Multiple source neutron measurement, device, system and use thereof
A multi-source neutron logging tool has been developed. The multi-source logging tool comprises axially aligned gamma and thermal neutron detectors on either side of a multi-neutron source. The multi-source neutron logging tool may be used to evaluate formations adjacent a borehole.
1. A method of evaluating a characteristic of a formation adjacent a wellbore comprising:
simultaneously emitting neutron fields into the formation from at least two axially aligned neutron sources;
obtaining data comprising a long neutron neutron (LNN), short neutron neutron (SNN), long neutron gamma (LNG) and short neutron gamma data (SNG) from the neutron fields emitted into the formation;
combining the long data with the short data to reduce error associated with the characteristic to be evaluated.
2. The method of claim 1 further comprising the step of:
optimizing a spacing of long detectors for detecting LNN and LNG relative to the multi-neutron source and short detectors for detecting SNN and SNG relative to the multi-neutron source to minimize the error associated to the characteristic to be evaluated.
3. The method of claim 1 , wherein the step of simultaneously emitting neutron fields into the formation is through a wellbore casing.
4. The method of claim 1 , wherein the combining step comprises the cancelation of the error associated to the long data with the error associated to the short data.
5. The method of claim 1 , wherein the characteristic is neutron porosity (QTP) and is evaluated according to:
QTP
=
A
*
ln
(
SNN
*
LNG
SNG
*
LNN
)
+
B
wherein A and B are determined empirically and are influenced by bit size and the amount of high dense material in a volume of investigation.
6. The method of claim 5 , wherein the characteristic is neutron liquid (QNL) and is evaluated according to:
QNL
=
A
*
ln
(
LNG
*
SNG
LNN
*
SNN
)
+
B
wherein A and B are chosen to create the best overlay of QNL to QTP.
7. The method of claim 5 , wherein the characteristic is water saturation (Sw) and is evaluated according to:
Sw
=
1
-
k
f
(
QEP
-
QEL
)
QEP
wherein
k=fluid factor;
f=formation factor;
QEP=Quad Effective Porosity (Clay Free); and
QEL=Quad Effective Liquid porosity (Clay Free); and
wherein
QEP and QEL are determined as follows:
QEP=QTP−QC and QEL=QL−QC
wherein
QTP=Quad Total Porosity;
QC=Quad Clay Porosity; and
QL=Quad Liquid Porosity;
the fluid factor k= 1/(MaxPor*QLgain)
wherein
MaxPor=WaterPor−OilPor;
WaterPor is determined as follows:
WaterPor
=
(
12.73
+
12.73
2
-
4
*
0.0966
*
(
403.04
-
Waterkppm
)
)
/
(
2
*
0.0966
)
OilPor is determined as follows:
OilPor=0.1333*oilAPI+71 and
QLgain is computed as
QLgain=QNL A value/32.5.
8. The method of claim 1 , wherein the characteristic is neutron clay (QNC) and is evaluated according to:
QNC
=
A
*
(
C
(
SNG
LNG
)
+
D
*
(
SNN
LNN
)
)
+
B
wherein A, B, C and D are empirically determined coefficients.
9. The method of claim 8 , wherein A, B, C and D are 0.004, 0, 1.9 and −1.5, respectively.
10. A method of determining the relative bulk density, or the nuclear caliper, or the chemical effect of a formation adjacent a wellbore through the wellbore casing comprising:
simultaneously emitting neutron fields into the formation from at least two axially aligned neutron sources;
obtaining long neutron neutron (LNN) field readings and long neutron gamma (LNG) field readings;
converting the LNN and LNG for porosity to provide LNNpor and LNGpor respectively; and
determining the relative bulk density by subtracting the LNGpor from LNNpor,
wherein LNNpor and LNGpor represent the porosity converted counts.
11. A system for evaluating a characteristic of a formation adjacent a wellbore comprising:
a logging tool for downhole use collecting data for evaluating a characteristic of a formation adjacent a wellbore, the logging tool comprising:
a multi-neutron source, each neutron source axially aligned for simultaneous emission of a respective neutron field into the formation;
short detectors for detecting a neutron gamma field and a neutron thermal neutron field; and
long detectors for detecting a neutron gamma field and a neutron thermal neutron field; and
a computer for combining the long data with the short data to reduce error associated to the characteristic to be evaluated.