How to Assess Shales From Well Logs - March 2012 IOGA Talk

8/9/2019 How to Assess Shales From Well Logs - March 2012 IOGA Talk

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Property of interest Core data Log data

Porosity Crushed dry rockHe porosimetry

Density (mostly)

TOC LECO or RockEval GR, density, resistivity

Water saturation As-received retortor Dean-Stark

Resistivity + kerogencorrected porosity

Mineralogy XRD, FTIR, XRF Density, neutron, Pe,

ECS-type logsPermeability Pulse decay on

crushed rockThis is tough

Geomechanics Static moduli DTC, DTS, RHOB, &synthetic substitutes

Geochemistry Ro, S1-S2-S3, etc. Resistivity (sort of)


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Porosity first

How much storage volume is there for freegas (or oil)?

Saturation next

Can we estimate Sw in shales?

Then Permeability

Is there sufficient permeability to flow gas, orbetter yet oil?

GIP & oil-in-place last


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SimplifiedRock

ClasticRock

ClasticComponents

Porosity

Effective porosity, Phie

Total porosity, Phit

Bulk volume water

solid matrix

porosity

non-

moveable

fluids

non-mov.

fluids

moveable fluids

grains

porosity

water

residual

HChydrocarbons

mud,silt

dryclay

OH-

water

water

matrixquartz, feldspar, etc. cl

ay-

bound

residual

HChydrocarbons

clay&OH-

capillary-bound

freewater

Bulk volume hydrocarbon

Solidity, 1 - Phit

Grain volume

Organic ShaleComponents

OrganicShale

mineral matrixkerogen

water

dryclay

OH-

cla

y-

bou

ndclay &

OH-

gas, oilwater

gas, oil

porosity

Freewater

quartz, carbonate,pyrite, etc.

kero

gen


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In organic shales, TOC looks likeporosity to a density log

Density of organic matter is close to thatof water or oil

Ranges from 0.9 to 1.4 g/c3 in ourexperience

Common default value is 1.3 or 1.35 g/c3

Conseq uenc e : density porosity is toohigh and needs to be corrected down


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Estimate from GR or density log

Estimate from deltaLogR (sonic-resistivity

overlay method) Run a specialty log to get a kerogen-free

grain density, and compare to total

grain density from density-neutron log Get it from cores


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Basically a sonic F overlay

Two universal eqns published by Passeyet al. (1990, AAPG Bull)

logR = log10(R/Rbl) +0.02(T-Tbl)This defines deltaLogR

TOC =logR * 10 (2.297 0.1688LOM)

This relates deltaLogR to TOC (wt%)

Most of us are concerned about theLOM parameter, but the second twoconstants were empirically determined


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Passey et al, 2010, SPE 131350


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TOC (wt %) =logR * 10 (2.297 0.1688*LOM)Pa sse y e t a l, 1990,

AAPG 74 (12) 1777-1794


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Deterministic suiteof regressioneqns to computemineral volumes

and kerogen-freegrain density

RhoMecs= a + b Si + c (Ca,Na) + d (Fe,Al )+ e S


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Not as simple as standard densityporosity determination

We need TWO grain densities, Density of the inorganic mineral matrix

Density of the organic fraction (a.k.a.kerogen density or TOC density)

And, we need to estimate the fluiddensity in the flushed zone (where thedensity log makes its measurement)


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(1 * / )ma b TOC TOC ma TOC T

ma fl

W W

+=

So nd e rg e ld e t a l, 2010, SPE 131768

(1 ) (1 )b hc T wT w T wT ma T TOC TOC TOC

S S V V = + + +


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Intercept at 0% porosity givesRhoMa inorganic = 2.76 g/c3


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New Albany example

Use core measured porosity, RhoB, andWtoc to solve for RhoTOC

RhoFl requires assumption about gassaturation and gas density at in-situconditions

(12.72

2.72

* 2.72 / )TOC TOC TOC bT

fl

W W

=

+


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Full solution for porosity

(1 * / )m b TOC TOC m TOC T

m fl

W W

+

=


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TOC correctedporosity


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Convert Wtocto volume of TOC using anassumed RhoTOC

Subtract Vtocfrom density porosity to get

a rough kerogen-corrected shaleporosity

/ *toc toc toc bV W =


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Calibrate a log model to as-receivedcore saturations

Best data we have for in-situ Sw Rw in shale is a guess

Usually assume an m and n, often 2 orsometimes less

Remarkably, we can usually fit the coredata without doing anything toostupid.


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GR porosity Sw TOC


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Perm is a can of worms

We are not very confident we canmeasure shale perms in core, but its the

best we have for now Log models for perm are weak at best

Most use phi-K correlation based on core


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Two components

Free gas or compression gas

Adsorbed gas

There is also some absorbed gasdissolved in kerogen or bitumen, but theway we calibrate the adsorption model

this is inside the other number


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Need to know porosity, saturation, & h:

w he re C is a un its c o nsta nt , A is a re a , f is p o ro sity , h is

th ic kne ss, Sw is w a te r sa tu ra tio n, & Bg is the fo rm a tio n

vo lum e fa c to r

High pressure, high porosity, high Sg, andlarge A greater GIP

* * (1 ) /w gGIP C A h S B=


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Adsorption is a surface phenomena

Characterized by adsorption isotherms:

Where Gcis the adsorbed gas content (scf/ton)

VLis the Langmuir volume (scf/ton)

PLis the Langmuir pressure (psia)

Where Ga bis the adsorbed gas-in-place (scf), A is area in acres,

h is thickness in ft, bis bulk density, and Gc is the averageadsorbed gas content in scf/ton

* / ( )L LGc V P P P= +

1359.7* bGab Ah Gc=


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Kerogen is highly adsorptive

Wet clays are minimally adsorptive

Quartz, calcite, dolomite: adsorption is nil


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Compute free gas

Compute adsorbed gas

Correct for volume of free gas occupiedby adsorbed gas (SPE 131772)

Add em up

NGLs and condensate are calculatedfrom the GOR


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Assume a vertical well tests 10 ft of a lowporosity zone at 2 BOPD.

Well go horizontal in that zone and drill

5,000 to 10,000 ft laterally

Kh scales with contacted interval, so weexpect 500 1000X the flow capacity

Real world = not that good. Not all ofthe lateral contributes equally.


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You can calculate porosity andsaturations in shales; they make senseand match core data

The apparent density porosity is too highin the presence of TOC

Dont try to just eyeball it, there are too

many moving parts Log analysis on the hood of a Chevy does

not work well in shales


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How to Assess Shales From Well Logs - March 2012 IOGA Talk

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