Chapter 5-Formation Evaluation

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CHAPTER 5

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COURSE CONTENTS

Mud Logging

Coring

Open-hole Logging

Logging While Drilling

Formation Testing

Cased Hole Logging


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Formation Evaluation

What is Formation Evaluation?

Formation Evaluation (FE) is the process ofinterpreting a combination of measurements takeninside a wellbore to detect and quantify oil andgas reserves in the rock adjacent to the well. FEdata can be gathered with wireline logginginstruments or logging-while-drilling tools .

Study of the physical properties of rocks and thefluids contained within them.

Data are organized and interpreted by depth andrepresented on a graph called a log (a record ofinformation about the formations through which awell has been drilled).

Formation Evaluation

Why Formation Evaluation?

To evaluate hydrocarbons reservoirs and predictoil recovery.

To provide the reservoir engineers with theformations geological and physical parametersnecessary for the construction of a fluid-flowmodel of the reservoir.

Measurement of in situ formation fluid pressureand acquisition of formation fluid samples.

In petroleum exploration and development,formation evaluation is used to determine theability of a borehole to produce petroleum.


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Mud Logging

Mud logging (or Wellsite Geology) is a well loggingprocess in which drilling mud and drill bit cuttingsfrom the formation are evaluated during drilling andtheir properties recorded on a strip chart as a visualanalytical tool and stratigraphic cross sectionalrepresentation of the well.

Provide continuous record of penetration rate,lithology and hydrocarbon shows.

These information supports wireline log data.

From the cuttings, an oil stains or odor of oil may bedetected, become an excellent qualitative indicator.

The fluorescent lamp is also a great help in detectingoil shows.

Mud Logging

The gas record and lithological sample are plottedalong with surface parameters such as rate ofpenetration (ROP), Weight On Bit (WOB),rotationper minute etc. on the mudlog which serve as a toolfor the drilling engineers and mud engineers.

Some problem: a discrepancy between the time therock was drilled and the time it reached the surface particularly for deep wells, where it take two ormove hours to reach the surface.


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Coring

One way to get more detailedsamples of a formation is by coring,where formation sample is drilledout by means of special bit.

This sample can provide: Detailed lithological decscription. Porosity, permeability, fluid

saturation and grain density.

These parameters are measured inthe laboratory and serve as a basisfor calibrating the response of theporosity logging tools and toestablish a porosity/permeabilityrelationship.

Coring

Two techniques commonly used atpresent. The first is the "wholecore", a cylinder of rock, usuallyabout 3" to 4" in diameter and up to50 feet (15 m) to 60 feet (18 m) long.

It is cut with a "core barrel", ahollow pipe tipped with a ring-shapeddiamond chip-studded bit that cancut a plug and bring it to the surface.

Taking a full core is an expensiveoperation that usually stops or slowsdrilling operation, and can be doneonly before the drilling has beendone.

Coring Tool & Core Barrel


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Coring

The other, cheaper, technique for obtaining samplesof the formation is "Sidewall Coring". In this method,a steel cylindera coring gunhas hollow-point steelbullets mounted along its sides and moored to thegun by short steel cables.

The coring gun is lowered to the bottom of theinterval of interest and the bullets are firedindividually and the core will be retrieved.

Advantages of this technique are low cost and the

ability to sample the formation after it has beendrilled.

Core Preservation

Once the core is retrieve to surface then it isimportant that it should remain as unchanged aspossible.

The core should be prevented from drying out,

coming into contact with oxygen or beingmechanically damaged. Core barrel is filled with resin to prevent the

core from moving and to minimize the exposedsurface area.

Freezing the core in freezer containers. Core sample is wrapped in a plastic film, aluminium

foil and then dipped in molten wax.


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Core Analysis

Can be divided into two categories:

Conventional Core Analysis. Special Core Analysis.

Conventional Core Analysis.

The core is usually slabbed, cut lengthwise tomake the structure visible.

Provides information on lithology, residual fluidsaturation, ambient porosity, ambient gaspermeability and grain density.

Gas Permeameter

Liquid Permeameter


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Porosimeter

Core Analysis

Special Core Analysis :Provides the following information:

Porosity and permeability at elevated confiningstress.

Electrical properties such as formation factorand resistivity index.

Capillary pressure.

Wettability and relative permeability.

Mechanical rock properties such ascompressibility.

Waterflood sensitivity for injectivity and wellperformance.


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Open-hole Logging

Open-hole logging, also known aswell logging is the practice ofmaking a detailed record (a welllog) of the geologic formationspenetrated by a borehole.

Open hole logs are run beforethe oil or gas well is lined withpipe or cased

Principal of Well Logging

A well log is a record of certain formation dataversus depth.

The appropriate downhole logging tools instrumentcalled sonde, about 3.5 inches in diameter islowered into mud-filled hole on logging cable.

This tools will measure the electrical, acoustic, andradioactive properties of the formation.

The result will be analyzed to determine which ofthe layers are porous and permeable, and likely tocontain hidrocarbon.

A depth calibration wheel records the length ofcable in the hole.


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Principal of Well Logging

Survey is normally donefrom the bottom up. Asthe sonde is pulled up thehole, a continuousmeasurement signal is sentto the surface where thedata is processed andrecorded as a curve.

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Electrical Logs

Developed by Conrad & Marcel Schlumberger (whofounded Schlumberger Limited), and intoduced tothe US in 1929.

Can be divided into two main types: measurementof natural electrical current in the rock (SP Log),and measurement of induced electrical current(Resistivity Log and Induction Log).

(1) Spontaneous Potential (SP) Log

Also known as Self PotentialLog.

SP Log record weak electricalcurrents that flow naturally inthe rock next to the wellbore(natural electricity).

The log shows the boundariesand thickness of each layer ofrock, especially permeable(sandstone) and impermeable(shale).

Because the SP Log is sosimple to obtain and providesuch basic information, it isthe most common log.

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(1) Spontaneous Potential (SP) Log

Useful for: Detecting permeable beds

and it thickness. Locating their boundaries

and permitting correlationof such beds.

Determining formationwater resistivity.

Qualitative indication ofbed shaliness.

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(2) Resistiv ity Logs

Use to measure the resistivity of the formation,and thus the possibility of hc shows.

A sonde sends an electrical signal through theformation and relays it back to a receiver at thesurface (induced electricity). The surface detectorwill measure the formations resistance to thecurrent.

A rock which contains an oil and/or gas saturationwill have a higher resistivity than the same rockcompletely saturated with formation water.


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(3) Induction Logs

Use to measure the conductivity of the formation,and thus the possibility of hc shows.

A rock which contains an oil and/or gas saturationwill have a lower conductivity than the same rockcompletely saturated with formation water.

Induction logs use an electric coil in the sonde togenerate an alternating current loop in theformation by induction.

Induction tools t give best results when mudresistivity is high with respect to formationresistivity, i.e., fresh mud or non-conductive fluid.In oil-base mud, which is non conductive, inductionlogging is the only option available.

(4) Dielectric Logs

Responds essentially to water and is unaffected bythe presence of hydrocarbons.

Particularly important in determining theirreducible water saturation when oil-based mudsare used.


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Nuclear Logs

Just as SP and resistivity logs record natural andinduced electrical currents, nuclear logs (alsocalled radioactivity logs) record natural andinduced radioactivity.

Three type of logs: Gamma Ray Log, Neutron Logand Formation Density Log.

(1) Gamma Ray Log

Record the natural -radioactivity of rockssurrounding the borehole.

The -radiation arises fromthree elements present in therocks, isotopes of potassium,uranium and thorium.

Useful for defining shale bedsbecause K, U and Th are largelyconcentrated in associationwith clay minerals.

It is used to define permeablebeds when SP log cannot beemployed (eg. When Rmf = Rw).

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(2) Neutron Log

To obtain a neutron log, a sonde sends atomicparticles called neutrons through the formation.

When the neutrons collide with hydrogen, thehydrogen slows them down.

The response of the devise is primarily a functionof the hydrogen nuclei concentration.

When the detector records slow neutrons, it meansa lot of hydrogen is present main component ofwater and hydrocarbon, but not of rocks.

Considered as porosity log because hydrogen ismostly present in pore fluids (water, hydrocarbons)the count rate can be converted into apparentporosity.

(3) Formation Density Log

This devise measure number of photon then berelated to electron density of the formation.

Electron density is related to an apparent bulkdensity which equivalent to formation bulk density.

Useable to detect formation lithology.


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Sonic or Acoustic Logs

Provide continuous record of the time taken inmicrosecond/foot by sound wave to travel from thetransmitter to the receiver n the sonde.

Velocity of sound through a given formation is afunction of its lithological and porosity.

Dense, low porosity rocks are characterized byhigh velocity of sound wave and vise-versa forporous and less dense formation.


One of the major drawbacks of wirelineinformation is that it is received several hours toseveral weeks after the borehole is drilled.

During this time period, the formation can undergo

significant alteration, especially in its fluidsaturation, effective porosity, and relative perm.

LWD allow wireline-type information to beavailable as near as real-time as possible.

Logging While Drilling (LWD) is a technique ofconveying well logging tools into the well boreholedownhole as part of the bottom hole assembly(BHA).


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Some available measurement in LWD technology:

Gamma Ray Resistivity Density Neutron Sonic (fairly recent) Formation pressure Formation fluid sampler

Borehole caliper (Ultra sonic azimuthal caliper,and density caliper).

Formation Testing

Is a means of obtaining information concerning theliquid and pressure in an open-hole formations.

Three methods:

Wireline testing Drill stem test (DST) Well Test Analysis


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Wireline Testing

Provide reservoir fluid samples, reservoir pressure,an indication of fluid mobility and information onreservoir continuity.

Two types: Repeat Formation Tester (RFT) andFormation Interval Tester (FIT).

The RFT is run into the hole and a continuousdigital readout of hydrostatic pressure is obtained.

At any point in the hole the tool may be actuated

to force a rubber pad against the wall of the hole,and a tube in the centre of the pad is forced hardagainst the formation.

The formation fluid will flow to the chamberthrough the tube.

Wireline Testing

The FIT is used for single test only one pressurereading and one fluid sample for each run.

A tool is actuated (a pad is tightly against theformation to form a seal against hydrostaticpressure of the fluid in the hole).

A shaped charge is then fired into the fm, openinga passageway for fm fluids to flow into a chamberin the tool. At he same time the fm pressure will berecorded.


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Drillstem Test (DST)

A drill stem test (DST) is a procedure for isolatingand testing the surrounding geological formationthrough the drill stem.

The test is a measurement of pressure behavior atthe drill stem and is a valuable way to obtainimportant sampling information on the formationfluid and to establish the probability of commercialproduction.

The test is made by lowering a valve, a packer, anda length of perforated tailpipe to the level offormation.

The packer set against the wall of the borehole sothat it seals off the test interval from the mudcolumn above.

Drillstem Test (DST)

The valve is then opened, andthe fm fluid will flow to thesurface through the drillpipe.

The amount of fluid producedwill represent the fluidproduction can be expectedfrom the well.


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Well Test Analysis

Two types of testing: pressure build-up and drawdown test.

The primary objectives of well testing are toestablish:

Permeability thickness (Kh) and permeability (K) Stratification (by sequential testing of layer). Well productivity. Investigate reservoir boundaries and size.

The amount of fluid produced will represent thefluid production can be expected from the well.

Cased-hole Logging

Two major areas of cased-hole logging:

Production logging. Reservoir monitoring.

Production logging refers to obtaining productionor injection profiles over a completed interval.

Reservoir monitoring refers to obtaining real timeinformation about changes in hydrocarbonsaturation.

Crucial for understanding water contact movement.

Other services include cement bond log which usedto evaluate the degree of isolation provided by thecasing cement.

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Chapter 5-Formation Evaluation