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

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.

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.

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

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

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.

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

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.

Conventional core barrel

Diamond core barrel

Core Barrel Detail

Wireline core barrel

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.

EXAMPLE OF CORE AND BOX MARKINGS