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Well Logging Salam Al Rbeawi

2011

Well Logging: Is a technique used for formation evaluation to determine the size of the reservoir and the amount of oil and gas in place. The following parameters can be estimated from different types of logging tools; 1- Borehole hole diameter. 2- reservoir thickness. 3- Porosity. 4- Water saturation. 5- Rock type (Lithology).

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Logging tools classification:

Based on the function, logging tools can be classified as follow;

1- Formation Fluid Indicators:

- Induction

- Laterolog

- Microfocused and microresistivity devices

2-Formation property-lithology Indicators:

- Acoustic

- Density and lithologic density

- Neutron

- Gamma ray

Logging tools classification:

3-Layer geometry Indicators: - Dipmeter - Borehole gravimeter

4-Auxiliary tools: - Spontaneous potential - Caliper

5-Specialty Tools: - Nuclear Magnetic Resonance - Dipole - Geochemical Tools

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Caliper Log:

Borehole geometry is controlled by:

- Lithology

- Mud type

- Formation Properties

- In-situ stresses Borehole size can be determined from caliper log. Caliper log can be an indication to one of the following cases: 1- Gauged hole – diameter of hole is about equal to the bit size Hard well consolidated and impermeable formation. borehole diameter = drill bit size

b- Keyseat – asymmetric oval holes, formed by wear against the drill string at points where the borehole inclination changes (doglegs) . c-Breakout – similar to keyseat but not due to doglegs, small brittle fractures due to existing stress regime of the country rock.

a- Washout – general drilling wear, esp. in shaly zones and dipping beds, both caliper larger than bit size, considerable vertical extent .

Unconsolidated formation borehole diameter > drill bit size

2- Increased borehole diameter which means;

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Permeable formation borehole diameter < drill bit size

a- Generally due to formation of mud cake Mud cake thickness = (bit size diameter – caliper diameter reading)/2 b- mud cake formation indicates permeability and involves loss of mud filtrate into a permeable formation – invasion.

3-Decreased borehole diameter means;

Resistivity Log:

Resistance is the opposition offered by a substance to the passage of electric current. Resistivity is the resistance measured between opposite faces of a unit cube of the substance at specified temperature. Resistivity is measured in ohm-meter2/meter, more commonly shortened to just ohm-meter. Resistivity logs do not always measure resistivity directly. Some resistivity logs (actually induction logs) measures conductivity instead which is the reciprocal of resistivity. Induction logs are used in wells drilled with a relatively fresh-water mud (low salinity) to obtain more accurate value of true resistivity.

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csr2006 10

The resistivity of a rock (R) is given by: R = r (A / L) Where; r = resistance (ohms) A = Cross sectional area (meters2) L = Length (meters) R = resistivity (ohm-meters) E = Voltage (Volt) I = current (Amp)

L

A r

E

I

r = E / I

Factors that influence Resistivity of Natural Porous Media: 1- Salinity of water 2- Porosity 3- Stress 4- Temperature 5- Pore geometry 6- Rock Composition 7- Wettability

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Formation Water Resistivity (Rw):

Formation water resistivity can vary widely from well to well. It can be estimated by the following methods; - Chemical analysis of produced water - Direct measurement in resistivity cell - Using Empirical equations The best method is direct measurement of resistivity. Chemical analysis: Resistivity of water is controlled by amount and type of ions present and temperature. Salinity is a measure of concentration of dissolved salts in water and is generally expressed as parts per million, grains/gallon or grams/liter. 1 grain/gallon = 17.118 ppm = .017118 grams/liter NaCl is the most common dissolved salt in formation water; the concentration of other dissolved ions is generally converted to equivalent concentration of sodium chloride;

n

i i

i 1

C MC

Where; C = equivalent concentration of NaCl. Mi=weight multiplier (can be estimated from graph) Ci= concentration of each ion.

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Based on equivalent concentration of NaCl and temperature, formation water resistivity can be determined using graph. The following equation also can be used to calculate (Rw).

w4

1R 0.0123

2.74x10 C

1w 2 w 1

2

T 6.77R (t ) R (t )

T 6.77

Arp’s equation:

T1 usually taken as 75 °F

Formation Resistivity (Ro):

The resistivity of the formation saturated 100% with formation water.

Archie equation:

wo

w

oR RR

R

RF 1

Where; = Formation factor RF

Formation porosity or the void space in the formation can be determined from formation factor using the following equation: mRF

1

Where; = cementation exponent≈2 =porosity

m

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HW Q.1 The chemical analysis of formation water as follow; Ion concentration (ppm) Na 14,000 Cl 12,000 Mg 10,000 Ca 8,000 SO4 11,000 Room temperature=75°F Calculate formation water resistivity at 75, 125 and 150 °F.

Q.2 Formation water contains 10,000 ppm of NaCL, 15,000 ppm of MgSO4 and 8,000 ppm of CaCl2 . Calculate the resistivity at formation temperature 200 °F.

HW Q.3 Calculate formation water resistivity at 150 °F if the concentration of NaCl 50,000, 100,00 1nd 150,000 using graph and equations.

Q.4 If the formation resistivity in the above cases (Q.3) is 2.4 Ω-m at 225 °F and the cementation factor is 2. Calculate the porosity for each case.

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True Resistivity (Rt):

The resistivity of the formation at any saturation of water less than 100% when the hydrocarbon displaces some water from pore space in the formation. The relationship between formation resistivity (Ro) and true formation resistivity (Rt) can be represented by resistivity index:

o

tR

R

RI

Where; = resistivity index

RI

Water saturation (Sw) which is defined as the percentage of the pore volume filled with water can be determined from the following equation:

n

t

ow

R

RS

1

Where; = saturation exponent≈2 n

HW Q.5 Calculate porosity and water saturation if the formation factor is 15, true formation resistivity 10 Ω-m and the concentration of the formation water at 75°F is 60,000 ppm. Use m=n=2 and formation temperature 200 °F.

Q.6 The resistance cylindrical core having 3 in diameter and 10 in height saturated 100 % with formation water is 10 Ω. The resistance of the core is increased to 85 Ω when oil is injected to it. Calculate water saturation of the core after the injection of oil.

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SP Log:

The spontaneous potential (SP) log is a measurement of the natural potential difference or self potential between an electrode in the borehole and a reference electrode at the surface . It represents a recording of naturally occurring physical phenomenon in in-situ rocks. The SP curve records the electrical potential (voltage) produced by the interaction of formation water, drilling mud and shale. Though relatively simple in concept, the SP curve is quite useful for a number of things: 1- Differentiates potentially porous and permeable reservoir rocks from nonpermeable shales 2- Defines bed boundaries and correlation of beds 3- Aids in lithology identification 4- Detection of hydrocarbon from suppression of SP response 5- Permits determination of formation water resistivity, Rw 6- Gives semi-quantitative indication of bed shaliness

SP Log:

Three factors are necessary to produce an SP current: 1. a conductive fluid in the borehole, 2. a porous and permeable bed surrounded by an impermeable formation, and 3. a difference in salinity (or pressure) between the borehole fluid and the formation fluid.

Resistivity of drilling mud filtrate (Rmf): The resistivity of drilling mud filtrate which is normally observed in the permeable layers.

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21

Rmf > Rw

Rmf > Rw

Rmf<Rw

(Normal)

The SP deflection is a reflection of contrast between the mud filtrate and connate water resistivity. The deflection is said to be normal or -ve when the mud filtrate is more resistive than the connate water and is reverse or +ve when the mud filtrate is less resistive that the connate water. It is quite common to find fresh water in shallow sands and increasingly saline water as depth increases. Such a progression is shown in the figure, where SP appears deflecting to left deep in the well but is reversed near to the surface.

(Reverse)

Shale baseline

Shale Baseline and SSP: SP has no absolute values and thus treated quantitatively and qualitatively in terms of deflection, which is the amount the curve moves to the left or to the right of a defined zero. The definition of the SP zero, called shale baseline, is made on thick shale intervals where the SP curve does not move. All values are related to the shale baseline. The theoretical maximum deflection of the SP opposite permeable beds is called the static SP or SSP. It represents the SP value that would be measured in an ideal case with the permeable bed isolated electrically. It is the maximum possible SP opposite a permeable, water-bearing formation with no shale. The SSP is used to calculate formation-water resistivity (Rw).

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SP = -K log(Rmfe/Rwe)

SP= SP value: this should be the SSP (Rmf)e = equivalent mud filtrate resistivity: closely related to Rmf (Rw)e = equivalent formation water resistivity: closely related to Rw K = temperature-dependent coefficient K = 61+ 0.133 * T T=formation temperature, °F

SP value measured is influenced by: - Bed thickness - Bed resistivity (Rmf, Rw, ) - Borehole and invasion - Shale content - Ratio of Rmf/Rw (amplitude and sign) - Temperature

24 Rw or Rmf, ohm-m

Rw

e o

r R

mfe

, oh

m-m

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PSP

SSP

csand ssandsh

csand

(SP) (SP)V

(SP)

= 1- PSP/SSP

PSP (Pseudo-static SP) – the SP value in the water–bearing shaly sand zone read from the SP log. SSP (Static SP)– the maximum SP value in a clean sand zone.

Q.7 Calculate water formation resistivity and shale volume if SSP=40 mv and PSP=15 mv. Reservoir temperature is 250 °F and Rmf=0.5 Ω-m.

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Gamma ray Log:

Shale is usually more radioactive than sand or carbonate, gamma ray log can be used to calculate volume of shale in porous reservoirs. The volume of shale expressed as a decimal fraction or percentage is called Vshales. Calculation of the gamma ray index is the first step needed to determine the volume of shale from gamma ray log. The gamma ray log has several nonlinear empirical responses as well a linear responses. The non linear responses are based on geographic area or formation age. All non linear relationships are more optimistic that is they produce a shale volume value lower than that from the linear equation. Linear response :

minmax

min

GRGR

GRGRI

Log

GR

Where; indexrayGammaIGR

shaleforraygammaGR

sandcleanforraygamaGR

fromrecordraygammaGRLog

max

min

log

For very hard compacted formation at depth of 8,000 ft or more, gamma ray index is considered equal to shale volume:

GRsh IV

For tertiary sediment rocks at depth of less than 4,000 ft, the shale volume is;

)12(083.07.3

GRI

shV

For older rocks at depth of 4,000-8,000 ft, the shale volume is;

)12(33.02

GRI

shV

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Sonic Log:

maf

ma

tt

tt

Where;

)/189(

)(

)/(log

ftsusuallyporesfillingfluidtheforetransittimt

matrixtyperockthefortimetransit

ftsreadingt

f

ma

1)Determine porosity of reservoir rock 2)Improve correlation and interpretation of seismic records 3)Identify zones with abnormally high pressures 4)Assist in identifying lithology 5)Estimate secondary pore space 6)Indicate mechanical integrity of reservoir rocks and formations that surround them (in conjunction with density data) 7)Estimate rock permeability

Wyllie’s time average equation can be used to determine porosity:

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HW Q.8 From the gamma ray log, the record is 200 API, gamma ray for shale zone is 120 API and gamma ray for clean sand is 40 API. Calculate the gamma ray index and shale volume if the rocks at depth 3,500 ft and 7,500 ft.

Q.9 Sonic log reading Calculate porosity.

ftstftstftst fma /190,/80,/100

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Density Log: The density log belongs to the group of active nuclear tools, which contains a radioactive source and two detectors. the Gamma Ray tool, which is a passive nuclear tool, contains no source and can only measure the natural radiation in the formation. The radioactive source is applied to the wellbore wall in a shielded sidewall skid and emits medium gamma rays into the formation. The gamma ray waves may be thought of as energy particles. As these energy particles (photons) collide with the electrons in the formation, the gamma ray loses some of its energy to the electron. This is called Compton scattering. The denser the formation, the more electrons are presented, and more energy is lost due to collisions. If the matrix density is known, then the energy loss is directly related to porosity.

1): Material Density(gm/cc) Fluid Density (gm/cc) Quartz 2.65 Fresh Water 1.00 Calcite 2.71 Salt Water 1.15 Dolomite 2.87 Oil 0.85 Anhydrite 2.96

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Neutron Log:

Nucleus of all elements except hydrogen have neutrons. Neutrons have same mass as protons but no charge. Their small size and electrical neutrality make neutrons ideal projectiles for penetrating matter. Two categories of neutron sources are found in the logging industry: chemical and pulsed sources. Chemical sources are composed of two elements in intimate contact that continuously emit neutrons, usually plutonium/beryllium or americium/beryllium. Such sources need to be heavily shielded when not in use. Pulsed sources incorporate a neutron accelerator and a target, and can be activated by simply switching on the accelerator. This source is used for pulsed neutron logging and in tools that measure inelastic neutron collisions .

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The following equation can be used to determine porosity from density log:

fma

bmaD

Where;

poresfillingfluidtheofdensity

matrixrocktheofdensity

fromdensitybulk

f

ma

b

)(

log

2

ND

Q.10

The bulk density reading from density log is (2.2 gm/cc). The

density of matrix is (2.45 gm/cc) and fluid density is (1.035

gm/cc). The density reading from neutron log is (15%).

Calculate formation density.