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This chapter presents an overview of coring tools, including guidelines for selecting coring tools for specific applications. Coring provides the only means of obtaining high quality samples for the direct measurement of rock and reservoir properties. Cores provide both geological and engineering information, and their analysis ultimately leads to a profitable field development.

This chapter presents an overview of coring tools, including guidelines for selecting coring tools for specific applications. Coring provides the only means of obtaining high quality samples for the direct measurement of rock and reservoir properties. Cores provide both geological and engineering information, and their analysis ultimately leads to a profitable field development.

Many coring systems exist. The system used depends on the objectives for the coring program and on the physical constraints of both the formation and the drilling location. A remote location might require a ready-made core-handling and preservation system such as a disposable inner barrel. An exploration program might profit by employing a wireline core barrel to core and drill alternately without the expense of tripping pipe. A mature field might need a sponge or pressure core to help pin down remaining reserves. These three situations describe three coring programs, each with different objectives and constraints and each with a different “best” way to get the job done.

Many coring systems exist. The system used depends on the objectives for the coring program and on the physical constraints of both the formation and the drilling location. A remote location might require a ready-made core-handling and preservation system such as a disposable inner barrel. An exploration program might profit by employing a wireline core barrel to core and drill alternately without the expense of [[trip]]ping pipe. A mature field might need a sponge or pressure core to help pin down remaining reserves. These three situations describe three coring programs, each with different objectives and constraints and each with a different “best” way to get the job done.

==Conventional coring systems==

==Conventional coring systems==

|+ {{table number|1}}Conventional coring systems

|+ {{table number|1}}Conventional coring systems

|-

|-

! Inner Barrel

! Inner Barrel || Core Length (ft) || Special Features

! Core Length (ft)

! Special Features

|-

|-

| Mild steel

| Mild steel || 30–120 || Ready-made core preservation system; high temperature applications

| 30–120

| Ready-made core preservation system; high temperature applications

|-

|-

| Mild steel

| Mild steel || 1.5 || Designed for short radius coring

| 1.5

| Designed for short radius coring

|-

|-

| High strength steel

| High strength steel || 120–>400 || Stronger barrel, includes additional inner and outer core barrel stabilization

| 120–>400

| Stronger barrel, includes additional inner and outer core barrel stabilization

|-

|-

| Fiberglass

| Fiberglass || 30–90 || Ready-made core preservation system; used for consolidated and unconsolidated formations

| 30–90

| Ready-made core preservation system; used for consolidated and unconsolidated formations

|-

|-

| Aluminum

| Aluminum || 30–90 || Ready-made core preservation system; high temperature applications

| 30–90

| Ready-made core preservation system; high temperature applications

|-

|-

| Steel with a plastic liner

| Steel with a plastic liner || 30 || Ready-made core preservation system; maximum temperature [[temperature::180°F]]; reduces core diameter by 0.5 in.

| 30

| Ready-made core preservation system; maximum temperature [[temperature::180°F]]; reduces core diameter by 0.5 in.

|-

|-

| Steel with a fiberglass liner

| Steel with a fiberglass liner || 30 || Ready-made core preservation system; maximum temperature [[temperature::250°F]]; reduces core diameter by 0.5 in.

| 30

| Ready-made core preservation system; maximum temperature [[temperature::250°F]]; reduces core diameter by 0.5 in.

|-

|-

| Steel with a steel liner

| Steel with a steel liner || 30 || Ready-made core preservation system; maximum temperature [[temperature::350°F]]; reduces core diameter by 0.5 in.

| 30

| Ready-made core preservation system; maximum temperature [[temperature::350°F]]; reduces core diameter by 0.5 in.

|}

|}

The ''core catcher'', the device that holds the core in the barrel, is tailored to the type of inner barrel and lithology expected. Table 2 lists core catchers by their common names and usages. In some cases, multiple catchers are used. Friable sandstone interbeddded with shale might require both slip and flapper type catchers. Full-closure catchers, run primarily to ensure success when coring unconsolidated sand, also incorporate split ring or slip type catchers to improve core recovery in the event that coring ends in hard rock.

The ''core catcher'', the device that holds the core in the barrel, is tailored to the type of inner barrel and lithology expected. Table 2 lists core catchers by their common names and usages. In some cases, multiple catchers are used. Friable [[sandstone]] interbeddded with shale might require both slip and flapper type catchers. Full-closure catchers, run primarily to ensure success when coring unconsolidated sand, also incorporate split ring or slip type catchers to improve core recovery in the event that coring ends in hard rock.

{| class = "wikitable"

{| class = "wikitable"

|+ {{table number|2}}Core catchers

|+ {{table number|2}}Core catchers

|-

|-

! Type

! Type || Recommended Usage

! Recommended Usage

|-

|-

| Split ring or spring

| Split ring or spring || Consolidated formations

| Consolidated formations

|-

|-

| Collet

| Collet || Where formation characteristics are unknown

| Where formation characteristics are unknown

|-

|-

| Slip

| Slip || Consolidated formations, normally run with flapper catcher or with orientation knives

| Consolidated formations, normally run with flapper catcher or with orientation knives

|-

|-

| Dog or flapper

| Dog or flapper || Consolidated, fractured, and unconsolidated formations where geology is unknown

| Consolidated, fractured, and unconsolidated formations where geology is unknown

|-

|-

| Basket

| Basket || Unconsolidated formations, normally run with another core catcher type

| Unconsolidated formations, normally run with another core catcher type

|-

|-

| Full closure

| Full closure || Friable to unconsolidated formations to provide full closure

| Friable to unconsolidated formations to provide full closure

|}

|}

A heavy duty core barrel should be considered when cutting long lengths of relatively homogeneous formations or when anticipating higher than normal torque loads. This system can also be especially attractive when rig time is the largest component of the coring expense.

A heavy duty core barrel should be considered when cutting long lengths of relatively homogeneous formations or when anticipating higher than normal torque loads. This system can also be especially attractive when rig time is the largest component of the coring expense.

The precursor of today's heavy duty core barrels is the marine core barrel. This tool was developed to be stronger than conventional systems for use in [[offshore rig|offshore]] settings. The marine barrel increases the margin of safety against tool failure, but is restricted to cutting a 3-in.-diameter core.

 

The precursor of today's heavy duty core barrels is the marine core barrel. This tool was developed to be stronger than conventional systems for use in offshore settings. The marine barrel increases the margin of safety against tool failure, but is restricted to cutting a 3-in.-diameter core.

Today's special heavy duty core barrels have been developed to core harder than normal formations and to cut extended length cores. These tools are designed to cut cores up to 5.25 in. in diameter. Heavy duty threads allow more torque to be applied to the bit and improve the margin of safety against tool failure.

Today's special heavy duty core barrels have been developed to core harder than normal formations and to cut extended length cores. These tools are designed to cut cores up to 5.25 in. in diameter. Heavy duty threads allow more torque to be applied to the bit and improve the margin of safety against tool failure.

===Core barrel liners===

===Core barrel liners===

PVC plastic, ABS plastic, and aluminum have all been used as inner core barrel liners. The liner slips inside a steel inner core barrel, reducing the effective inner diameter of the core barrel by approximately 0.5 in. The liner simplifies [[core handling]], especially for friable or unconsolidated core, and serves as a core preservation system. Liners are less expensive than disposable inner core barrels. However, besides reducing the diameter of the core, they restrict the length of the core to [[length::30 ft]].

PVC plastic, ABS plastic, and aluminum have all been used as inner core barrel liners. The liner slips inside a steel inner core barrel, reducing the effective inner diameter of the core barrel by approximately 0.5 in. The liner simplifies [[core handling]], especially for friable or unconsolidated core, and serves as a core preservation system. Liners are less expensive than disposable inner core barrels. However, besides reducing the diameter of the core, they restrict the length of the core to [[length::30 ft]].

The sponge coring system consists of a conventional core barrel fitted with a series of sponge-lined aluminum inserts ([[:file:conventional-coring_fig2.png|Figure 2]]). The system cuts a relatively small core, 2.5 or 3.25 in. wide and [[length::30 ft]] long. The sponge liner catches oil that “bleeds” out of the rock as the core is pulled (tripped) from the hole. The oil-wet sponge holds oil tightly, while allowing water and gas to move through the sponge and out vent holes drilled in the aluminum liner. Sponge and conventional cores may be cut one after another without making special trips to open the hole, so it is possible to “spot” a sponge core and still acquire larger diameter cores for other analyses.

The sponge coring system consists of a conventional core barrel fitted with a series of sponge-lined aluminum inserts ([[:file:conventional-coring_fig2.png|Figure 2]]). The system cuts a relatively small core, 2.5 or 3.25 in. wide and [[length::30 ft]] long. The sponge liner catches oil that “bleeds” out of the rock as the core is pulled (tripped) from the hole. The oil-wet sponge holds oil tightly, while allowing water and gas to move through the sponge and out vent holes drilled in the aluminum liner. Sponge and conventional cores may be cut one after another without making special trips to open the hole, so it is possible to “spot” a sponge core and still acquire larger diameter cores for other analyses.

[[file:conventional-coring_fig3.png|thumb|{{figure number|3}}Full-closure core catcher. (From <ref name=pt03r49>Whitebay, L. E., 1986, Improved coring and core-handling procedures for the unconsolidated sands of the Green Canyon area, Gulf of Mexico: SPE Paper 15385, 61st Annual Technical Conference and Exhibition, New Orleans, LA, Oct. 5–8.</ref>.)]]

[[file:conventional-coring_fig3.png|thumb|{{figure number|3}}Full-closure core catcher. From Whitebay.<ref name=pt03r49>Whitebay, L. E., 1986, Improved coring and core-handling procedures for the unconsolidated sands of the Green Canyon area, Gulf of Mexico: SPE Paper 15385, 61st Annual Technical Conference and Exhibition, New Orleans, LA, Oct. 5–8.</ref>]]

===Full-closure coring systems===

===Full-closure coring systems===

The difference between full-closure and other coring systems is that the core catcher is not exposed during coring. This allows the inner core barrel to slip over the soft core with a minimum of disturbance. After coring, a ball is pumped downhole activating the core catcher and sealing off the bottom of the barrel ([[:file:conventional-coring_fig3.png|Figure 3]]).

The difference between full-closure and other coring systems is that the core catcher is not exposed during coring. This allows the inner core barrel to slip over the soft core with a minimum of disturbance. After coring, a ball is pumped downhole activating the core catcher and sealing off the bottom of the barrel ([[:file:conventional-coring_fig3.png|Figure 3]]).

The hidden core catcher is both the major asset and major liability of this coring system. Since the core catcher is not exposed during coring, coming off bottom is likely to result in lost core. If the catcher is not activated after coring, the core will be lost during tripping. Against these risks, the benefits are a longer length and larger diameter core than is possible with a rubber sleeve core barrel and a less disturbed core than one cut with a conventional coring system.

The hidden core catcher is both the major asset and major liability of this coring system. Since the core catcher is not exposed during coring, coming off bottom is likely to result in lost core. If the catcher is not activated after coring, the core will be lost during [[trip]]ping. Against these risks, the benefits are a longer length and larger diameter core than is possible with a rubber sleeve core barrel and a less disturbed core than one cut with a conventional coring system.

===Rubber sleeve core barrel===

===Rubber sleeve core barrel===

===Wireline core barrel===

===Wireline core barrel===

The wireline core barrel allows drilling or coring without pulling the drill string. This reduces drilling costs by eliminating the need to trip pipe. The maximum size of a wireline core is [[length::30 ft]] long and 2.75 in. wide. The most commonly used tools produce a 2.36-in.-diameter core. For applications in which small diameter samples are adequate, this is a cost effective method.

The wireline core barrel allows drilling or coring without pulling the drill string. This reduces drilling costs by eliminating the need to [[trip]] pipe. The maximum size of a wireline core is [[length::30 ft]] long and 2.75 in. wide. The most commonly used tools produce a 2.36-in.-diameter core. For applications in which small diameter samples are adequate, this is a cost effective method.

The wireline core barrel is designed so that the inner core barrel and bearing assembly can be dropped through the drill string, locked in place for coring, and retrieved by wireline. Sequential cores may be cut or a diamond drilling plug can be dropped into place to drill ahead. The drilling plug may also be retrieved by wireline for inspection or to resume coring. During planning, it is important to check that the inner diameter of the drill pipe is large enough to allow the tool to pass through.

The wireline core barrel is designed so that the inner core barrel and bearing assembly can be dropped through the drill string, locked in place for coring, and retrieved by wireline. Sequential cores may be cut or a diamond drilling plug can be dropped into place to drill ahead. The drilling plug may also be retrieved by wireline for inspection or to resume coring. During planning, it is important to check that the inner diameter of the drill pipe is large enough to allow the tool to pass through.

[[Category:Wellsite methods]]

[[Category:Wellsite methods]]