Well Log Analysis & Consulting

About Us

Everett Petrophysics specializes in mineral-based well log interpretation For 28 years we have been providing consulting, coaching and analytical

services globally; previous 27 years spent with Schlumberger. Software has been developed to use Nuclear Spectroscopy (elements) and

Nuclear Magnetic Resonance (total and free porosity) to derive grain density, exponents m & n, Rw, permeability, porosity and saturation.

ROBERT (BOB) EVERETT P.Eng Over 55 years of experience using petrophysics analytical techniques to interpret oil and gas well logs Provided consulting services for Unocal, Z and S, Dresser, Baker Hughes, Schlumberger and the University

of Texas at Austin, as well as many international Energy companies B.Sc. In Mechanical Engineering

James (Jamie) Everett, B.A., M.Sc. 30 years experience developing highly specialized software Development of software for companies such as Verity, Autonomy and Hewlett Packard B.A. with High Honours in Physics as well as an M.Sc. in Biomedical Engineering

Analytical Services

Calculation of full mineralogy: full mineralogy is possible with elements from Nuclear Spectroscopy Important as the attributes of the minerals are used to provide a unique

solution and has value in obtaining the best petrophysical analysis possible accurate porosity from accurate grain density Accurate Sw over full range of Sw Better permeability without special shift-constants Provides results in same units as core analysis such as weight fractions of

minerals A system that can be relied on

Analytical Services

Calculation of full mineralogy: the best way to analyze logs

Aspect of analysis 2: rather than using Vsh, which is not measurable in routine core analysis, use Si, Ca, Al, Fe, Ti, S, Gd which are measurable. Convertible to minerals, CEC, m, n, Grain density etc.

Aspect of analysis 3: logs respond to minerals and elements; makes sense to use them

Our PhilosophyMinerals Are the Cornerstone Of

Efficient Log Interpretation

Conventional Approach vs Our Approach

Data Collected Guess Vsh,matrix Use Vsh Guess Sw

Data Collected Derive grain density

Derive m, n, Rw

Sw = core

Type of results: Vsh is empirical, wrong Sw conclusions, uncertainty is high

Type of results: Derive from core-based data, best conclusions, lower uncertainty

Conventional approach

Our approach

Why use guesswork from 1950’s when better methods available?

Example Case 1: Duvernay

Project Details: Duvernay Unconventional

Conventional Vsh results were valid only at low Sw

What services we provided: Offset Nuclear Spectroscopy added

What info we added: cation exchange capacity from each clay family results in

full range of Sw

CONCLUSION: Mineral-based interpretation works

Example Case 1: Duvernay

Conventional Vsh results were valid only at low Sw

Example Case 1: Duvernay Cation exchange capacity from each clay family results in full range of Sw

Example Case 2: Tight gas

Project Details: tight gas environment

Conventional Vsh results missed the zone

Mineral-based results located the zone

We added Nuclear Spectroscopy from offset well

CONCLUSION: Mineral-based interpretation works

Example Case 2: Tight Gas Conventional Vsh results missed the zone: Sw 100%, PHIE < 5%

Example Case 2: Tight Gas Conventional Vsh results missed the zone: Sw 100%, PHIE < 5%

NOTE: THE POINT IS SERIOUS ERROR IS POSSIBLE WITH A VSH MODEL. Depends too heavily on analyst’s preconceived ideas of pay/no pay.

WEAKNESS is Rw, m, n, Vsh, PermAll “guessed” with empirical models

Example Case 2: Tight gas

Mineral-based results located the zone: SW 30%, PHIT 12%: Productive with Frac.

Mineral-basedCalculation ok

elements/mineralsProvide cec, GD, perm& constraints for m, n.

STRENGTH is Rw, m, n, GD & Perm:All are Internally calculated,originally from core data

APPENDIX: Example How to obtain m, nSw calculation:

• The saturation equation used is called a Dual Water Equation, from the paper by Chris Clavier et al, (Ref 5).

• The components of the equation are:

m_zero, which is the cementation factor, dependent on m* (m_star), the Waxman-Smits cementation factor:

IF((m_star<=2.0356),(m_star/(0.1256*m_star+0.7781)),((m_star/(0.3764*m_star+0.2694)))), where

m_star=(1.653+(0.0818*(Surface area*RHOG)^0.5)), where

Surface area (SO) =sum of specific SO of each mineral. This is a very critical part of the calculation.

n_zero = tortuosity factor = m_zero =

IF((m_star <=2.0356),( m_star /(0.1256* m_star +0.7781)),(( m_star /(0.3764* m_star +0.2694))))

N_zero and m_zero are used in the Dual Water saturation equation; M_star would be used in the Waxman-Smits-Thomas equation, but we used the DW equation. In the lab, both DW and WS give the same results. However, DW also gives Swb, the clay water saturation. We use Swb as a quality control check because Swt cannot be lower than Swb.

APPENDIX: Example How to obtain SwWater Saturation Detail

Water saturation is calculated from the Dual Water equation, using cation exchange capacity to calculate Qv.

[See Clavier, C., G. Coates, and J. Dumanoir, 1977, the theoretical and experimental bases for the ''dual water'' model for the ... SPE Paper 6859, Society of Petroleum Engineers 52nd Annual Fall Meeting, Denver, Colorado, October 9–12, 1977. Dewan, J. T., 1983, Essentials of modern open-hole log interpretation: Tulsa, Oklahoma, PennWell Publishing Company, 361 p. ... Hill, H. J., O. J. Shirley, and G. E. Klein, 1979, Bound water in shaly sands: Its relation to Qv and other formation ...]

From the Petrophysics Designed to Honour Core (PDHC) program:

CEC = Sum(Wi*CECi)

Qv = (CEC/100*Rhog*(Rhob-1)/(Rhog-1)/Tpor)

M = salinity/Rhof_bw/58.45, where salinity is the formation water salinity and Rhof_bw is density of bound water, usually 1. [Molarity.]

W = (0.22+(0.084/M^0.5))

Swb = W*Qv

B = 0.03772*Temp_degF-0.6516

Cb = B*Qv/Swb

F = F_Ghanbarian or 1/TPORm_zero

Cw = 1/Rw_SP_used

TC_DW = ((Cb*Swb-Cw*Swb)/F)) [Term c in quadratic]

Co = (Cw/F)+TC_DW

Ro = 1/Co

Ct = 1/Rt

Swt = (Ct/Co)n_zero

Hence,

Swt = (Ct / ((Cw/F)+ ((0.03772*Temp_degF-0.6516*Qv/ (0.22+(0.084/ salinity/1/58.45^0.5))* (CEC/100*Rhog*(Rhob-1)/(Rhog-1)/Tpor) * (0.22+(0.084/ salinity/1/58.45^0.5))* (CEC/100*Rhog*(Rhob-1)/(Rhog-1)/Tpor) -1/Rw_SP_used * (0.22+(0.084/ salinity/Rhof_bw/58.45^0.5))

*Qv)/F)))) n_zero

APPENDIX: Example How to obtain Rw: step 1

The Rw_SP_USED is calculated in two parts. First an estimate is made, called RW_SP, from an estimated field value and a baselined SP. Second, this value is refined so that after a full ECS calculation is made, the CEC-Corrected Ro matches the recorded deep resistivity. The Rw process is repeated until a satisfactory match of Ro and Rt is obtained. One has to be careful as the density tool has a different vertical resolution than the deep resistivity. Hence, some experience is useful.

APPENDIX: Example How to obtain Rw: step 1

• Rw_SP is derived from an estimated field value and a based lined SP. The trick is how to baseline the SP.

• Other methods baseline the SP by drawing a line from whatever is considered shale, called a shale baseline.

• This method ‘calculates’ and average zero from the SP equation. This step is important as shales have an SP as they are never zero.

APPENDIX: Example How to obtain Rw: step 1a

• First, calculate temperature from this equation or any other that provides a geothermal reservoir temperature. Depending on circulation time, the geothermal temperature sis expected to be 10F to 20F above maximum recorded logged temperature.

• 0.0198*depth in ft + 42.8 degF • Modify as you see fit.

APPENDIX: Example How to obtain Rw: step 1b, SP_ZERO

• Next calculate the estimated field Rw using the temperature gradient; standard formula

• Rw_known = 0.025*(75+6.77)/(TEMP_DEGF+6.77)• Now calculate Rmf using the same formula

0.04*(87+6.77)/(TEMP_DEGF+6.77)• Then SP_ZERO = (61+0.133*TEMP_DEGF) *LOG(RMF/RW_KNOWN)+add• The “add” is a constant to make the average value zero. This SP_ZERO is

the key step.

APPENDIX: Example How to obtain Rw: step 1c, SP_shift & SP_baselined

• Next, calculate SP_SHIFT = SP + add2• Where add2 is zero to start with but will be changed later.• Now SP_BASELINED = SP_SHIFT-SP_ZERO• Finally RW_SP = RMF/(10^(SP_BASELINED/(-1*(61+0.133*TEMP_DEGF))))• Now test to see if Rw_SP comes close to Rw_KNOWN.• If not, supply a new add2 & iterate until they are close. See plot next.

APPENDIX: Example How to obtain Rw: step 1c, shift required

APPENDIX: Example How to obtain Rw: step 1c, shift made

Add2=+40

Now RW_SP_CALCset to RW_SP

APPENDIX: Example How to obtain Rw: step 2, compare Ro to Rt; re-shift

Dashed Rw_SP_USEDAfter “add2”changedso that Ro=~Rtat arrowi.e. at low Resistivity, water or shale,since Ro has CECcorrection.

APPENDIX: Example How to obtain Rw: step 2, compare Ro to Rt; re-shift

Dashed “Rw_SP_USED” after “add2”changedso that Ro=~Rt at blue arrow.i.e. at low Resistivity, water or shale,

since Ro has CEC correction.Now we have a Rw that can be used for theentire well as it is modulated by the SP.This is an oil-base mud & SP was predicted!

Conclusion

Why this information is important: log interpretation used to be more art than science;

times have changed.

Additional point: with Nuclear Spectroscopy and Nuclear Magnetic Resonance, better

science in interpretation has resulted: all parameters initially internally computed

without analyst bias.

When science-based interpretation is available, why not use it?The conventional Vsh approach was all we had before 1980s but now we have a better, more accurate way to provide Petrophysics Designed to Honour Core

Contact Information

Robert V Everett PetrophysicsConsulting, Teaching, Coaching

AddressPO Box 271 – 1589 Nurmi RoadMerville BCV0R 2M0

Robert: 250-442-9696 / roberteverettsupercomputer@gmail.comJamie: 403-620-2403 / jamie@everett-energysoftware.com