BasicPetro_4.ppt

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Core Data Analysis andComparison with Logs



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Objectives

Core Data Types

Comparison with logs

Advantages

Disadvantages



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Cores

Allow direct measurement of reservoir properties

Used to correlate indirect measurements, such as

wireline/LWD logs

Used to test compatibility of injection fluids

Used to predict borehole stability

Used to estimate probability of formation failure and sand

production



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Information from Cores

Porosity

Horizontal permeability to air

Grain density

Vertical permeability to air

Relative permeability

Capillary pressure Cementation exponent (m) and

saturation exponent (n)

Standard Analysis Special Core Analysis



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Coring

Whole-core Analysis

Full-Diameter Core Analysis

Core-Plug Analysis

Up to

2 ft 6 in.1-1.5 in.



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PDC Cutters

Fluidvent

Drill collarconnection

Inner barrel

Outer barrel

Thrust bearing

Core retainingring

Core bit

Coring Assembly and Core Bit



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(Whole Core Photograph, MisoaSandstone, Venezuela)

Whole Core



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Sidewall Sampling Gun

Core bullets

Core sample

Formation rock



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Sidewall Coring Tool

Coring bit

Samples




Comparison with Cores

Taking a core remains the only opportunity for the petroleumengineer of the geologist to physically examine a continuous

interval of reservoir rock. This technique has been continuously

improved to increase recovery and to keep the sample collected in

conditions as close as possible to the ones observed downhole

Three types of analysis are performed depending on the size of

the sample:

Fullbore analysis

Analysis of the plugs extracted from the fullbore core

Sidewall cores




Comparison of logs with drilled cores

The comparison of core and log results is comsuming largeamounts of the petroleum engineer and geologist energy and

time. There is a number of reasons why the parameters differ:

Physical configuration

Depth mismatch

Changes in fluid type and saturation Difference of volume of inverstigation and of vertical resolution

Porosity

Saturation exponent




Physical Configuration

A fullbore core extracted from a hole of nominal bitsize

b would have a diameter approximately equal to b/2.

The high resolution logging tools, generally of pad

type, have sensors directly against the borehole wall,

while the outside of core is several inches away from

the borehole wall during drilling. This discrepancy

causes a depth offset

hole hole

Logging toolFullbore Core




Depth Mismatch

Most laboratories match core depth with log depth byrunning a gamma ray survey over the core. However,

when the recovery is poor and the core is short, the

match is often unsatisfactory. It may even happen that

the cores are wrongly labelled or put upside down in

their boxes




Change in Fluid Type and Saturation

When a conventional core is cut with a standard diamond or corebarrel, it is partly flushed by the drilling fluid or mud filtrate. The

core cut at downhole pressure and temperature contains a mixtureof fluids including connate water, mud filtrate and residual oil orgas. When the core is brought to the surface, the pressure andtemperature are reduced till they reach atomospheric conditions.The gas in solution in the oil is liberated. The free gas or solutiongas expands forcing mud filtrate, oil and possibly connate water out

of the core. Consequently, a core taken from an oil bearing zone contains gas,

oil and a large amount of water (filtrate or connate). A core comingfrom a gas bearing zone contains gas, no oil and a larger amount ofwater

These changes of phase may be reduced by special coring

techniques using rubber or plastic sleeves. The cores aremaintained at bottom hole pressure at which they were cut until theyare analyzed in the laboratory




Difference of Volume of Investigation and ofVertical Resolution

Logs average porosity over a depth sample,

typically 0.3 m [1ft]. Core porosity may change by

several p.u. within a few inches, as shown in the

following table. Core plug porosity measurements

taken at 0.5m [2ft] intervals do not represent theaverage porosities every 0.5m [2ft]. In fractured or

vuggy carbonates, the core laboratory analysts cut

the plugs in homogeneous zones and high porosity

zones are often missed.




Porosity

Measured core porosity depends on the technique

used (table). For unconsolidated formations, thedifferences between porosities measured with and

without overburden can be even larger.

In shaly sands, core porosity includes bound water

volume and is equivalent to total porosity, a parameter

introduced in the Waxman-Smits and Schlumberger

shaly sands interpretation models. It could be

significantly higher than log porosity, equivalent to

effective porosity.




Saturation Exponent

If careful procedures are not followed, the rock fabric of

the core may be altered and the laboratory-derivedsaturation exponent is erroneous.




Comparison with Sidewall Cores

Sidewall samples are obtained by percussion. The

petrophysical properties of the formation such asporosity and permeability cannot be evaluated because

of compaction, mud invasion and shattering. Despite

these limitations, sidewall cores are a useful

complement to logs to evaluate lithology. Since thesidewall core and log information have different

resolutions, a potential difficulty in core/log comparison

is depth matching




Determination of Depth Shifts Between

Core and Log Data The method is similar to the correlation of

microresistivity dipmeter curves. It searches for an

optimal displacement between the two parameters




Comparison with Production Data:Capillary Pressure Curves In a homogeneous formation, the capillary pressure curve gives

the saturation profile . The pressure is proportional to the heightabove the free water level, precisely defined as the depth wherethe capillary pressure is nil. The threshold pressure is defined asthe lowest capillary at which water is displaced from the formation.It corresponds to the most shallow level with 100% water saturation computed from the logs.

The curved section of the capillary pressure curve represents thetransition zone. Corresponding to higher pressure, the zone abovethe transition displays the lowest water saturation values thatreach an asymptotic minimum, the irreducible water saturation.

Discrepancies between core and log-derived saturations are

mainly due to their respective resolution. Correct log valuescannot be obtained from low resolution tools in thin layers. In thatcase higher resolution logs, such as the ElectromagneticPropagation logs help resolve or explain the differences.




Uses of Logs

A set of logs run on a well will usually mean different

things to different people.

The Geophysicist The Geologist

The Drilling Engineer

The Reservoir Engineer

The Production Engineer



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The Geophysicist

As a Geophysicist what do you look for?

Are the tops where you predicted?

Are the potential zones porous as you haveassumed from seismic data?

What does a synthetic seismic section show?



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The Geologist

The Geologist may ask:

What depths are the formation tops?

Is the environment suitable for accumulation of Hydrocarbons?

Is there evidence of Hydrocarbon in this well?

What type of Hydrocarbon? Are Hydrocarbon present in comercial quantities?

How good a well is it?

What are the reserves?

Could the formation be commercial in an offset well?



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The Drilling Engineer

What is the hole volume for cementing?

Are there any Key-seats or severe Dog-legs in the

well?

Where can you get a good packer seat for testing?

Where is the best place to set a Whipstock?



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The Reservoir Engineer

The Reservoir Engineer needs to know:

How thick is the pay zone?

How homogeneous is the section?

What is the volume of Hydrocarbon per cubic meter?

Will the well pay-out?

How long will it take?



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The Production Engineer

The Production Engineer is more concerned with:

Where should the well be completed (in what

zone(s))?

What kind of production rate can be expected?

Will there be any water production?

How should the well be completed?

Is the potential pay zone hydraulically isolated?



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GeoFrame Applications

Petrophysics

Borehole Geology

Seismic Interpretation

Synthetics

Geology Office

WellPix

ResSum

Documents

BasicPetro_4.ppt