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Confidential

Shaly sand reservoirs-Impact on Porosity and Water Saturation;The Dual Water Model

Rubi Rodriguez

Sr. Petrophysicist

07th March 2019

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Introduction

Shale definition

Shale Effects on Logs

Porosity & Saturation

Dual Water (examples)

Summary

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Evolution of shaly sand models

1927

1942

19501953

19681984

Dual Water model by Clavier

Waxman & Smit model

“Double Layer” concept by W.M.McCardell

Archie’s law doesn’t apply to shaly sand by M.R. Wyllie

First electric Log by Conrad & Marcel

Schlumberger

Archie’s law published

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Archie’s equation

• First quantitative log analysis methodology

• Relating resistivity, porosity and saturation

The shaly sand effect

• Formation factor (F) was not intrinsic rock property, but instead was seen to vary with water salinity due to clay

• X called extra conductivity

The problem

“The most important problem that has received thus far no satisfactory

solution is that of shaly sands” - Henri Doll 1953

Water ConductivityR

ock

Con

duct

ivity

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How much hydrocarbon is there?

STB

RB=B

B

)hASφc=OOIP w

0

0

1 1( −OOIP=original oil in place

c1=7758 units conversion

h=height

=porositySw=water saturation

A=area

B0=oil formation volume factor

RB=reservoir barrels

STB=stock tank barrels

Shale presence in hydrocarbon reservoirs has a large impact on estimates of reserves

and producibility

The clay minerals present in the shale complicate the determination of and Sw

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Introduction

Shale definition

Shale Effects on Logs

Porosity & Saturation

Dual Water (examples)

Summary

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Type of Rocks

Earth

IGNEOUS

SEDIMENTATY

METAMORPHIC

SedimentaryClastic

Chemical

Conglomerate - Coarse-grained,

mixture of rounded pebbles and sand

ranging widely in size; well rounded

pebbles imply some transport in a

high energy system

Sandstone- commonly quartz,feldspar, or rock fragments;deposited in many environments

Shale, very fine grained; composedprimarily of clay; deposited in low-energy environments such as lakes,bays, lagoons, of deep marinesettings

Sedimentary Rocks- Most important for the oil industry as

they contain most reservoirs, seals and source rocks.

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Rock Classification

Clastic Rock Type Particle diameter

Conglomerate Pebbles 2 - 64mm

Sandstone Sand .06 - 2mm

Siltstone Silt .003 - .06mm

Shale Clay <.003mm

b N (thermal) Pe

Kaolinite 2.54 59.6 1.85

Illite 2.52 47.9 3.97

Smectite 2.02 87 1.70

Chlorite 2.73 59.6 4.07

Four main clay minerals, Kaolinite,Illite,Smectite as the most

common, with widely varying different log responses

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Introduction

Shale definition

Shale Effects on Logs

Porosity & Saturation

Dual Water (examples)

Summary

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They are often radioactive

They have porosity and it is filled with salted water

Resistivity logs exhibit shales as low resistivity

They show high Gamma Ray value

They exhibit high porosity in the Neutron porosity

logs

Density and sonic logs react to the porosity and the variation in the grains

Shale and Logs

Shales have properties that have important influences on log

readings:

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Lithology in shaly sands

The volume of shale (Vsh) in a sand

is used for the evaluation of shaly

sand reservoirs

GR - GRclean

GRshale - GRclean

Vshale =

Vsh can be computed from several

methods:▪ Gamma Ray

▪ Neutron-Density

▪ Spontaneous Potential

Most traditional method

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Lithology in shaly sands

Geochemical Logging tool is essential

in the evaluation of complex shaly

sand reservoirs

Elemental Capture Spectroscopy

(ECS)

*The figure compares elemental weight fractions measured by the

tool (black lines) to those derived from core analysis (red points).

*Radtke, Lorente, et al- A NEW CAPTURE AND INELASTIC SPECTROSCOPY TOOL TAKES

GEOCHEMICAL LOGGING TO THE NEXT LEVEL-SPWLA 2012

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Introduction

Shale definition

Shale Effects on Logs

Porosity & Saturation

Dual Water (examples)

Summary

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Porosity in shaly sands

▪ Porosity is defined as the ratio of pore volume to its total volume

Fluid

Matrix

(1-)

1 - = amount of rock (matrix) per unit volume

= amount of fluid per unit volume

𝜑 =Fluids volume

Total volume=𝑉𝐹𝑉𝑇

▪ Effective porosity (PHIE) is the pore space in the formation that is

neither in the shale nor in the clay

▪ Total Porosity (PHIT)

Where is the total porosity of shale/clay fraction

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Porosity in shaly sands

▪ The way shale is distributed through the reservoir rock

will have an impact on porosity

▪ Ignoring distribution leads to reservoir potential being

overestimated

▪ Shale distribution types: Laminar, Structural, Dispersed

▪ Quantification of shale distribution can be done using

*Thomas-Stieber model (among other models)

*THE DISTRIBUTION OF SHALE IN SANDSTONES AND ITS EFFECT UPON

POROSITY,

E. C. Thomas and S. J. Stieber, SPWLA SIXTEENTH ANNUAL LOGGING

SYMPOSIUM, JUNE 4-7, 1975

.

Clean formation Structural shale

Porosity

Porosity

Matrix

Matrix

Porosity

Matrix

Porosity Shale

Shale

Matrix

Porosity

Matrix

Laminar shale Dispersed shale

Sh

ale

Sh

ale

Total porosity vs Shale

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Lithology & Porosity

ECS ELAN+ECS

Integration of geochemical log (ECS) with multimineral solver (ELAN)

provides a better lithology evaluation hence more accurate porosity

▪ Elemental Log Analysis (**ELAN)

provides advanced multimineral log

analysis capabilities

▪ The mineral solver doesn’t rely on a

model to derive the clay volume

▪ It is a simultaneous equation solver

that combines log measurements and

parameters in response equations to

compute volumetric results

*Example shows a complex shaly sand formation (with calcite-

cemented zones) from Nile Delta.

*Van Steene, SPE 127965. ** Quirein et al,SPWLA 1986

Eeffective

Porosity

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◼ Any measurement that displays a contrast between water and

hydrocarbon properties can be used to estimate saturation

◼ Resistivity is dominant method for saturation evaluation from logs

◼ Archie’s Equation- works well for clean environments

◼ In shaly environments the basic assumption of non-conductive

formation does not apply

◼ Other methods needed to be found

Saturation in shaly sand

w

n

S =a

mRw

Rt

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Saturation in shaly sand

Which saturation equation?

◼ Many equations have been

developed to account for the

electrical properties of clays:

◼ Most of them are area specific

◼ They require Vsh

+

=

Sw

QvBRwRt

RwSw

m

n

1

1*

*

*

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Introduction

Shale definition

Shale Effects on Logs

Porosity & Saturation

Dual Water (examples)

Summary

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Dual Water Model

◼ Dual water model attempts to correct for

excess clay conductivity in the sands

especially at low formation water salinity

◼ It splits the conductivity of a water filled

rock into water conductivity and clay

conductivity

◼ Dual Water model refines Waxman-

Smits model by removing a fraction of

the pore volume water with clay water

◼ It divides the clay into dry clay and its

associated water, called bound water.

This avoids the necessity of determining

Qv.

Fo = F*(1 – vqQv)

Fo = Dual Water formation resistivity factor Vq = volume of water per counterion (cc / meq)Qv = charge concentration per unit pore volume, (meq / cc P.V)

Clavier, Coates and Dumanoir (1984)

SPE paper 6859

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Dual Water Model Concept

◼ The model divides the formation into

solids and liquids

◼ It proposes two distinct waters to be

found in the pore space; Free &

Bound water

◼ It suggests that the amount of bound

water is directly related to the clay

content

◼ In hydrocarbon bearing zones the

model leads to the following

relationship (in bound-water terms)

as:

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Saturation in shaly sand (example)

◼ Shaly sand zoneArchie (blue) shows the highest Sw

as Vsh increases (black), Swt from

D-W (green) reflects the shale

contribution and Swe from D-W (red)

shows the lowest Sw in the shaly

zone

◼ Clean sand zoneArchie and Dual Water give about the

same Sw value when Vsh is zero.

Swt=Swe because Swb is zero in

clean formations.

Vsh

Swe DW

Swt DW

Archie

Vsh increase

Shaly sand

Clean

sand

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Saturation in shaly sand (example)

Water Saturation (Sw) HPV=PHI*So*H

Zone Vsh Av PHI Archie Simandoux Dual WaterHPV

Archie HPV Sim HPV DW

% v/v v/v v/v v/v m m m

A 10 0.23 0.24 0.22 0.18 0.52 0.54 0.57

B 2 0.25 0.33 0.32 0.33 0.50 0.51 0.50

C 20 0.2 0.32 0.24 0.21 0.41 0.46 0.47

D 30 0.18 0.41 0.26 0.23 0.32 0.40 0.42

All 1.75 1.90 1.96

+ 12% reserves index

Dual Water model provides an increase of 12% in reserves compared

to Archie’s equation (in this specific case)

The difference in Hydrocarbon Pore volume (HPV) computed using

water saturation from Archie, Simandoux and Dual Water equations in

shaly sand reservoirs is not negligible

Where H=3m

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Introduction

Shale definition

Shale Effects on Logs

Porosity & Saturation

Dual Water (examples)

Summary

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Saturation in shaly sand- Model Validation

◼ All saturation models should be validated with core data, well test

data (DST) and production information

◼ The integration of conventional logs with more sophisticated

measurements like NMR, Dielectric and Spectroscopy will reduce

the uncertainty of the petrophysical model

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Summary

◼ Most sandstone reservoirs contain some clay or shale, which

reduces the effective porosity, lowers the permeability, and

alters the resistivity predicted by Archie’s equation

◼ Shaly sand evaluation is a complex task and a good geological

and local knowledge is important

◼ Integration of all available data is key in assessing the most

reliable petrophysical models in shaly sands evaluation

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