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Improved Workflow for Evaluation of Thinly Bedded Sandstones Revisiting the Normalised Qv equation of Juhasz

Jan van der Wal & Simon Stromberg

Thin Beds – Introduction 2/19

• What are ‘Thin Beds’?

• Laminations of sand and shale, with..

• .. beds so thin that logs do not read true properties.

• Why do we care? In Thin Beds..

• .. conventional evaluation can miss pay,

• .. phi & perm are too low,

• .. resistivity reads too low, and

• .. saturation height functions give too low HC

Case Study data – where is the HC?

Vshale

Original

Publication

Neutron &

Density

Deep Res

What steps in Thin Bed workflow? 4/19

• Aim: properties of sand lamination

1. Thomas Stieber (1975)

• Φ, Vsand, Vsh.disp

2. Resistivity of the sand lamination

• 3D-res 2000’s

• Rsand

3. Saturation computation

• Juhasz 1981

Definitions

• Shale types:

Step 1 6/19

Thomas & Stieber, 1975

• 2 endpoints + 1 P

oro

sit

y

Volume of Shale

Clean Sand

‘Pure Shale’

Step 2: 7/19

Resistivity of Sand Laminations 3D-res

• 2a) Tensor Model

• Horizontal and Vertical, or

• Parallel Conductivity and Serial Resistivity

• 2b) Anisotropy Model

• Smart Tensor Model;

• Inputs Thomas Stieber

Res HOR

+

Re

s V

ER

Step 3 8/19

Saturation Calculation

• Which saturation equation?

• Conventional (deterministic):

• Laminated Shaly sand eqs:

• Poupon, Indonesia, Simandoux

• Thin Beds (dispersed clay/shale in sand lamination) :

• Dispersed Shaly sand eqs:

• Dual Water, Waxman Smits, Normalised Qv Juhasz

sh

shn

w

m

w

sh

t R

VS

R

V

R

11

Poupon, parallel conductor

Saturation from Resistivity 9/19

• Waxman Smits equation

• For waterleg assume SWT = 1, (and a*=1):

Shale corr Archie

Waxman Smits in Xplot 10/19

Y = a X + b

Slope B

Y =

X =

(cousin of Pickett-plot)

Juhasz 11/19

• Juhasz: if no core Qv available,

• Qv = f(Vshale)

100% Shale

1/Rw_shale

1/Rw ~ C

onductivity

Qv_shale

Juhasz Normalised Qv 12/19

• Juhasz: Qv = f(Vsh), or Qv = f(Phi), f(1/Phi)

• Qv = f(1/Phi, Vsh) = f(RPD), (similar to ~Qvn)

• Relative Porosity Difference

• Assume Qv = RPD*C, substitute

To better pick BC 13/19

1/Rw

RPD

Slope B*C

= CWA

To better pick Rw 14/19

• Terms divided by RPD:

BC

1/RPD

Slope 1/Rw

CWA/RPD

Data Example 15/19

BC Rw

RPD in Thin Beds? 16/19

• RPD of Bulk not good enough

• We require RPD of sand lamination (RPDs)

• RPDs = f(1/PHIs, Vsh.disp), or

BVirr

H20 HC

POROSITY

Conv

POROSITY

Conv &

Par.Cond.

POROSITY

Thin Beds &

3D-Res

Vshale

Original

Publication

Neutron &

Density

H+V Res &

Parallel

Conductor

Case Study, data of Clavaud

Summary Workflow 18/19

• In case of: 3D resistivity, no core, water leg

• Optimise Thomas Stieber with Tensor Model

• Resistivity Sand from Anisotropy model

• Relate Qv to RPD

• Compute RPD for sand lamination only

• New Xplots for picking Rw and ‘BQv’

Conclusions 19/19

• Workflow can be based on log data only

• New form of Norm Qv of Juhasz applied to thin beds

• Qv estimate refined

• Conventional: low HC

• Conventional with 3D res (Parallel Res): more HC

• Thin Beds with 3D res: most HC

References

• Clavaud, J. B., Nelson. R., Guru, U. K. and Wang, H., 2005, Field Example of Enhanced

Hydrocarbon Estimation in Thinly Laminated Formation with a Triaxial Array Induction Tool: A

Laminated Sand- Shale Analysis with Anisotropic Shale, SPWLA 46th Annual Logging Symposium,

June 26-29, 2005.

• Thomas, E. C., Stieber, S. J., 1975, The distribution of shale in sandstones and its effect on

porosity. Transactions of the SPWLA 16th Annual Logging, Symposium, June 4-7, 1975.

• Juhasz, I., 1981, Normalised Qv. The Key to Shaly Sand Evaluation using the Waxman-Smits

Equation in the Absence of Core Data. SPWLA 22nd Annual Logging Symposium, June 23rd-26th,

1981.

• Cao-Minh, C., Clavaud, J., Sundararaman, P., Froment, S., Caroli, E., Billon, O., Davis, G. &

Fairbairn, R., Graphical Analysis of Laminated Sand-Shale Formations in the Presence of

Anisotropic Shales, 2008, PETROPHYSICS, Vol 49, No. 5, October 2008, pp. 395–405.

• Stromberg S., Nieuwenhuijs R., Blumhagen, C., Edwards, J., Ramamoorthy R., Herold, B.,

2007, Reservoir Quality, Net-to-Gross and Fluid Identification in Laminated Reservoirs from a new

generation of NMR logging tools. Examples from the Gharif Formation, Southern Oman.

Transactions of the SPWLA 1st Annual SPWLA Middle East Regional Symposium April 15- 19.

• Waxman, M.H. & Thomas, E. C., 1974. Electrical Conductivities in Shaly Sands-I. The Relation

between Hydrocarbon Saturation and Resistivity Index; II. The Temperature Coefficient of Electrical

Conductivity. J. Pet Tech. 213-23. Trans., AIME, 257.

What is RPD?

• Middle East for Carbonate stringers (PDO)

• Shaliness indicator

• Combines 1/PHIT and Vshale (~ Neu-Den separation)

• RPD= (Neu + Co – PhiT)/ PhiT

• How to get ‘Co’

• Use ND overlay

• For clean sand: RPDs ~ 0

• Clean but conduct: RPDs > 0

What if no 3D resistivity available?

• Make cases for vertical resistivity

• Check with Thomas Stieber

• Simplest: Rv = Rh * C

• Better: Rv = Rh * C * Vsh_lam, or

• Rv = RH + (RH – RshH)*RatioMax*Vshl

• RV = RH + 1/ ((1/RshH – 1/RH) * Vshl)

What is Parallel Conductor model?

• Ct = Vsand * Csand + Vsh.lam * Cshale, or

• 1/ RT = Vsand / Rsand + (1-Vsand) / Rsh.hor

Res HOR

When to apply Thin Beds

• Neutron Density Data

• Intermediate GR?

• Dispersed, Laminated, or

Both?

3 <

D

ensity >

2

3 < Neutron > 2

GR

When to apply Thin Beds

• Conductive dispersed shale?

3 <

D

ensity >

2

3 < Neutron > 2

DeepRes

What if you do have core?

• Are plugs of the sand lamination?

• Porosity:

• Calibrate Clean Sand endpoint to match the high porosity

• Optimise input PHIT

• Calibrate BC & RPD to match the predicted QV curve