1-Basic Log Interpretation

5/28/2018 1-Basic Log Interpretation

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Reservoir Rocks

Introduction to Log

Interpretation

Schlumberger 1999

A


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Log InterpretationInterpretation is defined as the action of

explaining the meaning of something.

Log Interpretation is the explanation of logs b,

GR, Resistivity, etc. in terms of well and reservoir

parameters, zones, porosity, oil saturation, etc.

Log interpretation can provide answers to

questions on:


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Why Run Logs


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The Reservoir


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Requirements of a reservoir

To form a reservoir needs

- source of organic material (terrestrial ormarine)

- a suitable combination of heat, pressure andtime

- an oxygen free environment

- a suitable basin


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Reservoir Geometry


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Reservoir Rocks

Schlumberger 1999

A


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Rocks General

There are three major classes of rock:

Igneous:

(e.g. Granite).

Sedimentary:

(e.g. Sandstone).

Metamorphic:

(e.g. Marble).


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Rock Cycle


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Igneous Rocks

Comprise 95% of the Earth's crust.

Originated from the solidification of molten

material from deep inside the Earth.

There are two types:

Volcanic - glassy in texture due to fast cooling.

Plutonic - slow-cooling, crystalline rocks.


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Igneous Rocks and Reservoirs

Igneous rocks can be part of reservoirs.

Fractured granites form reservoirs in some

parts of the world.

Volcanic tuffs are mixed with sand in some

reservoirs.

Example: Granite Wash - Elk City, Okla., Northern

Alberta,CA


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Metamorphic Rocks

2) Metamorphic rocks

formed by the action of temperatureand/or pressure on sedimentary or

igneous rocks.

Examples are

Marble - formed from limestone

Hornfels - from shale or tuff

Gneiss - similar to granite butformed by metamorphosis

Field Example: 1. Point Arguello - Monterey Formation is

actually layers of fractured Chert and Shale. Oil is in the

fractures

2. Long Beach, Calif. - Many SS producers on an Anticline

above fractured Metamorphic basement rock

3. Austin, TX eastward - Lava flows of Basalt (Serpentine) from

Volcanoes in ancient Gulf of Mexico


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Sedimentary Rocks

The third category is Sedimentary rocks. Theseare the most important for the oil industry as itcontains most of the source rocks and cap rocksand virtually all reservoirs.

Sedimentary rocks come from the debris ofolder rocks and are split into two categories

Clastic and Non-clastic.

Clastic rocks - formed from the materials of

older rocks by the actions oferosion, transportation anddeposition.

Non-clastic rocks -

from chemical or biologicalorigin and then deposition.


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Reservoir Rocks

Depositional Environments

The depositional environment can be

Shallow or deep water.

Marine (sea) and lake or continental.

This environment determines many of the

reservoir characteristics

Frigg Gas Field -

North Sea


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Depositional Environments 2

Continental deposits are usually dunes.A shallow marine environment has a lot of

turbulence hence varied grain sizes. It can also

have carbonate and evaporite formation.

A deep marine environment produces fine

sediments.


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Depositional Environments 3

The depositional characteristics of the rocks

lead to some of their properties and that of the

reservoir itself.

The reservoir rock type clastic or non-clastic.

The type of porosity (especially in carbonates) is

determined by the environment plus subsequent

events.

The structure of a reservoir can also be

determined by deposition; a river, a delta, a reef

and so on.

This can also lead to permeability and

producibility. of these properties are often

changed by further events.


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Depositional Environment 4

The environment is not static.

Folding and faulting change the structure.

Dissolution and fracturing can change the

permeability.


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Clastic Rocks

Clastic rocks are sands, silts and shales. The

difference is in the size of the grains.


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Sedimentation

Sediments settle to

the bottom of the

sedimentary basin.

As the sediments

accumulate

the temperature and

pressure increase

expelling

water from the

sediments.


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Sedimentation 2

Sedimentary muds become sedimentary rocks.

Calcareous muds become limestone.

Sands become sandstone.

Another effect involves both the grains in the

matrix and the fluids reacting to create new

minerals changing the matrix and porosity.Fluids can also change creating a new set of

minerals.

This whole process is called Diagenesis.


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Depositional Environment - Delta

Sediments are transported to the basins by

rivers.

A common depositional environment is the delta

where the river empties into the sea.A good example of this is the Mississippi.


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Rivers

Some types of deposition occur in rivers and

sand bars.

The river forms a channel where sands are

deposited in layers. Rivers carry sediment downfrom the mountains which is then deposited in

the river bed and on the flood plains at either

side.

Changes in the environment can cause thesesands to be overlain with a shale, trapping the

reservoir rock.


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Carbonates

Carbonates form a large proportion of all

sedimentary rocks.

They consist of:

Limestone.

Dolomite.

Carbonates usually have an irregular structure.


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Carbonate types

Chalk is a special form of limestone and is

formed from the skeletons of small creatures

(cocoliths).

Dolomite is formed by the replacement of some

of thecalcium by a lesser volume of magnesium

in limestone by magnesium. Magnesium is

smaller than calcium, hence the matrix becomes

smaller and more porosity is created.

Limestone CaCO3

Dolomite CaMg(CO3)2

Evaporites such as Salt (NaCl) and Anhydrite

(CaSO4) can also form in these environments.


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Depositional Environment

Carbonates

Carbonates are formed in shallow seas

containing features such as:

Reefs.

Lagoons.

Shore-bars.


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Rock Properties

Rocks are described by three properties:

Porosity - quantity of pore space

Permeability - ability of a formation to flow

Matrix - major constituent of the rock


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Definition of Porosity


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Porosity Sandstones

The porosity of a sandstone depends on the

packing arrangement of its grains.

The system can be examined using spheres.

In a Rhombohedral packing,the pore space accounts for

26% of the total volume.

With a Cubic packing

arrangement, the pore space

fills 47% of the total volume.

In practice, the theoretical

value is rarely reached

because:

a) the grains are not perfectly

round, and

b) the grains are not of

uniform size.


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Porosity and Grain Size

A rock can be made up of small grains or large

grains but have the same porosity.

Porosity depends on grain packing, not thegrain size.


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Diagenesis

The environment can also involve subsequent

alterations of the rock such as:

Chemical changes.

Diagenesis is the chemical alteration of a rockafter burial. An example is the replacement of

some of the calcium atoms in limestone by

magnesium to form dolomite.

Mechanical changes - fracturing in a

tectonically-active region.


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Carbonate Porosity Types 1

Interparticle porosity:

Each grain is separated,

giving a similar pore spacearrangement as sandstone.

Intergranular porosity:

Pore space is created inside

the individual grains whichare interconnected.

Intercrystalline porosity:

Produced by spaces between

carbonate crystals.

Mouldic porosity:

Pores created by the

dissolution of shells, etc.

Carbonate porosity is very heterogeneous. It isclassified into a number of types:


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Carbonate Porosity Types 2

Fracture porosity:

Pore spacing created

by the cracking of

the

rock fabric.

Channel porosity:Similar to fracture

porosity but larger.

Vuggy porosity:

Created by the

dissolution offragments, but

unconnected.


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Carbonate Porosity

Intergranular porosity is called "primary

porosity".

Porosity created after deposition is called

"secondary porosity".

The latter is in two forms:

Fractures

Vugs.


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Fractures

Fractures are caused when a rigid rock is

strained beyond its elastic limit - it cracks.

The forces causing it to break are in a constant

direction, hence all the fractures are also

aligned.

Fractures are an important source of

permeability in low porosity carbonate

reservoirs.


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Vugs

Vugs are defined as non-connected pore space.

They do not contribute to the producible fluid

total.

Vugs are caused by the dissolution of soluble

material such as shell fragments after the rock

has been formed.

They usually have irregular shapes.


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Permeability Definition

The rate of flow of a liquid through a formationdepends on:

The pressure drop.

The viscosity of the fluid.

The permeability.

The pressure drop is a reservoir property.

The viscosity is a fluid property.

The permeability is a measure of the ease at

which a fluid can flow through a formation.

Relationships exist between permeability and

porosity for given formations, although they arenot universal.

A rock must have porosity to have any

permeability.

The unit of measurement is the Darcy.Reservoir permeability is usually quoted in

millidarcies, (md).


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Darcy Experiment

The flow of fluid of viscosity m through a

porous medium was first investigated in 1856 by

Henri Darcy.

He related the flow of water through a unit

volume of sand to the pressure gradient acrossit.

In the experiment the flow rate can be changed

by altering the parameters as follows:


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Darcy Law

K = permeability, in Darcies.

L = length of the section of rock, in centimetres.

Q = flow rate in centimetres3/ sec.

P1, P2 = pressures in bars.

A = surface area, in cm2.

= viscocity in centipoise.


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Permeability and Rocks

In formations with large grains, the

permeability is high and the flow rate larger.


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Permeability and Rocks 2

In a rock with small grains the permeability is

less and the flow lower.

Grain size has no bearing on porosity, but has a

large effect on permeability.


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Reservoir Rocks

Reservoir rocks need two properties to be

successful:

Pore spaces able to retain hydrocarbon.

Permeability which allows the fluid to move.


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Clastic Reservoirs

Sandstone usually has regular grains; and is

referred to as a grainstone.

Porosity

Determined mainly by the packing and

mixing of grains.

Permeability

Determined mainly by grain size and

packing, connectivity and shale content.

Fractures may be present.


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Carbonate Reservoirs

Carbonates normally have a very irregular

structure.

Porosity:

Determined by the type of shells, etc. and

by depositional and post-depositional events

(fracturing, leaching, etc.).

Permeability:

Determined by deposition and post-

deposition events, fractures.

Fractures can be very important in carbonate

reservoirs.


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Cap Rock

A reservoir needs a cap rock.

Impermeable cap rock keeps the fluids trapped

in the reservoir.

It must have zero permeability.

Some examples are:

Shales.

Evaporites such as salt or

anhyhdrite.

Zero-porosity carbonates.


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Source Rocks

Hydrocarbon originates from minute organismsin seas and lakes. When they die, they sink to

the bottom where they form organic-rich

"muds" in fine sediments.

These "muds" are in a reducing environment or

"kitchen", which strips oxygen from the

sediments leaving hydrogen and carbon.

The sediments are compacted to form organic-

rich rocks with very low permeability.

The hydrocarbon can migrate very slowly tonearby porous rocks, displacing the original

formation water.


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Hydrocarbon Migration

Hydrocarbon migration takes place in two

stages:

Primary migration - from the source rock to a porous

rock.This is a complex process and not fully understood.

It is probably limited to a few hundred metres.

Secondary migration - along the porous rock to the trap.

This occurs by buoyancy, capillary pressure and hydrodynamics

through a continuous water-filled pore system.

It can take place over large distances.


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Rock Classification

Clastics

Rock type Particle diameter

Conglomerate Pebbles 2 - 64mm

Sandstone Sand .06 - 2mm

Siltstone Silt .003 - .06mm

Shale Clay


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Reservoir Structure

There are many other types of structure.

The criteria for a structure is that it must have:

Closure, i.e. the fluids are unable to

escape.

Be large enough to be economical.

The exact form of the reservoir depends on the

depositional environment and post depositional

events such as foldings and faulting.


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Traps General

Ghawar Oilfield - Saudi Arabia- Ls - 145 mi x 13 mi wide x260 ft

produces 11,000 b/d total 82B bbls

Gasharan Oilfield - Iran - Ls - 6000ft. Net pay total 8.5 B bbls


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Structural Traps

The simplest form of trap is a dome.This is created by upward movement or folding

of underlying sediments.

An anticline is another form of simple trap. This

is formed by the folding of layers of sedimentaryrock.


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Fault Traps

Faults occur when the rock shears due tostresses. Reservoirs often form in these fault

zones.

A porous and permeable layer may trap fluids

due to its location alongside an impermeablefault or its juxtaposition alongside an

impermeable bed.

Faults are found in conjunction with other

structures such as anticlines, domes and salt

domes.

Normal Faults- Nigeria,Hibenia (E. Canada), Vicksburg

Trends (Victoria, TX)

Drag Faults - Wyoming,

most Rocky Mountains


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Salt Dome Trap

Salt Dome traps are caused when "plastic" saltis forced upwards.

The salt dome pierces through layers and

compresses rocks above. This results in the

formation of various traps:

In domes created by formations pushed up by

the salt.

Along the flanks and below the overhang in

porous rock abutting on the impermeable salt

itself.

Example: Gulf of Mexico, Spindletop,TX, North Sea (Ekofisk

Dome with fractured Chalk as reservoir rock)


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Stratigraphic Traps

Point Bars - Powder River Basin, WY, Clinton SS in Western Ok,

Michigan - Belle River Mills

Devonian reefs (Barriers and

Atolls) - Alberta CA. (Leduc

& Redwater)

Midland Basin &Delaware

Basin of West TX - Barrier

Reefs


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Reservoir Mapping

Reservoir contours are usually measured to be

below Mean Sea Level (MSL).

They can represent either the reservoirformation structure or fluid layers.


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Wireline Well Logging

The Wellbore

Wellbore Fluids

Volumes Under InvestigationBasic Interpretation Technique

Interpretation Procedures


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Measuring in the borehole

Sensors+Electronics

Borehole

Formation

to beMeasured

The formation to be

measured is masked

by the borehole.

The boreholecontains fluids and is

of an irregular shape.

The sensor has to be

able to measure theformation property

accurately and send

the information to

surface.


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Borehole -Size and Shape

Perfect shape no problems

except if very large.

Ovalised hole; will give

problems for some tools.

Best to run two calipers.

Irregular borehole, gives

problems for most tools.


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Tool Positioning - 1

Formationto beMeasured

CentralisedTool

Some tools are run

centralised in the

borehole in order to

measure properly.

These include laterolog

and sonic devices.

Special centralisers are

put on the tool.


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Some tools are run

eccentred, pushed,

against the borehole

wall.

In some cases this is

done with an

eccentraliser.

In other cases a caliper

arm does this job.


EccentralisedTool


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Tool withStand-offs

Stand-Offs

Some tools are run

with stand-offs to

position them at a

fixed distance from

the wall.

The induction family

are usually run in this

manner.


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Borehole fluids 2

Oil based mud will not allow current to pass so

electrical logs will not work.

Foam and air muds will not transmit sonics

signals. Neutron tools are also affected.

Mud salinity affects electrical and induction tools

in different manners.

Additives such as barite affect density, gamma

ray and photoelectric effect measurements.

The mud type, salinity and additives must beknown so that the appropriate corrections can be

made.


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Borehole - Temperature

Increasing temperature affects the measurements

in some tools. The most affected is the thermal

neutron devices.

High temperature also affect the performance of

the electronics in the tools.

Temperature affects the mud resistivity (it

decreases with increasing temperature).

Temperature is measured during each logging

run.


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Volume of Investigation

The tool shown here measures all around the

borehole. It is omni-directional.

An example of this type of tool is the Gamma

Ray.

Some of the signal is in the borehole. Most

comes from the invaded zone.


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Volume of Investigation 3


Volume investigatedby the tool

Invaded Zone

VirginZone Virgin

Zone

This type of measurement has the sensor facing

in one direction only.

Examples of this are the neutron porosity and

bulk density measurements.


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Vertical Wells

In vertical wells, with homogeneous layers all

types of tool are reading in the same formation.

In horizontal (or highly deviated) wells the deep

reading resistivity tools may read a different layer

to the shallow reading tools.

In addition the omni-directional tools (e.g. GR)

may read different layers from the single

direction devices.


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Drilling Objective

A well is drilled to a pre-determined objective:

An explorationwell targets a suspected reservoir.

An appraisalwell evaluates a discovery.

A developmentwell is used for production.


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Pre-Drilling Knowledge

Exploration

Structural information obtained from surface

seismic data.

Rough geological information can be provided by

nearby wells or outcrops.Approximate depths estimated from surface

seismic data.


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Tool History

1927 - First electrical log recorded.

1930s - SP, Short Normal, Long Normal and Long Lateral

combined, Core Sample Taker.

1940s - Gamma Ray and Neutron, 3-arm Dipmeter using SP, then

electrical measurements, Induction tool.

1950s - Microlog tool, Laterolog tool, Sonic tool, Formation

Tester.

1960s - Formation Density tool.

1970s - Dual Spacing Neutrons, Advanced Dipmeters,

Computerised Surface Systems, Repeat Tester tools,

Electromagnetic Propagation tool.

1980s - Resistivity Imaging tool, Advanced Sonic tools

1990s - Advanced testing tools, Induction imaging tools,

Azimuthal Laterolog tools, Ultrasonic imaging tools,

Epithermal porosity tools, Magnetic resonance tools


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Early Interpretation

Early resistivity logs were used to find

possible producing zones.

high resistivity = hydrocarbon

SP was used to define permeable beds,

compute Rw and determine shaliness.

Resistivity was also used to determine

"porosity".

Archie developed the relationship between

resistivity, porosity and saturation.


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Basic Log ResponsePorosityResistivity

SP


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Interpretation Procedure 2

Resistivity PorosityGamma Ray

Water

Water

Shale

Hydrocarbon

The simplest evaluation technique consists of

recognising the hydrocarbon zone using theporosity and resistivity curves


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Interpretation Procedure


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Zoning

Zoning is the first step in any interpretation

procedure. During zoning, the logs are split into

intervals of:

1) Porous and non-porous rock.

2) Permeable and non-permeable rock.

3) Shaly and clean rock.

4) Good hole conditions and bad hole

conditions.

5) Good logs and bad logs.

Zoning Tools:SP.

GR.

Caliper.

Neutron Density-Pef.Resistivity.

1-Basic Log Interpretation

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