Coring.knk'nk

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Coring

Coring is the process of Cutting and removal of a rock sample from the well bore. (rocksamples are one of the most direct and valuable sources of data for the study of

subsurface rocks and reservoirs).

There are two types of coring:(I) coring axial to the well bore or conventional coring

(II) Sidewall coring.

Objective:

To bring a sample of the formation and its pore fluids to the surface in an unaltered form,

to preserve the sample and then transport it to laboratory for analysis.

These objectives are hard to meet since the very act of cutting a core will, to some extent,alter both properties of rock itself and saturation of fluids in its pores

Why Core Acquisition:

Particularly during the exploration phase of a field, coring presents an important means tocalibrate the petrophysical model and gain additional information about the reservoir not

obtainable by logs.

Usually the decision of when and where to core will be made in conjunction with thegeologist and operations department, taking into account the costs and data requirements

It is considered essential to at least attempt to core a part of the main reservoir formationduring the exploration and appraisal phases of drilling

Conventional coring

The process of collecting a core sample for testing In conventional coring, a diamond or PDC bit is used. The process is time consuming because a core bit drills much slower than

the conventional bit.

Preferred in case of hard rocks The sample is collected generally at the first indication of porosity. Cores can be from 10 to 6o ft long and have a diameter of 1 to 4

inches.

If unconsolidated formation is to be cored, rubber sleeve is used to holdthe friable material more securely.

The drill string edge can also be checked to detect the presence ofhydrocarbon


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For some application a special pressure core can be cut.

Core Barrel is designed so that after the core is cut it is maintained at original reservoir

pressure until it arrives at the laboratory for analysis.

Sidewall coring

Useful for soft or unconsolidated formations An explosive gun is used to push the barrel into the formation, which is supported

by an electric wire Small cylinders known as core barrel is used to collect the samples Can be used to supplement open-hole logs by checking various spots for

lithography and fluid saturations

It is much less expensive than continuous axial coring.

These samples arc about 2cm in diameter and up to 5 cm long.

Exact depth is known and samples are much larger than cuttings. Which enables

better evaluation of geological variation, depositional environment and definitionof hydrocarbon shows.

Some times the sample can be too small or fragmented to provide reliableinformation.


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Petrophysical measurements such as porosity and permeability on sidewallsamples ,should be treated with caution due to the probability of mechanical

damage during sampling especially with 'bullet' plugs.

These guns come in variety of shape and size. They are capable of retrieving 60 samples in one trip in the hole. The length of the core retrieved is a function of many variables.

o Strength of explosive charge usedo Type of core barrel selectedo Hardness of the formation determine the length of recovered

sample; It may as long as 2in. Occasionally, the retainer wires used to retrieve the core barrel will break and the

bullet will be lost in the hole.

Each core plug is stored and labeled with well name and depth from it was cut.

Subsequently these cores may be analyzed for the porosity, permeability and hydrocarbonsaturation.

Important information the petrophysicist can learn from such an inspection include:

The homogeneity of the reservoir and any variations that are likely to bebelow the resolution of logging tools.

The type of cementation and distribution of porosity and permeability The presence of hydrocarbons The types of minerals present Presence of fractures (either cemented, natural, or drilling induced) and

their orientation

Dip features that may influence logging tools response

The plugs are use to measured for porosity (using a helium porosimeter), horizontalpermeability, and grain density.

Additional plugs are cut in the axis of the core to determine vertical permeability.


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Core handling

In order to quantify mud redistribution while it sits in the core barrel during thetrip to surface the sponge core barrel was developed. Here the inner barrel is made

of aluminum and is lined with a special sponge material that absorbs expelled

liquids.

More recent developments include the use of the pressurized core barrel. wherethe core barrel is scaled and is designed to contain the full reservoir pressure atthe surface, and gel-coring to assist in preventing filtrate invasion.

Once the core is retrieved to surface. it is important that it should remain asunchanged as possible. A variety of techniques have been developed to meet this

requirement to prevent the core from drying out coming into contact with oxygenor being mechanically damaged.

One of the simplest methods is to cut up disposable core barrels such as fiberglass liner into :3 feet (~I m) lengths. The annulus between core and core barrel isthen filled with resin to prevent the core from moving during transportation and

also to minimize the exposed surface area.

This approach is often adopted with poorly consolidated core which isparticularly susceptible to damage during transportation. This problem of

handling un consolidated core can be further reduced by freezing the core eitherin dry ice or in freezer containers.

For consolidated rock the core seal approach can be adopted. Where samplewrapped in plastic film and then aluminium foil and finally dipped in molten wax.The wax solidifies and creates a seal against the nature.

Alternatively the sample can be placed in a jar and Immersed in liquid. Anextension of this method. for longer-term preservation is to establish a nitrogen

cap at low pressure to ensure oxygen can not enter the liquid surrounding thesample. It is recommended that the core analysis programme should start as soon

as the samples are received at the laboratory.

Core analysis can be divided into two categories:

Basic core analysis Special Core Analysis.


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Basic core analysis provides information on lithology, residual fluid saturation.porosity. permeability (horizontal and vertical) and grain density.

Special Core Analysis

SCAL measurements are typically performed on a special set of larger diameter (1.5 in.)plugs cut from the core. These may be cut at a regular sampling increment, or the

petrophysicist may specify certain depths based on the results of the conventionalanalyses. The most important criterion is obviously to obtain a broad spectrum of

properties that fully encompass the range of properties seen in the reservoir. It is hard tosay how many SCAL plugs are required for a typical program, since this depends on the

reservoir type, thickness, and homogeneity. In general a SCAL program may usebetween about 5 and 50 plugs.

Special Core analysis typically provides the following information -:

Porosity and permeability at elevated confining stress. capillary pressure electrical properties such as formation factor and resistivity index Wetability and relative permeability mechanical rock properties such as compressibility Water flood sensitivity for injectivity and well performance

Core Plugger

The core plugger uses a motorized circular bit to bore physically in to the wall offormation in order to retrieves it sample.

It presently capable of cutting up to 12 core samples in one run in the hole. With this tool, core size is 13/16 in. in dia (

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