GeoNeurale Newsletter




GeoNeurale Courses Program  2008                                

Double Duty with The Old and The New   (by Gene Ballay)


The Compact Theory

Carbonate Petrophysics Consulting

References and Links



17-19 June 2008             The Logic of Neural Networks for the Petrophysical, Seismic and Facies Estimation     PROGRAM

                                        Instructors:  Hansruedi Frueh  & Tino Perucchi                   

23-27 June 2008              Advanced Carbonate Petrophysics                                                                               PROGRAM       

                                         Instructor:  Gene Ballay 
1-5 September  2008        Applied Carbonate Stratigraphy                                                                                    PROGRAM

                                          Instructor:  Franz Meyer

6-10 October 2008            Geostatistical Applications and Petrophysical Analysis  

                                          Instructors:  Gene Ballay  & Jacques Deraisme

17-21 November 2008       Geostatistical Modeling for Petroleum Reservoir Characterization                                PROGRAM

                                           Instructors:  Hans Wackernagel &   Olivier Jaquet

November 2008                  Fundamentals of Petroleum Economics and Risk Analysis

                                           Instructor:  Tim James  &  Colin Howard

December 2008                  Seismic Interpretation in the Exploration Domain

                                            Instructor:  Tim Smith

Actual Courses

GeoNeurale Courses Schedule

DOUBLE  DUTY  with  THE OLD  and  THE NEW   

by  R.E. (Gene)  Ballay  PhD


It seems like only yesterday that I reported to work at Shell’s Bellaire Research Center. Gus Archie was already gone but Monroe Waxman (Waxman-Smit’s equation), Bob Purcell (developed mercury injection capillary pressure), Jerry Lucia (Rock-Fabric Carbonate Pore Classification) and many other fine professionals were always willing to take time to explain their trail-breaking work to a neophyte.

In those pre-computer days (Schlumberger’s CSU truck was not yet operational), petrophysical evaluations were, by necessity, very different than today. And yet with skill, insight and experience, petrophysicists such as Lou McPherson (who ran Shell’s Petrophysics Training Program) were able to evaluate intervals that, even today, would challenge the computer.

Computer interpretations, deterministic and probabilistic, typically place many evaluation options at our disposal, and are without doubt a major step forward. Unfortunately, in an era where animated power point slides are the norm, it’s also possible to over-look fundamental, under-lying petrophysical concepts that remain valid, whether applied with log-log graph paper or the latest probabilistic software.

                                                         Bulk Volume Water

The surface to volume ratio of a sphere is (4 p r 2 ) / (4 p r 3 / 3 ) ~ 1 / r . This relation reveals that pore surface area becomes a relatively larger issue, as the pore radii decreases (1 / r increases). In the case of water wet rock at some specific height above the free water level, one then anticipates an increased Sw, as pore size decreases.

Archie, who was in fact far more than Archie’s equation, documented this relation in his 1952 publication (Classification of Carbonate Reservoir Rocks and Petrophysical Considerations) with a graphical display of Porosity vs Water Saturation.

With Porosity along the vertical axis and Water Saturation plotted horizontally, Archie found that discrete pore sizes trended along approximately hyperbolic lines of roughly constant product value (ie Porosity * Water Saturation = Constant): Figure 1. Larger pores corresponded to lower constants, because the (water wet) surface area / volume ratio decreased.




FIG. 1B    

 FIG. 1
Bulk Volume Water

Reservoir performance can often be evaluated in terms of the Bulk Volume Water

BVW = Sw * Phi

Contour lines of constant bulk volume water may be used as cut-off boundaries
Permeability estimates may also be possible in favorable situations
The graphic consists of Water Saturation versus Porosity. Depending upon local conventions, either attribute (porosity or water saturation) may be along the vertical axis, with the other being along the horizontal
In the Log-Log world (such as used in a Pickett Plot), these BVW trends are straight lines

The value of the Constant (Irreducible Bulk Volume Water) came to be called a
Buckles Number, in recognition of the Imperial Oil (Calgary, Canada) man who tabulated the values for different pore systems, and used them to identify
when water free production might be expected, even though the water saturation might be high,
water cut might be expected, even though the water saturation might be low.

While Buckles Plots are not so common in today’s computer interpretations, the underlying relation is behind the common depth-oriented display of Phi * Sw and Phi * Sxo.

                                                                 Pickett Plot

Pickett Plots are another example of pattern recognition formation evaluation that can, as with Bulk
Volume Water, also be quantified.

The controlling relation behind the Pickett Plot is Archie´s equation, which can be rewritten as follows:

                                                       Sw ^ n = Rw / [ (Phi^ m) * Rt ]

                                             Log [Sw ^ n ] = Log { Rw / [ (Phi^ m) * Rt ] }

                                             m*Log(Phi) = Log(Rw) - n*Log(Sw) - Log(Rt)

 In the simple case of Sw = 1.00, Log(Sw) = 0 and one is left with :

                                                      m*Log(Phi) = Log(Rw) - Log(Rt)

A log-log plot of Phi and Rt
(Figure 2) will be linear with a slope related to "m" ( or 1/m
depending upon the choice of axes) and an intercept related to Rw @ FT

Interestingly, John Doveton of the Kansas Geological Survey tells us that Pickett himself never referred to the protocol as a Pickett Plot, and that the name was in fact attached by others.


Roberto Aguilera remembers G. R. Pickett as a true Giant and a great professor of petrophysics. He always tried to teach us to stay away from what he used to call the cookbook school of thought.


FIG. 2

 FIG. 2
Pickett Plot

Points of constant water saturation will plot on a straight line with slope related to cementation exponent  "m"
Saturation exponent  "n" determines the separation of the Sw=constant grids
Rw @ FT can be deduced from graphic
The same technique can be applied to the flushed zones, using flushed-zone measurements


                                                                 Double Duty

Although Bulk Volume Water and Pickett Plots are less commonly seen today, than in years past, they remain powerful, quantitative pattern recognition tools.

Furthermore, as pointed out by Aguilera, it’s possible to combine the two concepts into a single graphic, thereby compounding the utility of the graphic, achieving Double Duty: Figure 3.

FIG. 3

 FIG. 3
Double Duty

                                         In the case of  m = n, the porosity term [ (m - n)*Log ( Phi ) drops out leaving
Log(Rt) = Log(Rw) - n*Log(BVW) = Constant

BVW = Constant grids are vertical
BVW lines below Sw = 100 % line are a mathematical extrapolation (for visual reference) and not physically realistic

At some specific value of BWV
, the Archie equation becomes

Phi) = Log(Rw) - n*Log(Sw) - Log(Rt)

Phi) = Log(Rw) - n*Log(BVW/Phi) - Log(Rt)

                (m - n)*Log(Phi) = Log(Rw) - n*Log(BVW) - Log(Rt)

In the case of m = n, the porosity term drops out leaving

                 Log(Rt) = Log(Rw) - n*Log(
BVW) = Constant

The BVW = Constant (single pore size) grids are vertical lines, on the Pickett Plot.

If “m" and “n" are not equal, the BVW grids are no longer vertical, as the porosity dependence in the above relation does not drop out, but there remains a constraint, and a pattern, that will appear on the Pickett Plot:
Figures 4 & 5.


FIG. 4      - Blu Arrow shows how Rw @ FT remains the same
                      - Red Arrow shows Grids of constant BVW shift

 FIG. 4
"m"  Not Equal to "n"
                                                                               m=2.0 / n=2.0 vs m=2.5 / n=2.0
‘m’ relates to pore system tortuosity, and as ‘m’ increases, the resistivity of a specific porosity (10 pu in the graphic) at Sw = 100 % also increases

                                                                                 Rw @ FT remains the same

                                                                                 Grids of constant BVW shift

BVW lines below Sw = 100 % are a mathematical extrapolation (for visual reference) and not physically realistic

FIG. 5      Black Arrow: The Sw associated with a specific Porosity and Resistivity increases as " n " increases

 FIG. 5
"m"  not Equal to  "n"

                                                                                 m=2.0 / n=2.0 vs m=2.0 / n=2.5

"n" relates to the tortuosity of the conductive phase and as Sw decreases the associated rise in resistivity of a specific porosity is greater than what would have occurred at a lower  "n" value.

Alternatively, the Sw associated with a specific porosity & resistivity increases  as "n" increases.

Note that in some cases, this Double Duty situation could even allow one to deduce "m" from a water leg analysis, and "n" from the hydrocarbon zone response (actually "m - n", but with "m" known from the water leg, it will be possible to deduce  "n" ).

                                                                       The Old and The New

Although the two (BVW and PP) concepts were once common, they are less commonly seen today, than 30 years ago. In fact, during a recent course presentation I was told that the Bulk Volume Water concept was only useful in ‘big Middle East Fields’: even though the individual who made that statement was almost certainly receiving NMR interpretations that included Bulk Volume Irreducible.

Today’s nuclear magnetic resonance measurements typically yield a mineral independent porosity and a pore size distribution (and potentially yet more): Figures 6 & 7.


FIG. 6  Mineral independent Porosity

 FIG. 6
                                                Mineral Independent Porosity
BVI - Bulk Volume Irreducible water which includes water retained by capillary forces in small pores, and water wetting pore surfaces.

BVM - Bulk Volume Moveable, (Free Fluid Volume) which is porosity available for hydrocarbon storage and fluid flow



FIG. 7    - Blue Arrow : BVI  ( Bulk Volume Irreducible Water )

              - Yellow Arrow :  BWM  ( Bulk Volume Moveable Water )        

 FIG. 7

                                              Mineral Independent Porosity
BVI - Bulk Volume Irreducible water which includes water retained by capillary forces in small pores, and water wetting pore surfaces.

BVM - Bulk Volume Moveable, (Free Fluid Volume) which is porosity available for hydrocarbon storage and fluid flow.



The NMR pore size distribution is often further characterized in terms of Bulk Volume Irreducible; the fraction of the porosity expected to be filled by capillary bound water, above the transition zone.
When Phi * Sw(Archie) ~ Bulk Volume Irreducible (NMR), water free production can be anticipated, even if Sw is relatively high (high capillary bound water).
On the other hand, low BVI (NMR) could signal water cut, even though Sw (Archie) is relatively low (but greater than BVI (NMR)). Be aware that specific cutoffs should be tailored to the reservoir / rock type in question.

In those cases for which we have conventional porosity and resistivity logs, plus an NMR, it’s now possible to combine The Old and The New, by annotating (in the “z" direction) the Pickett Plot with NMR T2 results, thereby compounding the value of the two perspectives.


With today’s tool technology and computer power, it’s easy to over-look the Tried And True Methods of Yester-Year. A better approach is to retain the traditional methods (as appropriate) and build upon them with today’s technology and computer power.
In addition to ensuring internal consistency of data / interpretations, one could conceivably develop improved evalualuation for legacy data which does not include the modern tools.


- Archie, G. E. Classification of Carbonate Reservoir Rocks and Petrophysical  
  Considerations, AAPG Bulletin Vol 36 No 2 (1952): 278 - 296

- Aguilera, Roberto, 1990, Extensions of Pickett plots for the analysis of shaly
  formations by well logs: The Log Analyst, v. 31, no. 6, p. 304-313

- Aguilera, Roberto , Incorporating capillary pressure, pore throat aperture radii,
   height above free-water table, and Winland r35 values on Pickett plots.
   AAPG Bulletin, v. 86, no. 4 (April 2002), pp. 605–624

- Aguilera, Roberto, Integration of geology, petrophysics, and reservoir engineering for  
  characterization of carbonate reservoirs through Pickett plots.
  AAPG Bulletin, v. 88, no. 4 (April 2004), pp. 433–446

- Pickett, G R, A Review of Current Techniques for Determination of Water
  Saturation from Logs," paper SPE 1446, presented at the SPE Rocky Mountain
  Regional Meeting, Denver, Colorado, USA, May 23-24, 1966; SPE Journal of
  Petroleum Technology (November 1966): 1425-1435.




                                  CARBONATE PETROPHYSICS CONSULTING


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                                          Carbonates Petrophysical Analysis,
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                                            RESEARCH & OPERATIONS

                                          Light / Heavy Oil / Gas / Geothermal
                                         Single / Multi-Well
                                         Vertical and Horizontal Orientations

                                         SPECIAL APPLICATIONS

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                                         - Petrophysics-Geostatistical Applications

                                         - Imaging interpretation techniques

                                         - Petrophysical Input to the Static Geological Modeling

                                         - MWD/LWD Interpretation, quality control, diagnostics



Predicting Reservoir System Quality and Performance.
Dan J. Hartmann and Edward A. Beaumont

Public Domain Databases
National Energy Technology Laboratory Public Database

United States Geological Survey Rock Catalogue
Kansas Geological Survey Gemini Rock Catalogue

Kansas Geological Survey Abyss Rock Catalogue

Reference Material

Bureau of Economic Geology, University of Texas

Ross Crain's On-line Tutorial

Kansas Geological Survey (John Doveton) Tutorial

Schlumberger (OilField Review, ME Well Review, Oil Field Glossary)

Baker Hughes (InDepth Magazine, Oil Field Glossary)

Interactive Periodic Table


  GeoNeurale Contact



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