Rock Physics and Fluid Substitution Modeling

4D feasiblity study

Rock physics modeling The degree of changes in seismic response to production within the reservoir are highly dependant on the physical properties of the reservoir rocks. Accurate modeling of these properties from existing data will allow consideration of their effect on the 4D response. Modeled acoustic/elastic properties include Vp, Vs, bulk density, bulk Poisson's ratio, and reflectivity, both at normal incidence and non-vertical incidence. The rock physics model will allow variation in fluid saturation, fluid properties, and reservoir pressure, allowing fluid substitution modeling to represent realistic reservoir changes. Because many of the input parameters to rock physics modeling are often not well known, this task should include a sensitivity analysis, thus giving guidance toward what additional petrophysical data acquisition may be desirable prior to 4D data analysis.

Fluid substitution modeling Fluid substitution models can be used to assess the effect of different reservoir scenarios on the 4D response. Scenarios modeled will determine expectations of reservoir production changes. Examples include, but are not limited to:

Saturation change representing water influx from injection well or a natural aquifer Pressure change in areas where reservoir pressure is not maintained by injection and/or an aquifer Fluid property changes representing the release of solution gas as reservoir pressure falls below the bubble point A combination of saturation, pressure, and fluid property changes as predicted by simulation of reservoir production Changes in the overburden or reservoir physical rock properties brought about by production induced geomechanical changes.

Related Resources

The next generation of rock physics models (Hart's E&P September 2010)

Request More Information  about Rock Physics and Fluid Substitution Modeling.

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Introduction to Cation Exchange Capacity (CEC) - 3D Models and Shockwave Movies

IntroductionIn the previous tutorials we discussed primary and secondary minerals that are commonly found in soil environments: Primary Mineralogy and Clay Mineralogy. Many clay minerals found in soils have the ability to develop a net negative charge which is satisfied through the electrostatic adsorption of cations. This interaction is similar to how a magnet attracts iron filings. Although the filings are held by the magnet one can easily remove them. Similarly the cations adsorbe to colloid surfaces are exchangeable or available for plants and microorganisms. We can define Cation Exchange Capacity or CEC as "the sum total of the exchangeable cations that a soil can adsorb". Cation exchange capacity is an extremely important property of soils from both an agricultural and environmental standpoint. For example, without CEC essential nutrient cations such as potassium and calcium would have to be continually supplied as inorganic fertilizers throughout a growing season. In this tutorial we will discuss negative charge development on clay minerals, the distribution of cations around negatively charged clay surfaces, and cation exchange reactions that supply nutrient cations to plants and microorganisms.

Computer Requirements

The 3D models and CEC demos listed on this page require the use of Quick Time or Shockwave Plug-ins. If your computer does not have these plug-ins, they can be downloaded by clicking Quicktime or Shockwave . Follow the directions for downloading. Once the download is complete you may have to reboot your computer. If you are having difficulties e-mail me or talk to someone at the computer help desk by dialing 231-4357.

Charge Development on Clay SurfacesThe clay fraction of the soil consists of secondary minerals that are extremely small in size. These particles are too small to be seen with an ordinary light microscope and must be viewed using electron microscopes. Most clay particles are smaller than 2 um (2 millionths of a meter). Because of their small size soil colloids have a very high surface area or surface per unit mass. Soil colloids small size, high surface area, and net negative charge make their surfaces extremely reactive. Below are electron micrographs of the clay minerals montmorillonite (left) and kaolinite (right). The white and black bars in the pictures represent a length of 2 uM.

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Surface charge on soil colloids is developed in two ways: isomorphic substitution (permanent charge) and deprotonation of surface functional groups (pH dependent charge). pH dependent charge occurs on the edges of layer silicates, on variable charge minerals such as oxides of Fe and Al, and organic matter. It is called pH dependent charge because it increases in magnitude as the pH of the aqueous soil environment increases. Most of the pH dependent charge associated with agricultural soils is due to the deprotonation of organic functional groups. As the pH of the soil environment increases weak acid functional groups such as the carboxylic acid donate a proton and generate negative charge:

COOH + OH- = COO- + H2O

One practical way to increase the CEC of agricultural soils is to increase the organic matter content through tillage practices and increase the pH by adding lime.

Isomorphic substitution is the replacement of one atom by another of similar size in a crystal lattice without disrupting or changing the crystal structure of the mineral. If you remember from the clay and primary mineral tutorials, cations are coordinated to oxygen or hydroxyl anions in mineral structures. The negative charge of the anions is balanced by the positive charge of the cations that are coordinated to it. Net negative charge is developed when a cation of similar size and less positive charge substitutes for one of higher positive charge. Isomorphic substitution can also take place between cations of the same charge or a cation of higher positive charge. In the case of isomorphic substitution between cations of the same charge no charge is developed. In the case of isomorphic substitution between a cation of higher positive charge with one of lower positive charge a net positive charge is developed. The important thing to remember is that isomorphic substitution only occurs between cations of similar ionic radii. In the tutorials below we will be strictly dealing with permanent charge developed through isomorphic susbtsitution.

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Use the table below to identify the atoms and molecules which make up the clay minerals:

Color Key for Clay Mineral Models

Silicon Atom

Aluminum Atom

Magnesium Atom

EXAMPLE: Isomorphic substitution of Mg2+ for Al3+ in the octahedral layer of a 2:1 clay mineral. (Click on the buttons and use the mouse to rotate the mineral models.)

In this case Mg has a +2 charge, Al has a +3 charge, O has a -2 charge. If the rectangles below represent the octahedral layer in a 2:1 clay mineral then the substitution of one Mg+2 for one Al+3 will give rise to one negative charge. Add the charges generated by each aton to get the overall charge.

Al2O2OH2 (no charge)

AlMgO2OH2 (minus 1 charge)

Now let's take a 2:1 dioctahedral soil clay mineral. In this mineral there is no isomorphic substitution -- Si in the tetrahedral layer and Al in the octahedral layer. Now let's take the same mineral and isomorphically substitute 6 Mg2+ atoms for 6 Al3+ atoms in each of the octahedral layers. Each one of these Mg2+ cations will give rise to one negative charge. In soil science we express CEC as cmolc per Kg (that's centimols of charge). Each negative charge generated by isomorphic substitution gives rise to 1 molc. So based on the number of brown octahedrons (18 molc/mineral) and the overall molecular weight of the mineral picture (calculated by adding up each Si, Al, Mg, and O and multiplying each one by their molecular weight = 8741.1 g/mineral) this mineral will have an overall negative charge or CEC of:

18 molc

mineral

X

mineral

8741 g

X

1000 g

kg

X

100 cmolc

molc

=

206 cmolc

kg

This concept will become clearer when we talk about CEC and view the CEC demos.

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Electrical Double LayerThe net negative charge generated by clay particles will be balanced or neutralized by adsorbing cations. Additionally some of this negative charge will be used to exclude or repel anions from the area adjacent to the negatively charged clay surface. This repulsion or exclusion is often called negative adsorption. Therefore there will be a greater concentration of cations adjacent to the clay surface and an area farther away where there is a greater concentration of anions. Eventually, the distribution of cations and anions will be the same as the bulk solution. The combination of the negatively charged clay surface and unequal distribution of cations and anions (compared to the bulk solution) adjacent to the clay particle is called the electrical double layer (EDL) or diffuse double layer (DDL). Remember that we are talking about a phenomenon that occurs on the microscopic level, therefore the actual thickness and volume of the electrical double layer is extremely small. When viewing the shockwave demo for EDL notice that the 18 molc developed in the clay due to isomorphic substitution is balanced through the electrostatic attraction of cations and repulsion of anions. The EDL encompasses the negatively charged clay particle and the aqueous environment that is influenced by its charge.

COLOR KEY FOR CATIONS AND ANIONS IN SHOCKWAVE DEMOS

H+ K+ Ca2+ Cl- Al3+

In all of the demos below we scaled ions similar to their hydrated radii in aqueous environments. The hydrated radii of the cations used to scale the ions are listed below:

IonHydrated radii

in nm*

Al3+ 0.90

Ca2+ 0.60

K+ 0.30

Cl- 0.30

H+ 0.90

*From Lindsay (1979)

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When you click on the demo make sure that you observe the cations and anions that are influenced by the negatively charged clay particle. Also, notice that in the bulk solution there are an equal amount of cations and anions -- this is the concept of electroneutrality which must occur in soil environments.

Cation Exchange CapacitySo far we have talked about charge development in soil colloids and the distribution of ions around these charged surfaces. Since cations adsorbed to mineral surfaces are held via weak electrostatic interactions they are available to plants and microorganisms through exchange reactions. For example, plants can secrete protons (H+) that can exchange or displace a nutrient cation such as potassium on a colloid surface. The colloids negative charge is satisfied by the proton and the K+ enters the aqueous environment which can be absorbed by the plant via mass flow. The are several important characteristics of cation exchange:

1. The exchange reaction is rapid

2. The exchange reaction is diffusion controlled

3. The exchange reaction is reversible

4. The exchange reaction is stoichiometric

5. Selectivity of one ion over another

The first three points are pretty straight forward and we will discuss the fifth pooint in the next section. The forth point is important and merits further explanation. The negative charge generated on a soil colloid via isomorphic substitution can be satisfied by mono-, di-, or trivalent cations. In the case of trivalent cations such as Al3+, 1 cmol can satisfy 3 cmolc on the colloid surface. Similarly 1 cmol of Ca2+ can satisfy 2 cmolc while 1 cmol of K+ can satisfy 1 cmolc on the colloid surface. Or in other words it will take 1/3 cmol of Al3+ or 1/2 of a cmol of Ca2+ or 1 cmol of K+ to satisfy 1 cmolc on a colloid surface. This is what is meant by stoichiometry of the exchange reaction (point number 4). The important thing to understand here is the difference between cmol and cmolc. The cmolc of charge is used to quantify CEC. It is a fundamental unit used to normalize cations of different valence that can adsorb to colloid surfaces. So 1 cmolc of Al3+ = 1cmolc of Ca2+ = 1 cmolc of K+. This is the fundamental unit and puts cations of different valence on an equal playing field allowing soil scientists to quantify CEC.

This can get a little confusing so let's demonstrate this point using some models. The colloid surface in the models is the same surface used to demonstrate isomorphic substitution -- all of the brown polyhedra are Mg2+ cations that have isomorphically substituted for Al3+ in the octahedral layers. Therefore, the mineral shown in the model has a charge due to isomorphic substitution of 18 molc. If we expressed this as cmolc/kg (similar to the previous calculation for the section on isomorphic substitution) this mineral would have a CEC of 206 cmolc/kg. This amount of isomorphic substitution is greater than what is found in naturally occurring montmorillonites but

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is in the range found in naturally occurring trioctahedral vermiculites. We used this high CEC to demonstrate the concept of cation exchange in our demos. If you scroll up you will find the color key for the ions used in these models. When you click on the CEC models you will notice cations in the interlayers of the clay mineral as well as cations in the bulk solution. If you remember from the EDL demo some of the negative charge on the surface is used to repel anions (negative adsorption). Also, cations in the bulk solution need to be neutralized by anions (concept of electroneutrality). Again for the purposes of our model all the negative charge of the colloid is satisfied by cations adsorbed in the interlayer. Also, only cations (not associated anions) are shown in the bulk solution that will be involved in exchange reactions.

One more concept and we will get to the CEC demo -- Base saturation. This is a fairly simple concept -- the percent of the exchange complex that is occupied by base cations. Base cations are calcium, potassium, magnesium, and sodium. They are called base cations because they can be associated with strong bases (i.e NaOH or KOH). Acid cations are H+ and Al3+. Al3+ is considered an acid cation because it can generate protons through hydrolysis reactions:

[Al(H2O)6]3+ + H2O = [Al(OH)(H2O)5]2+ + H3O+

When you view the CEC demos pay particular attention to the nature of the exchange complex. If you want to calculate base saturation add up the molc of base cations adsorbed in the interlayers by the molc of acid cations and multiply by 100 -- you should come up with about 22%. This is typical for highly weathered Southeastern soils that have not been managed (i.e. lime additions). We will discuss ion selectivity and the nature of the exchange complex after we view our first 2 models. The first model (monovalent exchange) demonstrates a proton displacing potassium from the exchange complex -- a +1 cation exchanging for a +1 cation. In the second model (monovalent-Divalent exchange) Ca2+ is exchanged by 2 +1 protons illustrating point number 4 above (stoichiometry).

Nature of the Exchange ComplexNow let's finish by talking about point #5 above. Some cations will have a preference over others for the negative surface charge of a soil colloid. In general this will be related to the charge of the cation and its hydrated radii. For example the following order of preference is observed for cations with a different valence (mixed series):

Al3+ > Ca2+ = Mg2+ > K+ = NH4+ > Na+

In the case of cations with the same charge or valence the order of preference follows the order of decreasing hydrated radii:

Cs+ > Rb+ > K+ > Na+ > Li+

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The above orders of preference can be summed up using Coulombs law which states "The force of attraction between opposite charges is directly proportional to the charges on the ions and inversly proportional to the square of the distance between the charges". In other words the greater the charge the greater the force of attraction and the greater the distance between the charges the lesser the force of attraction. In highly weathered soils such as the Southeastern USA and the tropics the high rainfall leaches monovalent and divalent cations and the exchange complex becomes dominated by Al3+ (low base saturation). In arid and semi-arid climates base cations are conserved and the exchange complex will be dominated by mono and di-valent cations (high base saturation). Therefore, in order for many highly weathered soils to be productive from an agricultural standpoint we must lime them to increase their base saturation. The first farmer in the USA to recognize the importance of lime was a Virginia Farmer named Edmond Ruffin who used oyster shells on his fields. Today there are a variety of liming sources available to farmers. When lime is applied to soils it dissolves releasing a cation (usually Ca2+) that displces Al3+ from the exchange complex. Although Al3+ is preferred the concentration of Ca2+ in the soil solution is so great that it can overwhelm or displace the Al3+ by mass action. Once the Al3+ enters the soil solution it can hydrolyze (see above reaction) releasing protons that are neutralized by the carbonate and bicarbonate ions generated from the lime.

The demo below demonstrates how Ca2+ can displace Al3+ from the exchange complex. Pay careful attention to the stoichiometry of the reaction.

That's it!! -- a relatively simple introduction to charge development and CEC on soil colloids. Remember that the actual reactions represented in the above demos are much more complex in natural environments. Understanding charge development and CEC of soils allows us to better manage our soils for crop production and understand the bioavailability and mobility of contaminants in the subsurface.

How are the cation exchange capacity (CEC) and percent base saturation calculated for the soil test report?

To determine the cation exchange capacity (CEC), calculate the milliequivalents of H, K, Mg, and Ca per 100g of soil (meq/100g soil) by using the following formulas:

H, meq/100g soil = 8 (8.00 - buffer pH)

K, meq/100g soil = lbs/acre extracted K ÷ 782

Mg, meq/100g soil = lbs/acre extracted Mg ÷ 240

Ca, meq/100g soil = lbs/acre extracted Ca ÷ 400

Na, meq/100g soil = lbs/acre extracted Na ÷ 460

The total CEC will be the sum of the calculations from the 5 previous equations.

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EXAMPLE

LAB NO.

SAMPLENO.

SOILCODE

SOILPH

BUF.PH

P K Mg Ca Na

113282 3 4 5.1 7.70 168VH

221H+

28L+

400L+

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H, meq/100g soil = 8 (8.00 - 7.70) = 2.40

K, meq/100g soil = 221 ÷ 782 = 0.28

Mg, meq/100g soil = 28 ÷ 240 = 0.12

Ca, meq/100g soil = 400 ÷ 400 = 1.00

Na, meq/100g soil = 12 ÷ 460 = 0.03

--------

Total CEC = 3.83 meq/100g soil

To calculate the percent base saturation, divide the sum of the K, Mg, Ca, and Na (the bases) in meq/100g soil by the CEC (all these values were calculated above). Multiply the result by 100%.EXAMPLE

K = 0.28 meq/100g soil

Mg = 0.12 meq/100g soil

Ca = 1.00 meq/100g soil

Na = 0.03 meq/100g soil

CEC= 3.83 meq/100g soil

Total for bases = K + Mg + Ca + Na = 1.43 meq/100g soilPercent base saturation = (1.43 ÷ 3.83)(100%) = 37%

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Vclay Determination- The ConceptDefinitions:

Vclay Vs Vshale

These two terms can have different meaning with different users.

In PETROLOG we consider:

Vclay = Clay bound water plus dry clay solids. (Vclay + Vmatrix + PHIE = 1.0)

A sand is a formation that contains less than 30% clay.

A Shale is any clay that has less than 30% sand.

V dry clay  = Vclay - VBW

VBW = PHIT - PHIE

 

Concept

The determination of Vclay is probably the most difficult part of any interpretation as all clay indicators tend to be pessimistic.

Most users use only the GR as a clay indicator however the GR is often a very poor clay indicator particularly in thin laminated sand shale sequences, in the presence of glauconite and where the formation fluid is hot (Uranium rich)

Using GR only as a clay indicator can result in lost pay and/or missed reservoirs.

To avoid missed reservoirs Petrolog, by default, will compute all clay indicators and it is up to the users to disallow some of them should the final Vclay be not to their satisfaction.

 

IMPORTANT: This approach has the additional advantage to show if the clay points from different indicators are consistent since the same clay points are used in the computation of PHIE and PHIT.

 

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Petrolog has a build-in logic to turn off any clay indicators that have insufficient resolution to be meaningful.

 

The clay indicators uses are:

Gamma Ray Spontaneous Potential Sonic Neu tron RT Density Neutron Sonic Density Neutron Sonic M / N External EVCLAY

 

The final Vclay can be computed using any of the following options:

Minimum Functioned Hodges-Lehman Modified Hodges Lehman Weighed

 

 

Petrolog  v10.2 Help Manual

VGR DeterminationSee gr Logging tools

Input curves:

GR

Output curves:

VGR:

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The computed VGR is unlimited and can be either negative or greater than 1.0. This is to allow users to finely adjust the parameters to fit the results between 0 and 1.0.

Input parameters:

GR clean GR clay Vclay GR type

 

Threshold or condition for computations:

VGR is not computed if (GR clay - GR clean) < 20 API.

 

Many users confuse GRmin with GR clean and GRmax with GRclay.

If a sand contains a minimum of (say) 10 % clays, then GRclean should be lower than GRmin.(e.g. Gas sands of Queensland)

(e.g. some GR logs recorded in the Middle East will show intervals with GR = 5 API, others with 20 API and others reading 45 API. GRmin is none of these since the lower value represent Salts or Anhydrite, The middle value are clean limestone and the high GR values are clean dolomites. There is no clay in any of these formations and GRmax is never seen.)

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FIGURE 1

Figure 1 shows the 8 zone parameters that can be used for VGR determination. For most cases, only the first two entries are needed. The last 5 entries are needed for some of the special functions. (e.g. GR50% is used on the user defined Steiber equation)

All entries except for Vclay GR type can be interactively selected from the GR vs RWA X-Plot shown in Figure 2

Calculations:

IGR = (GR - GR clean)/(GR clay - GR clean)

Linear:

VGR = IGR

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FIGURE 2

Figure 2 shows the GR vs RWA Z-Plot.

The lower point represent the GR clean and RW values for this zones. Moving this point will automatically update the zone parameters and modifying the zone parameter will alternatively move the point on the screen.

The upper point represent both GR clay and RWB for this zone.

The line joining the two point represent the equation used. (Here = Linear.)

Asymmetric:

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This equation requires 4 sets of points as shown in figure 3 namely: (GR clean, 0.0), (GR1, Vcl1), GR2, Vcl2), (GR clay, 1.0)

Vclay is computed as follows:

      If (GR < GR clean) then VGR = 0.01

else if (GR < 0.5*Gr1)  then VGR = 0.5*Vgr1*(GR-GR clean)

else if (GR < Gr1)         then VGR = 0.5Vgr1+ (GR - 0.5*Gr1)/(0.5*Gr1) * (0.5*VGr1)

else it (GR < Gr2)         then VGR = VGr1 +((GR- GR1)/(Gr2-Gr1)) * (VGr2-VGr1)

else if (GR < (GR2 + 0.5*(GrClay - Gr2))) then VGR = Vgr2 + ((GR-Gr2)/(0.5*(GR clay-Gr2)) * (0.5*(1.0 - Vgr2)

Else if (GR < GR clay)  then VGR = (0.5 * (1.0-Vgr2) * Vgr2) + (GR-0.5*(GR clay - Gr2))/(GR clay - 0.5*(GR clay - Gr2))

else VGR = 1.0

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FIGURE 3

Figure 3 shows GR Vs VGR Z-Plot and the shape of the asymmetric equation before processing.

The Asymmetric equation is a variant of the Steiber equation, Larinov and Clavier equation that is both more flexible and more accurate in the high Vclay range where all other equations tend to read too high a Vclay.

The Lower part of the asymmetric equation is ideal to handle Glauconitic equation to make the VGR match the core results.

 

The upper section works best in the Far Eats where thick beds of shales with different GR readings overlay each other. (Say 200 m with GR clay = 150 API followed by 150 m GR reading 180 API.

Both 150 and 180 values can be made to read near 95% clay in the same zone using the asymmetric equation.

The same equation can be used to give the low volume of clays in the sand a completely different property to the adjacent shales.

Steiber 1, 2, 5 and user defined equations:

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FIGURE 4

Figure 4 shows the curvature of the Steiber 2 equation compared with the Vclay computed with the Asymmetric equation.

The Steiber equation is as follows:

 

IGR = (GR - GR clean)/(GR clay - GR clean)

VGR = IGR / (IGR + A * (1.0 - IGR))

Where:

A = 1 for Steiber 1

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A = 2 for Steiber 2

A = 3 for Steiber 25

 

The Steiber 50% is the value of A which can be interactively and graphically selected using Z-Plot shown in Figure 4.

 

Larinov Old Rock

VGR = (2^(2*IGR) - 1.0) / 3.0

This equation is very similar to the Steiber 1 Equation

Larinov Tertiary

VGR = 0.083 * ( 2.0^ (3.7058 * IGR) - 1.0)

This equation is very similar to the Steiber 3 equation

Clavier

If IGR < 1.0) then

  VGR = 1.7 - SQRT(3.38 - (IGR + 0.7)^2)

else

VGR = IGR

Endif

This equation is very similar to a Steiber 1 equation.

VSP Determination

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See SP logging tools

Input curves:

SP

Output curves:

VSP

Input parameters:

SSP SP clay

Threshold or condition for computations: VSP is not computed if ABS (SSP) < 20 mv.

FIGURE 1

Figure 1 shows the 2 zone parameters that can be used for VSP determination. If the SP log has been normalized properly SP clay should be 0.

 

SSP is the difference between the SP log values in shales and in sand.

It is recommended to normalize the SP log using the SP drift corrections graphically when viewing the Composite log plot.

The SSP is reduced in the presence of hydrocarbons

SSP is not fully developed in thin laminated sand/shale sequences.

SSP is unreliable in tight formation.

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Calculations:

VSP = 1.0 - (SP - zSPclay)/zSSP

Other Application for SSP

The SSP value is also used to compute RW(SP).

VS DeterminationSee dt logging tool

Input curves:

DT

Output curves:

VS:

Input parameters:

Dt Matrix min Dt clay

Threshold or condition for computations:

VS is not computed if (Dt clay - Dt Matrix min) < 30 us/ft.

FIGURE 1

Figure 1 shows the 2 zone parameters that a used for VS determination. These values can either be entered interactively from the Sonic-Density or Sonic-Neutron Z-Plots or manually entered in the zone control file.

 

IMPORTANT: DT clay is used for other clay indicators using the sonic log and for other computations like the pseudo DTc and DTSc.

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DT clay is also used to compute the M clay in the M - N clay indicator.

Calculations:

Linear:

VS = (DT - Dt Matrix min)/(DT clay - Dt Matrix min)

 

Petrolog  v10.2 Help Manual

VSD DeterminationVSD is the Vclay from the Sonic Density X-Plot.

Input curves

RHOB (g/cc) DT       (us/ft)

Output curves

VSD:

Input parameters

RHOMA (default = 2.65 for sandstone) (IMPORTANT: Also used for SSS and TWA models and to set the lithology when RHOB is missing or if the PEF is bad or missing)

DT Matrix Min. (Dt matrix minimum = Dtm) DTclean @ RHOB = 2.2 g/cc ( or DT2) RHOB clay (Also used to compute PHIT and PHIE after clay corrections). DT clay

Threshold or condition for computations: VSD is not computed if Vsdd > - 8.0 where Vsdd = ( DT clay - DT Matrix min ) * ( 2.2 - RHOMA ) ( RHOB clay - RHOMA ) * ( DT2.2 - DT Matrix min)

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FIGURE 1

Figure 1 shows the 6 zone parameters that can be used for VDN determination.

A clean line for VSD = 0.0 is traced between the following pair of points: (RHOMA, DT matrix min) and (2.2, DT @ 2.2)

The 100 clay line passes through a parallel line to the clean line and through the RHOB clay and DT clay point as shown in figure 2

Calculations: X = ( DT - DT min ) * ( 2.2 - RHOMA ) ( RHOB - RHOMA ) * ( DT2.2 - DT matrix min) VDN = X / Vdnd

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FIGURE 2

Figure 2 shows the Sonic  Density Z-Plot with Vclay in the Z axis.

The Clean line passes through (DT matrix min, RHOMA) and (DT2, 2.2) and is plotted as a continuous orange line in Figure 2.

Rhobc is also known as the wet clay points. It is critical in the calculation of PHIE and PHIT, to determine the M and N clay points and to compute RHOMA clay used in the reconstructed RHOB.

DTc is the Sonic clay point also used to compute VS Sonic. It is also used to reconstruct the a pseudo sonic and pseudo DT shear.

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RHOMA: Default = 2.65 is used for the clean line and also as the matrix density to compute porosity in the shaly - sand models. CAUTION: Lead RHOMA at 2.65 for sandstone as it is the parameter that will be used in the porosity determination.

Quality Control:

If both the RHOB and DT clay points are correct, the M and N clay points will also be correct.

Check the VSD curve on the right of the X-Plot to see if VSD reads 100% over shale beds. If negative Vs and VSD are measured then readjust your clean line.

Where the density is affected by bad hole effects, the VSD will show Negative values. If the Density is bad for Vclay determination it is also bad for PHIE and PHIT and the cut-off values for DRHO, CALI and RUGOS

 

Petrolog  v10.2 Help Manual

VRT DeterminationSee ild lld logging tools

Input curves:

RT

Output curves:

VRT:

Input parameters:

RT clean RT clay

Threshold or condition for computations:

VRT is not computed if ABS (Log10(RT clean) - Log10(RT clay)) < 0.7.

FIGURE 1

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Figure 1 shows the 2 zone parameters that a used for VRT determination. These values can either be entered interactively from the GR Vs RWA Z-Plot or directly from a composite log plot with RT plotted as a curve.

 

IMPORTANT: RT clay is also used in the resistivity saturation equations and is therefore critical for both VRT and SW.

Calculations:

Linear:

VRT =(Log10(RT) - Log10(RT clay) / (Log10(RT clean) - Log10(RT clay))

Interactive Selection of RT clay:

 

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FIGURE 2

Figure 2 is a typical Z-Plot of the GR-VS RWA  with Vclay plotted in the Z axis and all Vclay curves traced on the right.

The upper circle will determine the following zone parameters:

GR Clay as used in the Gamma Ray clay indicator Rwb as used in the Modified Waxman  Smits equation RT clay. (Computed using PHICP clay, RWB and the Archie equation.) This value of RT

should be equal to RT measured.

Quality Control:

Plot all the Vclay indicator next to the Z-Plot and check the VRT.

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In Figure 2 VRT is the dark green trace. It is falling on the Vclay = 100 line which confirms the good selection of RT clay.

VRT matches the cleanest sand in the water bearing sands.

Note that in the above example, VRT = 100% in the hydrocarbon bearing interval above the shale bed. This is because, in this reservoir, RT oil = RT clay = 12 Ohmms.

The default for RT clean = 1000 Ohmms and this should be adjusted to fit the interval.

 

VDN DeterminationVDN is the Vclay from the Density Neutron X-Plot.

Input curves

RHOB (g/cc) NPHI   (v/v)

Output curves

VDN:

Input parameters

RHOMA (default = 2.65 for sandstone) (IMPORTANT: Also used for SSS and TWA models and to set the lithology when RHOB is missing or if the PEF is bad or missing)

PHIN min. NPHI @ RHOB = 2.2 g/cc ( or PHIN2.2) RHOB clay (Also used to compute PHIT and PHIE after clay corrections). PHIN clay

Threshold or condition for computations: VDN is not computed if Vdnd < 0.06. where Vdnd = ( PHIN clay - PHIN min ) * ( 2.2 - RHOMA ) ( RHOB clay - RHOMA ) * ( PHIN2.2 - PHIN min)

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FIGURE 1

Figure 1 shows the 6 zone parameters that can be used for VDN determination.

A clean line for VDN = 0.0 is traced between the following pair of points: (RHOMA, PHIN min) and (2.2, PHIN @ 2.2)

The 100 clay line passes through a parallel line to the clean line and through the RHOB clay and PHIN clay point as shown in figure 2

Calculations: X = ( PHIN - PHIN min ) * ( 2.2 - RHOMA ) ( RHOB - RHOMA ) * ( PHIN2.2 - PHIN min) VDN = X / Vdnd

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FIGURE 2

Figure 2 shows the Density Neutron Z-Plot with Vclay in the Z axis.

The Clean line passes through (Phinmin, RHOMA) and (PHIN2, 2.2) and is plotted as a continuous orange line in Figure 2. In the presence of Gas, PHIN2 should be moved to the left of the gas points.

Rhobc is also known as the wet clay points. It is critical in the calculation of PHIE and PHIT, to determine the M and N clay points and to compute RHOMA clay used in the reconstructed RHOB.

NPHIc is the neutron clay point also used to compute VN Neutron

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RHOMA: Default = 2.65 is used for the clean line and also as the matrix density to compute porosity in the shaly - sand models.

Quality Control:

If both the RHOB and NPHI clay points are correct, the M and N clay points will also be correct.

Check the VDN curve on the right of the X-Plot to see if VDN reads 100% over shale beds.

Where the density is affected by bad hole effects, the VDN will show Negative values. If the Density is bad for Vclay determination it is also bad for PHIE and PHIT and the cut-off values for DRHO, CALI and RUGOS should be altered accordingly. see Bad Hole Logic

 

VSD DeterminationVSD is the Vclay from the Sonic Density X-Plot.

Input curves

RHOB (g/cc) DT       (us/ft)

Output curves

VSD:

Input parameters

RHOMA (default = 2.65 for sandstone) (IMPORTANT: Also used for SSS and TWA models and to set the lithology when RHOB is missing or if the PEF is bad or missing)

DT Matrix Min. (Dt matrix minimum = Dtm) DTclean @ RHOB = 2.2 g/cc ( or DT2) RHOB clay (Also used to compute PHIT and PHIE after clay corrections). DT clay

Threshold or condition for computations: VSD is not computed if Vsdd > - 8.0 where Vsdd = ( DT clay - DT Matrix min ) * ( 2.2 - RHOMA ) ( RHOB clay - RHOMA ) * ( DT2.2 - DT Matrix min)

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FIGURE 1

Figure 1 shows the 6 zone parameters that can be used for VDN determination.

A clean line for VSD = 0.0 is traced between the following pair of points: (RHOMA, DT matrix min) and (2.2, DT @ 2.2)

The 100 clay line passes through a parallel line to the clean line and through the RHOB clay and DT clay point as shown in figure 2

Calculations: X = ( DT - DT min ) * ( 2.2 - RHOMA ) ( RHOB - RHOMA ) * ( DT2.2 - DT matrix min) VDN = X / Vdnd

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FIGURE 2

Figure 2 shows the Sonic  Density Z-Plot with Vclay in the Z axis.

The Clean line passes through (DT matrix min, RHOMA) and (DT2, 2.2) and is plotted as a continuous orange line in Figure 2.

Rhobc is also known as the wet clay points. It is critical in the calculation of PHIE and PHIT, to determine the M and N clay points and to compute RHOMA clay used in the reconstructed RHOB.

DTc is the Sonic clay point also used to compute VS Sonic. It is also used to reconstruct the a pseudo sonic and pseudo DT shear.

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RHOMA: Default = 2.65 is used for the clean line and also as the matrix density to compute porosity in the shaly - sand models. CAUTION: Lead RHOMA at 2.65 for sandstone as it is the parameter that will be used in the porosity determination.

Quality Control:

If both the RHOB and DT clay points are correct, the M and N clay points will also be correct.

Check the VSD curve on the right of the X-Plot to see if VSD reads 100% over shale beds. If negative Vs and VSD are measured then readjust your clean line.

Where the density is affected by bad hole effects, the VSD will show Negative values. If the Density is bad for Vclay determination it is also bad for PHIE and PHIT and the cut-off values for DRHO, CALI and RUGOS

 

VNS DeterminationVSD is the Vclay from the Sonic Density X-Plot.

Input curves

NPHI   (v/v) DT       (us/ft)

Output curves

VNS:

Input parameters

DT Matrix Min. (Dt matrix minimum = Dtm) DTclean @ RHOB = 2.2 g/cc ( or DT2) DT clay PHIN min. NPHI @ RHOB = 2.2 g/cc ( or PHIN2.2) PHIN clay

Threshold or condition for computations: VSD is not computed if Vnsd > 5.0 where Vnsd = ( PHIN clay - PHIN min ) * ( DT2.2 - DT Matrix min) ( DT clay - DT Matrix Min ) * ( PHIN2.2 - PHIN min)

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FIGURE 1

Figure 1 shows the 6 zone parameters that can be used for VDN determination.

A clean line for VSD = 0.0 is traced between the following pair of points: (RHOMA, DT matrix min) and (2.2, DT @ 2.2)

The 100 clay line passes through a parallel line to the clean line and through the RHOB clay and DT clay point as shown in figure 2

Calculations: X = ( PHIN - PHIN min ) * ( DT2.2 - DT Matrix min) ( DT - DT Matrix Min ) * ( PHIN2.2 - PHIN min) VNS = X / Vnsd

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FIGURE 2

Figure 2 shows the Neutron Sonic Z-Plot with Vclay in the Z axis.

The Clean line passes through (PHIN min,DT matrix min) and (PHIN2.2, DT2.2) and is plotted as a continuous orange line in Figure 2.

DTc is the Sonic clay point also used to compute VS Sonic. It is also used to reconstruct the a pseudo sonic and pseudo DT shear.

NPHIc is the neutron clay point also used to compute VN Neutron

PHIN2.2 and DT2.2 are the same clean points as used in the D-N and S-D X-plots

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Quality Control:

If both the PHIN and DT clay points are correct, the M and N clay points will also be correct.

Check the VNS curve on the right of the X-Plot to see if VNS reads 100% over shale beds. If negative Vs and VSD are measured then readjust your clean line.

In the above example, the clay point is too close to the clean line and VNS would automatically be rejected as a clay indicator.

 

VMN DeterminationInput curves

RHOB (g/cc) NPHI   (v/v) DT       (us/ft) MFACT NFACT

The MFACT (M factor) and NFACT (N factor) outputs are computed automatically and saved as curves in the -pro.logdata file.

MFACT = 0.01 * (DTf - DT) / (RHOB - RHOF)

NFACT = (1.0 - NPHI) / (RHOB - RHOF)

with RHOB in g/cc. NPHI in V/V and Dt in us/f

Output curves

VMN

Input parameters:

M50 RHOB clay NPHI clay DT clay M clay N clay

M clay = 0.01 * (DTf - DT Clay) / (RHOB clay - RHOF)

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N  clay = (1.0 - NPHI clay) / (RHOB clay - RHOF)

Threshold or condition for computations: VMN is not computed if Vmnd < 0.085. where Vmnd = M50 - 0.11 - M clay + (0.22 * N clay) M50 = M at N=0.5 on the M-N Z-Plot. The clean line is a line that passed through the M50 point and is parallel to the average Sand, LS and DOL matrix points.

FIGURE 1

1. Figure 1 shows the 3 zone parameters that a used for VMN determination. These values can either be entered interactively from the M Vs N Z-Plot as shown in Figure 2

IMPORTANT: M clay and N clay are automatically calculated whenever the clay points for RHOB, NPHI and DT are changed in the previous Z-Plots. This is to make sure that there is a balance between all clay points.

The M and N clay points can be used for quality control of the RHOB, NPHI and DT clay points since all three should compute the M and N clay point to fall correctly on the M/N Z-Plot.

Calculations:

VMN = (M50 - 0.11 - M + 0.22 * N) / Vmnd

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Interactive Selection of M clay and N clay:

FIGURE 2

Figure2 shows the clean line that can be repositioned by moving the orange circle positioned at m50 and N = 0.5

The large blue circle represent M clay and N clay

The orange line is the clean line for VMN = 0.0

Place an imaginary parallel line through the blue circle for VMN = 100%

Quality Control:

If both the clay points for DT, RHOB and NPHI the blue circle should pass through the green points at Vclay = 100%.

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Check the VMN curve on the Vclay curve display on the right and check that VMN plots closed to the 100% Vclay opposite shale beds.

Clean intervals with either the presence of gas or secondary porosity will plot above the Sand, LS and Dolomite points and the M50 line should be moved upwards to avoid measuring negative VMN

 

External Vclay DeterminationInput curves:

EVCL (v/v)

Output curves:

VCL

Input parameters: None

 

Calculations:

Vclay = EVCL

 

The external EVCL curve, when available and selected as an input curve in the log analysis over rides all other clay indicators and will be used in preference.

 

IMPORTANT: Even if a user has an externally computed Vclay to be use in the interpretation, it is important to also carefully select all the clay points from all the logs as the clay points like RT clay, RHOB clay, NPHI clay, DT clay are required in subsequent phases of the interpretation.

 

When to use EVCL

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All clay indicators are pessimistic and tend to read too high a Vclay when all the clay parameters have been properly selected.

The GR clay is probably the best clay indicator but is often a very poor clay indicator in the presence of solution Uranium and in thin shale-sand beds.

 

Vclay computed from an NMR tool can give a much better Vclay in many cases.

The Vclay from micro scanner images can also be used effectively when all other clay indicator fail.

 

 

Final Vclay MinimumOnce all clay indicators have been computed the default final Vclay is the lowest of all clay indicators.

Input curves

VGR VS VSP VN VRT VDN VNS VSD VMN

Only the indicators that have been computed and allowed will be used in the averaging.

Output curves

VCL:

Input parameters

Vclay Flag

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FIGURE 1

Figure 1 shows the zone parameter that selects the Clay Flag options. This parameter is at the very top of all clay parameters.

Equation

The final Vclay

Limit all input clay indicators between 0.0 and 1.0 to remove negative Vclay and Vclay > 100%

Takes the 5 lowest values of the 9 clay indicators (Say: V1, V2, V3, V4, V5)

If there are less that 5 clay indicators available the number of average pairs is reduced accordingly

Create 10 averages pairs of new Vclays using the 5 Vclay used:

e.g. V6 = (V1+ V2)/2, V7 = V1 + V3)/2,  V8 = (V1+ V4)/2 etc

Add the original 5 clay indicators and sort all 15 clay indicators in increasing order.

The final Vclay is the middle value of the sorted averages.

 

Example 1:

Only 4 Indicators are available:

VGR  = 0.10

VS     = 0.80

VDN = 0.20

V4 = ( VGR + VS)    / 2.0 = 0.45

V5 = ( VGR + VDN) / 2.0 = 0.15

V6 = ( VS + VDN)    / 2.0 = 0.5

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The sorted values are: 0.1, 0.15, 0.2, 0.45, 0.50, 0.8 and the middle value is between 0.2 and 0.45 (0.2 is used in Petrolog)

 

Example 2:

VGR = 0.0

VSP = 1.0

V1   = (0.0 + 1.0)/2.0 = 0.5  and the middle value will be Vclay = 0.5

 

The Modified Hodges-Lehman average or the CDP Weighed average should be used in preference to the Hodges-Lehman Average.

 

Non-Linear VclayThe final Vclay default is the Minimum of all Vclay measurements made and limited from 0.0 to 1.0 v/v

The final Vclay can also use different combinations of Vclay indicators such as Hodges-Lehman, Modified Hodges Lehman or Weighed equations

The final Vclay can be recomputed using any of the non-linear equations used for the GR log.

This option should not be used if it has already been used on the GR

This option should be used with caution and only over intervals where its use can be justified from core data.

 

The concept of this Non-Linear option is made on the assumption that, like for the GR, the clay properties within the sands themselves do not necessarily have the same propertied of the clays in the thick shales above and below. The Clean-Clay relationship is therefore non-linear and can be corrected using the options described here below.

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Input curves:

VCL

Output curves:

VCL:

The computed VGR is unlimited and can be either negative or greater than 1.0. This is to allow users to finely adjust the parameters to fit the results between 0 and 1.0.

Input parameters:

Vclay Type Vclay inp1  (Used for asymmetric equation only) Vclay out1  (Used for asymmetric equation only) Vclay inp2  (Used for asymmetric equation only) Vclay out2  (Used for asymmetric equation only) Vclay 50% (Used for the user defined Steiber equation only)

Threshold or condition for computations: none

FIGURE 1

Figure 1 shows the non-linear Vclay types input parameters.

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FIGURE 2

Figure 2 shows the list of available non-linear equations.

Calculations:

Vclay = F * Vclay

Linear:

F = 1.0

Asymmetric:

This equation requires 2 sets of points as listed in the input parameters above.

 

Vclay is computed as follows:

If       (Vclay < 0.5*Vclay inp1)  then Vclay = 0.5*Vclay out1*(Vclay)

else if (Vclay < Vclay inp1)         then Vclay = 0.5Vout1+ (Vclay - 0.5*Vclay inp1)/(0.5*Vclay inp1) * (0.5*Vclay inp1)

else it (Vclay < Vclay inp2)         then Vclay = Vclay out1 +((Vclay- Vclay inp1)/(Vclay inp2-Vclay inp1)) * (Vclay out2-Vclay out1)

else if (Vclay < (Vclay inp2 + 0.5*(Vclay - Vclay2))) then

Vclay = Vclay out2 + ((Vclay-Vclay out2)/(0.5*(Vclay-Vclay out2)) * (0.5*(1.0 - Vclay out2)

else Vclay = (0.5 * (1.0-Vclay out2) * Vclay out2) + (vclay-0.5*(1.0 - Vclay out2))/(1.0 - 0.5*(1 - Vclay out2))

 

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Steiber 1, 2, 5 and user defined equations:

FIGURE 3

Figure 3 shows the curvature of the user defined Steiber equation compared with the linear input Vclay. The blue circle (Vclay50, 0.5) can be displaced at the user's choice to get the final Vclay along the new line. Figure 2 would calculate A = 2 here above.

The Steiber equation is as follows:

Vclay out = Vclay/ (Vclay + A * (1.0 - Vclay))

Where:

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A = 1 for Steiber 1

A = 2 for Steiber 2

A = 3 for Steiber 3

A = any other value when using the User defined Steiber equation.

The Steiber 50% is the value of A which can be interactively and graphically selected using Z-Plot shown in Figure 4.

 

Larinov Old Rock

Vclay out = (2^(2*Vclay) - 1.0) / 3.0

Larinov Tertiary

Vclay out = 0.083 * ( 2.0^ (3.7058 * Vclay) - 1.0)

Clavier

  Vclay out = 1.7 - SQRT(3.38 - (Vclay + 0.7)^2)

 

 

 

Hodges-Lehman VclayThis averaging is ignored if EVCL (External Vclay) is used and available in the processed interval.

Input curves

VGR VS VSP VN VRT

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VDN VNS VSD VMN

Only the indicators that have been computed and allowed will be used in the averaging.

Output curves

VCL:

Input parameters

Vclay Flag

FIGURE 1

Figure 1 shows the zone parameter that selects the Clay Flag options. This parameter is at the very top of all clay parameters.

Equation

The Hodges-Lehman average works like this.

Takes the 5 lowest values of the 9 clay indicators and calculates 10 average pairs + the 5 lowest clay indicator values.:

(say: V1 = VDN, V2=VGR, V3=VSP, V4=VS, V5=VRT will give 15 results as follows; V11= (V1+V1)/2, V12=V1+V2)/2 etc ... etc..

The 15 results are sorted and the middle value is chosen as the final Vclay. (for 5 indicators the pair is position 8 is therefore chosen.)

Where fewer than 5 clay indicators are available this method can give pessimistic values of Vclay and the modified Hodges/Lehman average can give better results.

 

Example 1:

Only 4 indicators are available:

VGR  = 0.10

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VS     = 0.80

VDN = 0.20

V4 = ( VGR + VS)    / 2.0 = 0.45

V5 = ( VGR + VDN) / 2.0 = 0.15

V6 = ( VS + VDN)    / 2.0 = 0.5

 

The sorted values are: 0.1, 0.15, 0.2, 0.45, 0.50, 0.8 and the middle value is between 0.2 and 0.45 = (0.2+ 0.45)/2 = 0.325

 

Example 2: Only 2 indicators used:

VGR = 0.0

VSP = 1.0

V1   = (0.0 + 1.0)/2.0 = 0.5  and the middle value will be Vclay = 0.5

 

Modified Hodges-Lehman VclayThis averaging is ignored if EVCL (External Vclay) is used and available in the processed interval.

Input curves

VGR VS VSP VN VRT VDN VNS VSD VMN

Only the indicators that have been computed and allowed will be used in the averaging.

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Output curves

VCL:

Input parameters

Vclay Flag

FIGURE 1

Figure 1 shows the zone parameter that selects the Clay Flag options. This parameter is at the very top of all clay parameters.

Equation

The Modified Hodges-Lehman average works like this.

Preferred when fewer than 4 clay indicators are available.

It uses the same concept as the Hodges-Lehman average to pick the mid point value after sorting all the clay indicators.

With the mid value = X and the minimum indicator = Y

The modified Hodges/Lehman weighed reduces this value as follows:

      Vclay = X - (X - Y/2)/N

      Where N = number of clay indicators (here 3)

It is easy to see that the larger the number of clay indicators the smaller the second term becomes.

 

Example 1:

Only 3 Indicators are available:

VGR  = 0.10

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VS     = 0.80

VDN = 0.20

V4 = ( VGR + VS)    / 2.0 = 0.45

V5 = ( VGR + VDN) / 2.0 = 0.15

V6 = ( VS + VDN)    / 2.0 = 0.5

 

The sorted values are: 0.1, 0.15, 0.2, 0.45, 0.50, 0.8 and the middle value is between 0.2 and 0.45 X = (0.2 + 0.45) / 2 = 0.325 = Hodges-Lehman

 

Vclay = 0.325 - (0.325 - 0.1)/2) / 3 = 0.2875

 

Example 2:

Only 2 indicators used:

VGR = 0.0

VSP = 1.0

X = (0.0 + 1.0)/2.0 = 0.5

X - (X - Y/2)/N = 0.5 - (0.5 - 0.0)/2 = 0.25.

In the above example the Vclay would still be too high and the Weighed average would work better.

 

Weighed Average VclayThis averaging is ignored if EVCL (External Vclay) is used and available in the processed interval.

50

Input curves

VGR VS VSP VN VRT VDN VNS VSD VMN

Only the indicators that have been computed and allowed will be used in the averaging.

Output curves

VCL

Input parameters

Vclay Flag

FIGURE 1

Figure 1 shows the zone parameter that selects the Clay Flag options. This parameter is at the very top of all clay parameters.

Equation

Sort all clay indicators used and take only the three lowest values L ( Low), M (Mid) H (High) and apply the following equation:

Vclay = ( 3*L + 2*M + H)/6  where N = Number of clay indicator used)

 

Example 1:

VGR  = 0.10

VS     = 0.50

51

VDN = 0.20

Vclay = ( 3* 0.1 + 2 * 0.2 + 0.5 ) / 6 = 0.2

 

Example 2:

VGR = 0.0

VSP = 1.0

Vclay = ( 2* 0.0 + 1.0) /4 = 0.25

This is half the value that would be calculated using the Hodges-Lehman Average.

 

52

Bulk Density Determination Introduction / Procedures / Calculations / Data Sheets / Comments /  References

Introduction

Bulk density is a measure of the weight of the soil per unit volume (g/cc), usually given on an oven-dry (110° C) basis (figure 1). Variation in bulk density is attributable to the relative proportion and specific gravity of solid organic and inorganic particles and to the porosity of the soil. Most mineral soils have bulk densities between 1.0 and 2.0. Although bulk densities are seldom measured, they are important in quantitative soil studies, and measurement should be encouraged. Such data are necessary, for example, in calculating soil moisture movement within a profile and rates of clay formation and carbonate accumulation. Even when two soils are compared qualitatively on the basis of their development for purposes of stratigraphic correlation, more accurate comparisons can be made on the basis of total weight of clay formed from 100 g of parent material than on percent of clay alone. To convert percent to weight per unit volume, multiply by bulk density (Birkeland, 1984). The determination usually consists of drying and weighing a soil sample, the volume of which is known (core method) or must be determined (clod method and excavation method). These methods differ in the way the soil sample is obtained and its volume determined.

Figure 1: Sketch of soil sample to show solid particle and void space distribution. Particles shown in white, voids in black. The mineral grains in many soils are mainly quartz and feldspar, so 2.65 is an adequate average mineral specific gravity for the sand fraction. Bulk density and porosity are calculated as follows:

A different principle is employed with the radiation method. Transmitted or scattered gamma radiation is measured; and with suitable calibration, the density of the combined gaseous-liquid-solid components of a soil mass is determined. Correction is then necessary to remove the components of density attributable to liquid and gas that are present. The radiation method is an in situ method (Blake and Hartge, 1986). Clod and core methods have been used for many years. Excavation methods were developed in recent years, chiefly by soil engineers for bituminous and

53

gravelly material. More recently the excavation method has found use in tillage research where surface soil is often too loose to allow core sampling, or where abundant stones preclude the use of core samplers. Radiation methods have been used since the 1950's, especially in soil engineering (Blake and Hartge, 1986). All the earlier mentioned methods have advantages and disadvantages according to the samples that are available and the sampling method. This method of discussion here is the clod method. The bulk density of clods, or peds, can be calculated from their mass and volume. The volume may be determined by coating a clod of known weight with a water-repellent substance and by weighing it first in air, then agin while immersed in a liquid of known density, making use of Archimedes' principle. The clod or ped must be sufficiently stable to cohere during coating, weighing and handling (Blake and Hartge, 1986). The clod method is applied commonly by pedologists or paleopedologists.

Procedure

1. Separate 3 peds from each sample (make the volume of each ped ~3-5cm3) 2. Tie a string around each ped with thread so that it can hang freely from a 2" length of thread with a loop on the end. 3. Place each ped in a numbered and weighed beaker, recording the sample and beaker number on the data sheet. 4. Place the beakers containing the peds in the oven and allow them to dry over night, remove the beakers, cool in the dessicator, weigh the beakers containing the peds, and record the data (subtracting the weight of the beaker from the combined weight of the ped and beaker). 5. Melt a cup of paraffin (wax), stabilizing it between 55° - 60°. 6. Dip each ped in the paraffin and allow to dry, making sure that the ped is entirely sealed. If there are any holes noticed, dip a rod in the melted wax and apply a drop of hot wax to patch the hole. Do not redip the whole ped, because the wax coating will be too thick. 7. Weigh the coated ped without the beaker and record its weight. 8. Immerse the ped in water and weigh the beaker and ped on a triple beam balance using a ring stand to hold the beaker of water positioned just above the balance pan. (note: if bubbles appear on the surface of the coated ped and then break free and rise to the surface, note this on the data sheet by writing "BBL" next to the "submerged weight". If the ped floats, write "Floater" in the space for "submerged weight"). 9. Peel the coating off each ped and return it to its beaker. Fill each beaker with water so that the peds will get soggy and fall apart. 10. Wet sieve the contents of each beaker through a >2mm sieve. Discard all but the >2mm fraction. Return portion to the beaker and place in the oven to dry. 11. Weigh the >2mm contents of each beaker and record the weights on the data sheet.

Calculations

1. Adjusted dry weight = (dry weight of the ped) - (dry weight >2mm fraction) 2. Weight of paraffin = (weight of dry ped) + (paraffin) - (dry weight of ped) 3. Adjusted immersed weight = (weight of ped with paraffin in water) + 0.1 (weight of paraffin) - 1.65 (weight of the >2mm material/ 2.65)* 4. Specific gravity = (adjusted weight)/ ((adjusted dry weight)-(adjusted immersed weight))

Record the dry weight of the ped, ped with paraffin weight, ped with paraffin in water weight, and >2mm weight.

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*Note: 0.1(ped with parafin weight-dry weight ped) ----corrects for the buoyant force of the wax 1.65(>2mm weight/2.65)---- corrects for the >2mm material assuming a density of 2.65 g/cm3 for that material.

Comments

· The clod method typically yields higher bulk-density values than do other methods (Tisdall, 1951), because it does not take the interclod spaces into account. The method mentioned here tries to reduce this error by removing the coating of each ped, thereby weighing each of the fractions. · Care should be practiced to get naturally occurring peds. Peds on or near the soil surface are likely to be unrepresentative, due to tilling/plowing in agriculturally used soils. Peds should be sampled at other depths or from areas that may not be used for agricultural purposes. · Should bubbles appear on the paraffin-covered ped while submerged in water, or if the weight in water increases with time, water is penetrating the clod, therefore the sample must be discarded. · Precision in calculating the bulk density would require a correction for the difference of the weight of the wire in air and in water. However, the error is negligible with thread or a 28-gauge wire. · In terms of overall accuracy, the greater number of samples used for each determination will significantly reduce the standard deviation. · If paraffin is not available, rubber, saran, wax mixtures, and oils may be substituted (Blake and Hartge, 1986)

References

Birkeland, P.W., 1984, Soils and Geomorphology: Oxford University Press, New York, p. 14-15.

Blake, G.R., and K.H. Hartge, 1986, Bulk Density, in A. Klute, ed., Methods of Soil Analysis, Part I. Physical and Mineralogical Methods: Agronomy Monograph no. 9 (2nd ed.), pp. 363-375.

Brasher, B.R., D.P. Franzmeier, V. Valassis, and S.E. Davidson, 1966, Use of saran resin to coat natural soil clods for bulk density and moisture retention measurements: Soil Science v. 101, p. 108.

Tisdall, A.L., 1951, Comparison of methods of determining apparent density of soils, Australian Journal of Agricultural Research, v. 2, pp. 349-354.

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1.1: NPHI vs RHOB "Density-Neutron" CrossplotSee   Crossplot options and operations on for more information on the crossplot toolbar options.

All crossplots are fully interactive. Changing a constant on one crossplot automatically changes the value in other crossplots and in the zone control file. Changing the zone parameter in the zone control file automatically changes the corresponding point on the crossplot.

A constant can be interactively changed on the crossplot by grabbing any of the points marked

with . To move a point click once on it then move it and click on the new location to position it.

It is recommended to click on to shown only the affected zone parameters left of the crossplot.

IMPORTANT: There is a threshold value which is the distance between the wet clay point perpendicular to the clean line. Petrolog will automatically reject this cross plot if the threshold is below a cut of value fixed in the program. This is to avoid obtaining noisy Vclay when the resolution is too small to be representative.

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FIGURE 1: Density-Neutron-Vclay Crossplot

Figure 1 is the default presentation of the Density-Neutron-Vclay crossplot for all models except for the SSS model which has 2 extra points displayed. See SSS below.

The D-N crossplot is the most critical cross plot since it is not only used as a clay indicator but also to compute PHIT and PHIE. The positioning of the dry and wet clay points are critical.

There are 5 points that are can be modified and from left to right:

Point 1: (Phinm, Rhoma). This is the clean matrix line. For Sandstone RHOMA should be set at 2.65. If a fixed matrix model is used for carbonates use the appropriate

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RHOMA. It is important to note that the RHOMA is not used to compute porosity when the CPX or CXP No PEF models are used. See Compute Porosity

Point 2 (Phin2, 2.2) This point is used to position the clean line when calculating the Vclay from D-N . See Density Neutron

Point 3 (-, RhoDryCl) This is the dry clay point and only the RHOB dry is needed since the point must fall on the line joining the wet clay point to the 100% point outside the chart. The Dry clay point is critical in calculating PHIT and since logs do not ever record dry clay since it does not exist in nature, this point must be determined from core measurements in a laboratory. Most companies take cores in sands with limited clay content and the clays within a sand do not necessarily have the same properties as the adjacent shales. Errors in PHIT can be expected in high Vclay if insufficient core results are available.

Point 4 (PhinMax, -) This set a PHImax cut off to limit the porosity calculated See Compute Porosity The Neutron point value is calculated automatically so that the lines fall on the appropriate Neutron chart used. It is different for different logging companies and Neutron tools.

Point 5 (Phinc, Rhobc) This is the wet clay point use both in the determination of Vclay from D-N, D-S and SN and also in computing PHIE and PHIT see Compute Porosity When changing the wet clay point the M clay and N clay values are automatically recomputed see 1.4: MFACT vs. NFACT Phinc is also used in calculating VN. See Neutron

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FIGURE 2: Density Neutron Crossplot with SSS model on

The 5 points described above are complemented by two new points.

Point 6 (PhinSilt, RhobSilt) This point is used in the Vsilt calculation see Compute Lithology and the Porosity determination. See Compute Porosity

Point 7 (PhinSand, RhobSand) This point is used in the Vsilt calculation see Compute Lithology and the Porosity determination. See Compute Porosity

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The SSS model has other applications than only computing the Silt. Regardless of the position of the Silt point, the porosity of the silt point uses RHOMA for determining PHI silt. PhiSilt is automatically calculated and changed in the zone control file shown in figure 3. However, the user can manually change the value of PhiSilt for specific applications.

Example: The presence of heavy minerals like pyrite nodes will affect almost exclusively the density log and the points will fall downward towards an apparent silt point. Correspondingly, the calculated porosity will be pessimistic. One solution is to increase the value of PhiSilt so that the porosity remains high even if RHOB increases and fall to 2.65 or higher.

FIGURE 3: D-N crossplot zone parameters

Changing the values in Figure 3 will move the points in Figure 2 and vice versa.

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The Bad Hole Display flag is normally off and only the D-N points that are in good hole conditions are plotted when Off.

The PHIN matrix is set so that the clean line falls parallel to the sandstone line where porosity is above 15%. This value is negative to compensate for the Neutron log non-linear sandstone response in lower porosity readings and may be different depending on the Neutron tool used. PHIN matrix is used only for the Density-Neutron clay indicator and it is not used for any other purpose.

IMPORTANT NOTES about the Density-Neutron log

The density-Neutron log is probably the best tool to help the Petrophysicist identify the type of formation or lithologies of a given zone. The appearance of this cross plot is very useful and here are some useful indicators:

Points fall in a relatively straight line from the clean sandstone line to the wed clay points: This is typical of a laminated clay sequence.

Points falls downward towards the dry clay point. This is typical of a dispersed clay sequence.

Points falls along the sandstone line then towards the wet clay point. This is typical of a Silty-Shaly-Sand model.

The distribution pattern or the direction of the clay points are also affected by the clay type and many clay types are listed by name in figure 1. The density value for most clay types are reasonably constant and in the general direction shown in Figure 1. On the other hand, the Neutron response to a clay type can vary enormously since it is directly dependant on the amount of de-watering a clay has been subjected to.

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