulating rates. Also, severe erosion of the water
courses and bit matrix may occur.
Pump pressure should be closely watched during diamond coring as an
indication of whether drilling fluid is passing over the face of the bit. With the
bit on bottom, pressure should be higher than when the bit is off-bottom. This
is essential to hit cleaning and performance. A sudden pump pressure increase
not alleviated by raising the bit off bottom, may mean that the core barrel is
plugged by trash in the mud; if this happens, it should be pulled for inspection.
12. Core Handling Process
• Prepare adequate number of boxes, have rags.
• Set up “rig floor” boxes to receive core.
• Supervise core layout to ensure correct orientation and order.
• Wipe core clean with clean dry rags.
• Fit core. Space rubble between ends of core. Put rubble in sample bag and
mark depths.
• Strap core, uncovered interval understood to be at bottom unless known
otherwise.
• Mark core with reference lines and depths, using marker pens.
• Describe the core and shows.
• Box core and mark boxes.
