Evaluation of Thin Beds

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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)

, Vsand, Vsh.disp

2. Resistivity of the sand lamination

3D-res 2000s

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. 395405.

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

2

3 < Neutron > 2

GR

When to apply Thin Beds

Conductive dispersed shale?

3

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