10.-log Interpretation Methods_lw.pdf

Log Evaluation and QA/QC Seminar Log Interpretation Methods

Lukas Wihardjo Apr-2014

QUALITATIVE INTERPRETATION - QUICKLOOK

Qualitative Interpretation - Quicklook • Three quick look (qualitative) methods exist for rapidly locating pay (oil or gas) from logs without performing calculations (next section). • The three quick look methods are: – Side by side technique – Overlay technique – Neutron-Density Crossover

• All are effective in experienced hands and are widely used. It is recommended to use them to have a general idea of the presence of hydrocarbons in the well

Side by Side Technique (1) • Logs are scaled so both the resistivity and porosity curves move in the same direction (left or right) in water filled zones. • Resistivity increases to the right. • Porosity decreases to the right (decreasing porosity causes increasing resistivity). • As the porosity varies, both the porosity and resistivity curves will move in the same direction (right or left) as long as the rock is water filled. • If the rock contains hydrocarbons, the resistivity and porosity curves will go in opposite directions. • These last two statements are the basis for qualitative hydrocarbon detection. • Shale reduces the amount of separation, but the effect still holds true.

Side by Side Technique (2) • Lay the resistivity and porosity logs side by side with depths aligned. • Only three things cause the resistivity to go to high values:

– Low porosity – Hydrocarbons – Fresh Water (discarded by local area knowledge)

• Look for any place where the resistivity increases. Check to see if the porosity decreases there. If the porosity decreases, the zone is most likely water bearing (resistivity increase due to decreasing porosity). If the porosity increases (or remains the same), this is a potential hydrocarbon bearing zone. • The zone one wants to find has high resistivity AND high porosity (hydrocarbon bearing). • This side by side technique is a good, first, fast-look method.

Exercise - Side by Side Technique Find the pay zone, what kind of hydrocarbon is there? Is there a water zone, where is the OWC?

Other Techniques • Overlay Technique –

The overlay technique consists of laying the resistivity log on top the porosity log on a light table, with depths aligned. Slide the resistivity log left or right (sideward) to align the deep resistivity curve on top the porosity curves in a clean high porosity zone.

–

Maintain this same relative position and examine the overlaid logs over the entire log.

–

The logs (deep resistivity and porosity) should track each other fairly well, except in hydrocarbon bearing zones. In hydrocarbon bearing zones the resistivity will lie significantly to the right of the porosity curves. Look for this separation of resistivity curve to the right of the porosity curves, making sure the two curves remain on top each other in water sands.

• Neutron-Density Crossover –

It consists of looking at only the neutron-density curves for crossover and mirror imaging. Such crossover with mirror imaging means gas is present. Nothing else causes such a response. Be sure the mirror imaging is present, as washouts or lithology can cause mere crossover .

Low Resistivity Pay • A certain type of pay sand exists fairly commonly in certain type environments that is not as obvious on the resistivity logs as conventional pay sands. • This type sand is called a Low Resistivity Pay sand. It is marked by a much lower resistivity than would be expected for a pay sand. • Basically, shales or very fine pores reduce the resistivity to much lower than normal values for hydrocarbon bearing sands. • Low resistivity Pay sands can be quite prolific producers. • The key to spotting them is a very careful study for even a small resistivity increase over what should be there for a water sand. The use of image logs and cores are also a good help to identify these sanda. Detecting these sands requires experience, and preferable specific experience in the area of question

SATURATION

Saturation • The saturation of a formation represents the amount of a given fluid present in the pore space.

S w = S w irr water

+

S o = S oresidual

oil

Matrix

• The porosity logs react to the pore space. • The resistivity logs react to the fluids in the pore space. • The combination of the two measurements gives the saturation

S w "free" +

S o"free"

Saturation Basics (1) • Rw = resistivity of water in the pore space. • Define Ro = resistivity of a rock totally filled with water.

R0 F Rw – F: Formation Factor.

– At constant porosity F is constant. – As porosity increases, Ro decreases and F decreases.

• Experiments have shown that F is inversely proportional to m.

a F m 

– m: is called the "cementation exponent". – a: is called the "lithology" constant.

Saturation Basics (2) Saturation can be expressed as a ratio of the resistivities:

Snw

R0  Rt

where n is the "saturation exponent", an empirical constant. Substituting for Ro:

Snw

FR w  Rt

Substituting for F:

S

n

w



a



m

Rw Rt

Archie’s Saturation Equation

S

n

w



a



m

Rw Rt

• The Archie equation is hence very simple. It links porosity and resistivity with the amount of water present, Sw.

• Increasing porosity,, will reduce the saturation for the same Rt.

• Increasing Rt for the same porosity will have the same effect.

Invaded Zone • The same method can be applied to the invaded zone. The porosity is identical, the lithology is assumed to be the same, hence the constants a, n, m are the same.

• The changes are the resistivities which are now Rxo and Rmf measured by the MSFL tool.

• The equation is then:

n Sxo

aR mf  m  R xo

Other Relationships • Dividing for Sxo and Sw, with n set to 2 1 2

Sw  R xo R t    Sxo R mf R w  • Observations suggest:

1 5

Sxo  Sw

• Hence:

5 8

 R xo R t Sw    R mf R w 

• providing a quick look saturation answer when porosity is not available.

Archie parameters • Rw

= resistivity of connate water.

• m

= "cementation factor", set to 2 in the simple case.

• n

= "saturation exponent", set to 2 in the simple case.

• a

= constant, set to 1 in the simple case.

• Two common sets of numbers for these constants are:

–

In a simple carbonate, the parameters are simplified to: • m = 2, • n = 2,

• a=1 –

In a sandstone the following values are often quoted: • m = 2.15, • n = 2, • a = 0.62

S

n

w



a



m

Rw Rt

Rw determination • Rw is an important parameter.

– Sources include: • Client. • Local tables / knowledge. • SP. • Resistivity plus porosity in water zone. • RFT sample. • From Rxo and Rt tools.

Rw from Rwa • If Sw = 1, the saturation equation can become:

Rw   Rt 2

• Assuming simple values for a, m, n.

• Procedure is to: – Compute an Rwa (Rw apparent) using this relationship. – Read the lowest value over a porous zone which contains water – This is the method employed by most computer based interpretation systems.

Rw from resistivity 1 2

Sw  R xo R t    Sxo R mf R w 

• In a water zone Sw = 1, thus the alternative saturation equation becomes:

• The value of Rmf is measured; Rxo and Rt are measured, the value of Rw can be calculated.

Other Archie Parameters • The constants a, m, n are an integral part of Archie's saturation equation. They can, and do, vary. They are usually taken from local knowledge if at all possible.

– n is dependent on the wettability of the rocks; in the common water wet case it is usually close to 2.

– a and m are dependent on the lithology and pore systems of the rock.

F Relation chart

Computing Saturation The standard saturation equation can be used with special attention taken to obtain the correct value for the cement exponent ‘m’: 

In vuggy formations this will be greater than 2. The resistivity logs see read higher as the “pathway” is more tortuous. Saturations calculated with an ‘m’ of 2 will show too much hydrocarbon 

In fractured formations ‘m’ will be less than one as the resistivity pathways are straight. In this case saturations computed with ‘m’ = 2 will show too much water. 

Variation of m m reflects the tortuosity of the formation, the pathway for electrical current flow Carbonates have complex porosities and hence current pathways an values of m 

Variable m Hence in a carbonate the major problem is the determination of ‘m’ 



A good method of determining m is as follows:



In a water zone, rearranging Archies formula



Log Rt = - m log  + log (a Rw)

Plotting on a log-log scale, slope will give ‘m’, and the intercept ‘a’ . The assumption is that m is constant through the entire reservoir. 

M relationship to secondary porosity

This chart gives the value of the fracture or vug porosity as a function of the total porosity and the cementation factor, m.

DUAL WATER MODEL

Shale and Saturation • The Archie equation has to be changed to take account of the shale effect.

• The shale looks like low resistivity so another term is added to the equations.

• The result is an equation which will can be used to compute water saturation in shaly sands.

• All these equations return to Archies equation if there is no shale present.

Saturation Equations (1) • Indonesia Equation

Sw 

1   Vc l   1     2   Vcl

Rc l





* e

1 Rt

Rw

• Nigeria Equation

 V 1.4 1   cl  Rt  Rcl

e

2

 n  Sw aRw  m

2

• Waxman-Smits Equation 2 1 Sw B Qv Sw   * * Rt F Rw F

• Dual Water Equation

Ct

 

n Swt   Swb Cw  Cwb  Cw   a Swt  

m t

Saturation Equations (2) • One of the difficulties is the number of equations available for shaly sands.

• They are often “country” oriented, Nigeria, Venuzeula..

• The choice of equation is dictated by local practice.

• Waxman-Smits (WS) and Dual Water (DW) approach the problem from experiments on the clay properties and are thus more realistic and universal.

Dual water • The Dual Water Model takes the basic work of Waxman Smits and expands it for use with logged information

• It divides the formation into solids and fluids.

• It splits the clay into dry clay and its associated water, called bound water

• The standard definitions for porosity and saturation to describe the fractions of fluids in the formation are expanded to include the new model.

Dual water model definitions hy drocarbon

total porosity

t f luids unit v olume

f ar water

hy ef f ectiv e porosity

wf

bound water

wb

dry clay

Vdcl

solids

clean matrix

e

=

wf +  hy

Vcl wet clay

Clean to Shale t M atrix

Far Wate r

t M atrix

t M atrix

Dry Colloid

t Dry Colloid

Bound wate r

Dual Water definitions The total porosity is given by

 t   e   wb   t 1  Swb   t Swb

the porosities are combined to give the saturations of the fluids present

 wb t  wf  t

Swb 

saturation of bound water

Swf

saturation of free water (this is Sw)

Sh y

 hy  t

Hydrocarbon saturation

Swt  Swf  Swb

Total water saturation is the sum of the saturations of the two waters

Swt  Shy  1

total water saturation plus hydrocarbon saturation must be one

V cl  V dcl 

 t Swb

wet clay volume includes the volume of bound water

Simplified DWM (1) Archie Equation can be generalized into the following form; 2 S wt 

Rw t2 Rt

where; Swt

- total water saturation

t

- total porosity

Rt

- true formation resistivity

Rw

- resistivity of the water(s)

The equation can be solved if Rf is known.

Simplified DWM (2) • 1) Clean water bearing zone – Swt = 1 

t2

*

Rt = Rw

– This is Rw, the resistivity of Free water

• 2) Clean 100% shale zone – Swt = 1 

t2 * Rt = Rw

– This is Rwb, the resistivity of Bound water

• These are the two end points. To give a universal solution they are combined linearly using the volume of shale.

Practical DWM The standard equation for the water saturation is expressed in terms of the conductivity, as it is linear. n   m S S C t  t w t Cw f  w b Cw b  C w f  a  Sw t 

This equation is in terms of measured quantities, porosity and resistivity and parameters that can be found, the far and bound water conductivities.

DWM Saturation solution The solution to the equation is

Swt  x 

x

2

Ct F0  Cw

where

x 

Swb Cw  Cwb

and

Fo 

2Cw

a



m



Practical outputs The equations give total water saturation Swt and total porosity t. These have to be transformed into effective saturation, Sw and effective porosity, wf (or e)

Sw

Swt  Swb  1  Swb

 wf   t Swt  Swb 

Dual water equation solution This derivation of the Dual Water equations is valid for any rock with any mixture of fluids 

It is possible to use the Dual Water Model to make a manual computation of a shaly zone. 

However computer programs are best equipped to handle the calculations. 

The selection of key parameters is essential to obtain the correct answers,  Cwf - free water conductivity  Cwb - bound water conductivity  Swb - bound water saturation 



t - total porosity

SHALES

Shale Deposition Types Matrix

Shale

Porosity

.

Clean formation

Structural shale

Porosity

Matrix

Matrix

Laminar shale

Dispersed shale Porosity

Shale

Porosity

Matrix

Shale

Porosity

Matrix

Shale

Clay Minerals b

N (thermal)

Pe

• Kaolinite

2.54

59.6

1.85

• Illite

2.52

47.9

3.97

• Smectite

2.02

87

1.70

• Chlorite

2.73

59.6

4.07

• Most shales are comprised of these clay minerals. • Clay minerals frequently occur together in "mixed layers", e.g. Illite Montmorillonite. • Kaolinite

Al, Si, little K

• Illite

K, Fe, Mg, Si

• Smectite

Very high porosity.

• Chlorite

Fe, Mg, no K

Shale and Logs (1) • Shales have properties that have important influences on log readings: – They have porosity. – The porosity is filled with salted water.

– They are often radioactive. – Resistivity logs exhibit shales as low resistivity zones.

Shale and Logs (2) • Neutron porosity logs exhibit shales as high porosity.

• Density and sonic logs react to the porosity and matrix changes. • Gamma ray logs react to shale radioactivity.

Shale Corrections • The electrical properties of shales greatly influence the calculation of fluid saturations. • A layer of water close to the clay surface is electrically charged.

• Archie's equation assumes that the formation water is the only electrically-conductive material in the formation. • The clay layer requires an additional term in the saturation equation. • Porosity tools can be corrected for the shale effect. An "effective porosity" can be computed as compared to a "total porosity" which includes the shale effect.

Shale Volume (1) • The volume of shale must be computed to correct the tool readings.

• This is achieved using simple equations such as:

Vcl 

GRlog  GRmin GRmax  GRmin

• or

Vcl 

SPlog  SPmin SPmax  SPmin

Shale Volume (2) • However, as every tool reacts to shale, each tool is a shale indicator. For example:

b   wSw   h 1  Sw    ma 1    Vcl   clVcl • Shale volume can be computed from different sources and from crossplots of different kinds of log data.

• The ideal method of computing shale volume is to use the Neutron Density plot.

LITHOLOGY AND POROSITY

Lithology and Porosity • The next major step in the procedure is lithology identification. Lithology data gives information on porosity and other parameters. • Lithology of a formation can be:

• Simple

• Dirty

• Complex

Lithology Determination • The lithology can be obtained in several ways: – From the cuttings (depth problems). – From local knowledge (good during development). – From the known depositional environment (good in general basis). – From a log Quicklook (good starting point). – From individual log readings (difficult if there are no areas of zero porosity). – From crossplots (the best method).

Lithology and Porosity Tools • All tools react to lithology - usually in conjunction with the porosity. • Major lithology tools are: – Neutron - reacts to fluid and matrix. – Density - reacts to matrix and fluid. – Sonic - reacts to a mixture of matrix and fluid, complicated by seeing only primary porosity. – SGT - identifies shale types and special minerals. – NMR - magnetic resonance reacts to the porosity with a small element if lithology.

Crossplots

• Combines properties from both measurements, thus eliminating ambiguities. The most common crossplot is the Density Neutron.

Volume • Formation model: • Water-bearing, mono-mineral.

• This formation can be described by the density tool and the neutron tool.

 b   mf    ma 1     n   mf    ma 1    • 2 equations for 1 unknown: • system is over-determined.

–

for limestone:

Nma = 0

–

for sand:

Nma = 0.04

Crossplot Solution • The plot is a straight line from the matrix point to the 100% porosity, water point. It is scaled in porosity.

Neutron-density X-plot (1) • This crossplot has b plotted against the corrected neutron porosity. Fluid density in this plot is 1.0g/cm3.

Neutron-density X-plot (2) • This plot is the same as the previous one except that the fluid density here is 1.19 g/cm3.

Dual Mineral model

B Ø

N

= Ø

mf

+ Vm1

m1

+ V m2

m2

= Ø Ø N mf + V Ø + V Ø m1 N m1 m2 N m2

1 = Ø + Vm1 + V m2 (Material Balance Equation) • 3 unknown : Ø, Vm1 , V m2 , 3 equations system is just determined

Dual Mineral plot

The plot now has two lines, one from each matrix point. The equi-porosity lines join the lines, any point falling between can be assigned its porosity the zero porosity line is scaled in ratio (or percent) of the two minerals. This can be extended to the water point. Points falling inside the lines can be subdivided in mineral percent

Dual mineral plot expanded

Crossplot example

This is a typical frequency crossplot. The lines are the limestone, sandstone and dolomite lithology lines

Z-axis Plot

Other Crossplots •There are numerous other crossplots to identify minerals using combinations of tools.

•

ma - Uma

•

b - Pe

•

MID plot (n, b, t)

•

MN plot (n, b, t)

•The z -axis is used for clarification.

Pe - b Crossplot • This plot is ideal to identify the lithology in conjunction with the neutron density plot.

ma - Uma (1)

ma - Uma (2) Uma determination

Matrix Identification Plot

• The Matrix Identification Plot uses neutron, density and sonic data as inputs. An apparent crossplot porosity is found on a density-neutron and a sonic neutron crossplot. The values are entered into the relevant section of the following chart and the values of tmaa and maa read;

MN plot

• The MN plot uses data from the neutron, density and sonic logs to solve complex lithology. Used when Pef is not available or as extra information.

Hydrocarbon Effect • The presence of light hydrocarbons especially gas, in the invaded zone seriously affects the main porosity tools, the density and neutron. • Both tools are calibrated to read correctly in water-filled rock.

• Light hydrocarbon has a lower hydrogen index, hence the neutron reads low and the low density of the fluid makes the density low. • Points exhibiting this problem plot above and to the right of the lithology line on the crossplot.

Hydrocarbon Effect Correction

Complete Well Evaluation Perform a complete well evaluation, determining VSH, f, RT, RW, SW, lithology type, fluid contents on the attached log, assume all environmental corrections have already been made