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Pulse Neutron Neutron (PNN) tool – logging for porosity

Some theoretical aspects

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Rules For The Direction Of The Mind-Descartes, Rene

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RULE IThe aim of our studies must be the direction of our mind so that it may

form solid and true judgments on whatever matters arise.

RULE IVThere is need of a method for investigating the truth about things.

RULE XX Having found the equations, we must perform the operations which we have left out, never making use of multiplication when there is scope for

division.

RULE XXI If there are several such equations, all of them must be reduced to a single one: namely to the one whose terms occupy fewest places in a

series of magnitudes in continued proportion; and its terms must be set out in the order followed by the series.

Introduction

• Response of neutron logs:– borehole size, type of fluid in the borehole or the position

of the logging instrument in the borehole, effect of mudcake in open-hole logging and cementing in cased-hole applications.

• On a relative basis the strong attenuator for both epithermal and thermal neutrons is the liquid-filled borehole and a weak attenuator is the formation.

• To increase sensitivity to the properties of the formation (or decrease borehole sensitivity) the detector should be positioned as far from the source as possible, allowing those neutrons which travel toward the detector within borehole to be attenuated relative to those traveling in the proper direction within the formation.

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Theoretical Considerations

0.2

. SD epithlrepithermaepithepith

0.2

. epithermallrepithermathermalrthermalermalthermalthD

r

e

D

Qr

Lepith

r

epithepith

4

)(

Slowing down length Lepithermal and thermal neutron diffusion length Lthermal are defined by:

r

e

r

e

LLD

QLr

Lthermal

rlLepitherma

r

thermalepithermalthermal

thermalthermal )(4

)(22

2

Solution of two equations for the case of a point source in an infinite homogeneous medium is:

Two group neutron diffusion equations:

Φ - neutron flux, S is the neutron source strength per unit volume, D is the diffusion coefficient and Σr is removal cross section.

Q-is the point neutron source strength (neutron/sec), r is distance source-detector.

lrepitherma

epithermalepithermal

DL

rthermal

thermalthermal

DL

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Theoretical Considerations – cont.

Theoretical values for silicate sandstone 30 pu, 100000ppm of NaCl. Theoretical values for silicate sandstone 5 pu, 100000ppm of NaCl

Red color epithermal part of equation, blue color thermal part of equation , black color is difference between epithermal part and

thermal part of equation.

r

e

LLD

QLr

lLepitherma

r

thermalepithermalthermal

thermalthermal )(4

)(22

2

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Theoretical Considerations – cont.

r

e

LLD

QLr

lLepitherma

r

thermalepithermalthermal

thermalthermal )(4

)(22

2

Although the spatial shape of this expression is governed by slowing down length, the magnitude of thermal neutron flux is still scaled by thermal neutron parameters. Clearly, the essentially all dependence of a thermal neutron measurement upon thermal neutron parameters can be eliminated simply by making measurements at two sufficiently distant points r1 and r2 and taking their ratio;

epithermalL

rr

er

r

r

r21

1

2

21

12

)(

)(

This ratio is not only independent of thermal neutron diffusion length and coefficient, but Q (neutron source strength) as well.

There are several reasons for wanting to make a thermal neutron measurement which is sensitive only to slowing down length. - slowing down length is principally determined by the concentration of hydrogen in a medium and hydrogen concentration can be frequently be related to rock porosity. (Notable exceptions are zones containing low pressure gas or substantial amounts of bound hydrogen.) -response should be independent of diffusion length because diffusion length varies significantly with water salinity and rock type. -the measurable thermal neutron density at any point in the borehole or formation usually exceeds the measurable epithermal neutron density

Porosity Epithermal parameters Thermal parameters

Fresh Water Salt water 100000 ppm

L(cm) D(cm) L(cm) D(cm) L(cm) D(cm)

3 17.8 91.4 14.3 1.07 13.1 1.08

11 13.7* 85.4 10.7 0.742 8.5 0.750

23 11.5* 80.4 7.7 0.514 5.6 0.523

34 10.5* 77 6.2 0.393 4.2 0.401

50 9.1 73.6 5.8 0.298 3.1 0.304

100 7.0* 68.8 2.8 0.167 1.7 0.171

Experimental and computed neutron parameters, for fast neutrons mean energy 4.46MeV.

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Field Experiment – Example A

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It has shown theoretically that thermal neutron measurements made at large source detector spacing respond almost exclusively to changes in Lepithrmal.

Above conclusions was used on field example for modeling PNN response.

In august 2007, PNN was logged in open hole, immediately after OH logging. Open hole was logged with Western Atlas 3600 series equipment.

Logged was CN2435 and compensated density, CDL 2227, with other logs.

Based on porosity log, L and D for epithermal and thermal parameters were calculated for PNN based on the data shown in table. Parameter r – detector source distance is set as for PNN, rnear was 40cm, and rfar was 54cm. Applying formula (4) with these parameters, blue curve was created as a ratio between count rates on two detectors. Red curve represents logged ratio curve from PNN log. Black curve is logged ratio curve from compensated neutron CN 2435.

Red is bulk density on the scale 1.95-2.95, and blue is CN 2435 in scale 60-0,green GR in the scale 50-350

Field Experiment-Example A

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According to conclusions listed above, values for rnear=70cm and rfar=92cm were chosen.

Field Experiment – Example A

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Same open hole well where Total Ratio defined as ratio SS/LS from first channel to 60, is used for porosity calculation, modified with GR index.

Field Experiment – Example B

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Well drilled 8.5” bit, produced from open hole, perforated from 1287-1377, water salinity inside borehole 138000ppm, two years before PNN logging, well was treating with acid from 1287m to 1358m. Bottom of the well 1367m. Casing shoe 1287m.

Field Experiment – Example B

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Well drilled 8.5” bit, produced from open hole, perforated from 1287-1377, water salinity inside borehole 138000ppm, two years before PNN logging, well was treating with acid from 1287m to 1358m. Bottom of the well 1367m. Casing shoe 1287m.

Blue is PNN Ratio in scale 4-1.2, pink is PNN porosity curve in scale 60-0, red is CN curve in scale 60-0

Field Experiment – Example C

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In the well is tubing 2-7/8” to 2720m, and depth in casing is up to 2772m. Perforated interval 2725m – 2735m. Fluid in the well was water, and formation water salinity 40000-50000ppm. Well logged in shut in condition.

Blue = PNN Ratio 10-0Red = Normalizied ratio 5-0Green = GR normalizied 1-0Light Blue CNPOR = CasedHole CN – 60-0Violet = NPHI – open hole CN 60-0

Field Experiment – Example C

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In the well is tubing 2-7/8” to 2720m, and depth in casing is up to 2772m. Perforated interval 2725m – 2735m. Fluid in the well was water, and formation water salinity 40000-50000ppm. Well logged in shut in condition.

CONCLUSIONS

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Two-group neutron diffusion theory was used in this paperwork to study theoretical response of PNN tool for porosity evaluation. In undisturbed open hole conditions, theoretical results obtained thru simple simulation were very similar to data recorded on the field.

Relation between ratio curve and porosity curve from open hole was established through same polynomial presentation as for the Western Atals CN tool series 2435. Changing distance between neutron generator and two detectors was shown that relative number of thermal neutrons detected on two detectors (their ratio) can be used as a measure of single epithermal neutron parameter, the slowing down length of the formation. For real data keeping CN2435 polinomial as a basic, it is possible to use only one polynomial presentation for all PNN tools with

porosity calculations in the limits of acceptable. Well conditions must be taken in account in porosity evaluation.

Rules For The Direction Of The Mind-Descartes, Rene

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RULE VIII If in the series of subjects to be examined we come to a subject of which our

intellect cannot gain a good enough intuition, we must stop there; and we must not examine the other matters that follow, but must refrain from futile

toil.