Note: Descriptions are shown in the official language in which they were submitted.
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NEW NUCLEAR MAGNETIC RESONANCE METHOD AND
APPARATUS FOR REMOTE DETECTION AND
VOLUMÆTRIC MEASUREMENT OF PETROLEUM RESERVES
This invention relates to a new nuclear magnetic
resonance method and apparatus which remotely senses
and measures the fluid volume of petroleum reservoirs.
Nuclear paramagnetism was first measured at very low
temperatures on solid hydrogen (B.G. ~asarew and L.W.
Schubnikow, Phys. Z~ Sowjet, 11, p. 445, 1937). In
this experiment the sample's paramagnetism was
observed during temperature cyclingO However, for
ordinary sized laboratory samples at room temperature,
nuclear paramagnetism becomes extraordinarily feeble
and difficult to measure. Consequently, many other
experimental techniques known collectively as nuclear
magnetic resonance techniques were developed to
observe the nuclear magnetic moments of nucleons in
solids and liquids.
Nuclear magnetic resonance measurements are classified
into "continuous wave" and "pulse" methods. Continu-
ous wave observations of nuclear magnetic resonance
are performed using power absorption measurements or
nuclear induction arrangements (E.R. Andrew, Nuclear
Magnetic Resonance, Cambridge Univ. Press, N.Y.I p.
34, 1955). Pulse methods, also known as free preces-
sion techniques, have been developed in recent years
--2--
(T.C. Farrar and E.D. Becker, Pulse and Fourier
Tran~form ~M~, Academic Press, N.Y., p. 1, 1971~.
Common to al these nuclear magnetic resonance tech~
niques are various means used to apply an A.C magnetic
~ield at the Larmor frequenc~ to the collection of
magnetic moments in a sample. All of these techniques
rely on initially producing a coherent magnetization
of the sample which moves dynamically in time and
which eventually decays due to relaxation phenomena.
This initially coherent magnetization executes complex
motion away from the axis of the original static
magnetic field. Furthermore, this type of time
~arying coherent magnetization produces large observ-
able effects in a laboratory environment such as
significant induced A.C. voltages in pick-up coils.
The measurement of the presence of this coherent
magnetization is, of course, an indication that the
sample is in the condition of resonance.
It is well known that a single nuclear magnetic moment
executes a precessional motion around a static
magnetic field. In the normal 1/2 gauss earth's
magnetic field, the proton's magnetic moment precesses
at a Larmor frequency of approximatel~ 2.1 kilohertz.
The tip of the magnetic moment vector traces out a
cone shaped motion around the static magnetic field.
If a large number of magnetic moments are placed in
the earth's magnetic field, similar motion ensues
except that the tips of the magnetic vectors fan out
around the same cone. The fact that some of the
nuclear magnetic moments have components along the
direction of the magnetic field causes the sample's
paramagnetism.
--3--
A microscopic description of the fluid in formation
provides the reason why these fluids exhibit the
phenomenon of paramagnetism. Nucleons which are
chemically bound in hydrocarbons and water possess
maynetic moments. These moments tend to line up in
the earth's magnetic field. Consequently, the earth's
magnetic field in the vicinity of the hydrocarbons and
water in formation is increased by their presence.
This alteration of the strength of the magnetic field
of the earth in the vicinity of fluids in the forma-
tion is the phenomenon of paramagnetism. Conversely,
if this alignment of nucleons in the earth's magnetic
field is caused to disappear by any mechanism, then
the paramagnetism of the formation would disappear and
the magnetic field adjacent the formation would
change.
Heretofore, it has been noticed theoretically that as
the conditions for nuclear magnetic resonance are
reached, the original paramagnetism of the sample is
reduced or eliminated (T.C. Farrar and E.D. Becker,
op. cit., p. 14). However, this fact has not been
used experimentally to actually measure whether the
sample has attained the conditions of resonance.
Consequently, a new nuclear magnetic resonance method
is proposed whereby the condition of resonance is
measured by the reduction or disappearance of the
original paramagnetism of the sample. Furthermore, it
is proposed to use this new type of nuclear magnetic
resonance method to detect the presence of unknown
petroleum reservoirs in geological formations and also
allow the direct measurement of significant portions
of oil and water fluid volume contained within the
formation. It is to be emphasized that the practi-
cality of the method is directly attributed to the
enormous volume of liquid contained in a petroleum
reservoir.
-4~
There have been prior attempts to utilize standard
nuclear magnetic resonance techni~ues in-situ and on
entire bulk petroleum reservoirs. In U.S. Pat. #
3,019,383 ~1962), Russell H. Varian proposes using a
pulse type free precession nuclear magnetic resonance
technique to ln~icate ~he presence of oil. In U.S.
Pat. # 3,060,371, Jonathan Townsend (1962) proposes
performing resonance experiments on unpaired elec-
tronic moments to locate petroleum reservoirs. In.
U.S. Pat. # 3,398,355, a pulse type nuclear magnetic
resonance experiment is proposed to be flown in
aircraft to locate oil deposits. All of these methods
rely on the coherent precession of magnetization after
conditions appropriate for resonance have been
ohtained. Furthermore, all of these methods require
relatively large magnetic fields and are consequently
impractical.
Other techniques have been proposed to remotely sense
bulk oil deposits by monitoring the absorption of
radiation at the Larmor frequency. ~xamples of these
methods are given in U.S. Pat. # 3,411,070 ~1968) and
in U.S. Pat. # 3,437,914 (1969). Here again, numerous
theoretical and experimental flaws make these methods
impractical.
Standard nuclear magnetic resonance methods are
currently being used to measure the properties of oil
reservoirs immediately adjacent to boreholes. See for
example U.S. Pat. Nos: 4,035,718 (1977); 3,667,035
(1972); 3,657,730 (1972); 3,617,867 (1971); 3,508,438
(1970); 3,483,465 (1969); 3,439,260 (1969); 3,395,337
(196B); etc. The reason that these representative
techniques are used immediately adjacent to boreholes
is that the standard magnetic resonance methods used
heretofore require applying relatively strong magnetic
-5-
fields. Although these are useful measurements, they do not
directly measure the amount of li~uid petroleum available over
large volumes of oil bearing formation.
Accordingly, an object of the invention is to provide
a new and practical nuclear magnetic resonance method for the
remote detection and direct volumetric measurement of petroleum
reserves.
It is yet another object of the invention to provide
new and practical nuclear magnetic resonance apparatus for the
remote detection and the direct volumetric measurement of
petroleum reservoirs.
Still further, it is another object of the invention
to provide new nuclear magnetic resonance methods and apparatus
for remote detection and volumetric measurement of petroleum
reservoirs which contain chemical constituents with short
relaxation times, particularly, those with short transverse
relaxation times.
And further, it is still another object of the invention
to provide new nuclear magnetic resonance methods and apparatus
for the remote detection and volumetric measurement of petroleum
reservoirs which also measure characteristic dimensions of the
reservoirs.
According to the invention an apparatus is provided
for remote detection and volumetric measurement of a petroleum
.,
, .
-5a-
reservoir in a formation which comprises means for placing a
significant portion of a petroleum reservoir in the formatlon
lnto a state of saturation and means for simultaneously rneas~
uxing the resulting variation in -the earth's magnetic field :in
the vicinity of the formation.
The disclosed method detects khe presence of petroleum
reserves in a formation by applying an a.c. magnetic field at a
frequency near and including the Larmor frequency of the nucleons
in any oil present which is at an angle with respect to the earth's
magnetic field to bring a portion of the reserve into nuclear
magnetic resonance to reduce the nuclear paramagnetism of said
portion of the petroleum reserve and simultaneously detecting any
change in the total magnetic field of the earth in the vicini~y
of said portion of the reserve to sense a change in the nuclear
paramagnetism of said portion of the reserve whereby to indicate
the presence or absence of petroleum reserves.
More particularly, there is provided an apparatus for
the remote detection of unknown petroleum reserves and volumetric
measurement of petroleum in a formation comprising means for
generating and applying an a.c. magnetic field to a portion
of the formation at frequencies near and includiny the Larmor
frequency of the nucleons within any petroleum present at an angle
with respect to the earth's magnetic field and means for
measuring the total magnetic field in the vicinity of said
portion of the formation and means for indicating changes in the
.~
-5b-
earth's magnetic field in the vicini-ty of said portion of the
formation whereby to indicate the change in the nuclear para-
magnetism of said portion of the formation to ind.icate the presence
and volume of fluid contained within said portion of the for~
mation.
There is further disclosed a method of determininy the
relative amounts of oil and water contained in an oil reservoir
in a geological formation which comprises the steps of applying
a periodically sweeping magnetic field -to a portion of the
formation which sweeps from a frequency below the Larmor frequency
to a frequency above the Larmor frequency repetitively at periods
which are less than the longitudinal relaxation time of the nucleons
within any oil present and simultaneously measuring the amplitud~
modulation of the earthls magnetic field in the vicinity of said
portion of the formation where said Larmor frequency is appropriate
for nucleons within the formation and then applying a period-
ically sweeping magnetic field to said portion of the formation
which sweeps from below the Larmor frequency to a frequency
above Larmor frequency repe-titively at periods which are greater
than the longitudinal relaxation time of the nucleons within said
oil present and simultaneously measuring the amplitude modulation
of the earth's magnetic field in the vicinity of said portion of
the formation whereby the difference in said measurements perfor-
med for different repetition rates are used to infer the relative
amounts of oil and water present within said portion of the
oil bearing formation.
`!~ ;`^
-5c~
Figure 1 is a section view of one preferred embodimenk
of the invention for the remote detection and direct volumetric
measurement of petroleum reservoirs~
Figure 2 is a diagram used to describe the motion of
magnetic moments in an oil bearing formation during nuclear
magnetic resonance conditions.
$~
--6--
Fig. 3 describes frequency sweeping the A.C. maynetic
field applied to the oil bearing formation.
Fig. ~ shows the decrease in the magnetic ield of the
earth as the oil bearing formation is swept khrough
nuclear ma~netic resonance.
Fig. 5 shows the voltage induced in the induction
magnetometer due to the variation in the earth's
magnetic field as the oil bearing formation is swept
through resonance.
Fig. 6 describes the magnetic field variations for
different distances above the oil bearing formation.
Fig. 1 shows a preferred embodiment of the apparatus
for remote sensing and volumetric measurement of
petroleum reserves. This particular embodiment is
appropriate when drilling has already occurred in the
oil field. Two boreholes, 10 and 12 respectively have
been drilled from the earth's surface 14. As is
shown, the boreholes have drilled through the oil
bearing formation 16. A standard frequency sweep
oscillator 18 has an output 20 which drives an A.C.
power amplifier 22. This frequency sweep oscillator
(F.S.O.) must be capable of slowly frequency sweeping
around approximately 2.1 kHz. and the power amplifier
(P.A.) must be capable of providing significant A.C.
current near the frequency of 2.1 kilohertz. One
output of the power amplifier is attached to a cable
24 which is lowered into borehole 10 and is connected
to a means 2~ of introducing current into the forma-
tion. The other output of the power amplifier is
attached to a cable 28 which is lowered into borehole
12 and is connected to a means 30 of introducing
current into the formation. ~herefore, A.C. current
--7--
is conducted through the oil bearing formation in
paths collectively identified as 32 in Fig. 1. The
total A.C. current passing through the oil bearing
formation produces an A.C. maynetic field throughout
the oil bearing strata. For example, at a location
labeled 34 within the formation, there exists an A.C.
magnetic field whose vector is primarily out of the
drawing. The magnitude of the A.C. magnetic field but
not the frequency depends on location in the forma-
tion. The frequency of 2.1 kHz. is the Larmorfrequency of protons in the nominal strength of the
earth's magnetic field of 1/2 gauss. Therefore, a
means is provided whereby a significant portion of the
oil bearing formation may be subjected to an A.C.
magnetic field at the ~armor frequency of protons in
the earth's magnetic field. This significant portion
of the formation subjected to the condition of nuclear
magnetic resonance is also called the "excitation
zone" for brevity. Furthermore, a means is provided
where by a significant portion of an oil bearing
formation may be swept through a condition o~ nuclear
magnetic resonance.
A borehole magnetometer assembly labeled as 36 in
Fig. 1 is lowered into borehole 12 a distance Z above
the center of the oil bearing formation. In this
particular embodiment, the borehole magnetometer
assembly includes a large number of turns of insulated
wire 38 which are wound around a very high perme-
ability magnetic core material 40 which is in turn
connected to an ampli~ier 42. This amplifier must be
stable, have high gain, extremely low noise, narrow
bandwidth, and excellent low frequency re~ponse.
relatively new integrated circuit which is very well
-8~
suited for this purpose is the OP-27A/E (Precision
Monolithics Inc., 1500 Space Park Dr., Santa Clara,
Ca. 95050). This operational amplifier has a ver~ lo~
input noise voltage density of 5 nanovolts per square-
root hertz at a ~requency of 10 hertz. The requiredlow noise and hi~h gain is obtained using standard
electronic design prlnc.iples and several OP-27A/E
integrated circuits. The output of thi.s amplifier is
connected to a shielded cable 44 which lea~es the
borehole and is connected to the input of a standard
signal averager 46. The signal averager (S.A.)
obtains its reference sync. pulse via a cable 48 which
is connected to the sync. pulse output of the
frequency swept oscillator. Consequently, a means has
been provided which measures the low frequency change,
variation, or amplitude modulation, of the earth's
magnetic field. Measurements at various positions Z
provide a means whereby the amplitude modulation of
the earth's magnetic field can be measured in the
vicinity of the excitation zone of the formation.
As has been briefly discussed, as matter is swept
through the condition of nuclear magnetic resonance,
it is expected that the original paramagnetism of a
sample should decrease or vanish under certain circum-
stances. In this embodiment of the invention, a means
has been provided to sweep the oil bearing formation
through a condition of nuclear magnetic resonance.
Therefore, as the formation is swept through resonance,
the paramagnetism of the oil formation is decreased or
eliminated. The small paramagnetism of the oil
formation contributes to the total magnetic field
measured by the magnetometer. Consequently, as the
oil formation is swept through resonance conditions, a
small decrease in the earth's magnetic field in the
vicinity of the excitation zone is observed. This
- 9 -
decrease in the magnetic field strength is directly
related to the volume of liquid petroleum in the
excited portion of the formation. As the formation is
repetitively swept through resonance, the signal
averager is used to increase the signal and decrease
the noise using standard signal processing techniques.
It is now necessary to more precisely define the
conditions which result in a reduction of the paramag-
netism during resonance conditions. To do this, some
additional physics must be described. In what
follows, it is shown that there are requirements on
both the frequency sweep rate and A.C. magnetic field
strength which depend on the physical properties of
the bulk petroleum reservoir. The following physics
is also necessary to demonstrate that the invention
provides a practical means of sweeping the oil forma-
tion through resonance and furthermore provides a
practical means to measure the resulting variation in
the earth's magnetic field. It will be understood,
howe~er, that the invention is not to be specifically
limited by the theory which follows.
The motion of a collection of nuclear magnetic moments
is described with reference to a coordinate frame as
shown in Fig. 2. The magnetic field of the earth is
Bo and lies along the Z axis of the coordinate frame.
An individual magnetic moment U precesses in a conic
motion around the direction of Bo in the absence of
other magnetic fields. The magnetic moment executes
this type of motion because the time rate of change of
the vector angular momentum (dL/dt) must equal the
vector torque on the magnetic moment (U X Bo)~ as
shown in Eq. 1.
dL/dt = U X Bo Equation 1.
-10-
This type of motion is described by locating the
angular position of the tip of the magnetic moment U
with respect to the X axis in Fig. 2. This angular
position is definecl by the angle ~ in Fig. 2. The
time rate of change of the angle is called the angular
precession frequency wO. This angulax fre~uency is
related to the earth's magnetic ~ield and to the
gyromagnetic ratio of the proton y by the followiny
equation:
wO = ~Bo Equation 2.
As is shown in many elementary physics texts, Equa-
tion 2 is a consequence of Equation lo Of course, the
many individual magnetic moments fan out around the
cone at different angles ~ as shown in Fig. 2.
However, the fact that the many magnetic moments have
components along the Z axis produces a net magnetiza-
tion Mz along the Z direction. This effect of course
produces the nuclear paramagnetism of petroleum. An
external magnetic field Bl is applied to the preces-
sing magnetic moments as shown in Fig. 2. For simpli-
city only, Bl is confined to the X-Y plane, has
constant magnitude and it rotates in time through the
angle ~ which is defined in Fig. 2. In general, Bl
must only have a non-parallel component to Bo for the
validity of the following analysis (Bl must be at an
angle to be Bo)~ Defining the time rate of change of
the angle ~ to be the quantity w, it is well known
that the condition of magnetic resonance occurs when:
w = wO Equation 3.
The physical significance of resonance is that energy
may be coupled from the Bl field into the precessing
magnetic moments thus altering their motion. If the
frequency of the applied magnetic field w is swept
--ll--
slowly through resonance, the solution to motion of
the vector magnetlzation of the sample is given by the
steady state solution to the "Bloch equations". The~e
equations and their solution under these conditions
are given in E.R. ~ndrew, op. cit., page 28~
In this analysis it is shown that the paramagnetism of
the sample may be reduced significantly or eliminated
at resonance. This phenomenon is called "saturation".
In this condition, the magnetization in the Z direc-
tion, Mz is reduced or eliminated. In the aforemen-
tioned reference it is shown that saturation occurs if
the follo~ing mathematical condition is obeyed:
2 B 2
~ 1 1 2 ~ Equation 4.
In this equation, ~ and Bl have already been defined.
The quantity T1 is the longitudinal relaxation time
and T2 is the transverse relaxation time. These
relaxation times are of importance and the physical
significance that these times have upon the motion of
the collection of magnetic moments is well described
in T.C. Farrar and E~Do Becker, op. cit., pages 7-15.
Briefly, in this case T1 is the time it takes for the
disoriented magnetic moments to achieve thermal
equilibrium and hence realign along the Z axis after
- the conditions of saturation have been achieved.
Therefore, T1 is also called the thermal relaxation
time. T2 is the time it takes for a hypothetical
group of magnetic moments which have been originally
aligned in one single direction in the X-Y plane of
Fig~ 2 to become disoriented with their vectors
fanning out and pointing at random in that X-Y plane.
T2 is also known as the spin-spin relaxation time.
-12-
If the embodiment ln Fig. 1 is to be practical, the
magnitude of the magnetic field B1 as specified in
Eq. 4 must not be impractically large. r~ost measure-
ments of the thermal relaxation time T1 of crude oil
in formation are known -to fall within the following
range of tlmes (J.D. Robinson, et. al., J. Pet. I'ech.,
26, p. 226, 1974):
.1 sec ' T1 < 5 sec Equation 5.
In U.S. Pat. No. 3,395,337 R.H. Varian (196~) has
measured the following limits on T2:
-5
10 sec ' T2 ' 60 sec Equation 6.
In the worst possible case corresponding to the
minimum times as specified for Tl and T2, the minimum
required strength B1 is approximately 37 milligauss.
Consequentlyl if a magnetic field of 37 milligauss is
applied to an oil bearing formation at the Larmor
frequency, saturation will occur. Furthermore, it is
well known that most transverse relaxation times are
longer than 10 4 sec Isee Eq. 6). This therefore
would require an A.C. field strength of only 11.7
milligauss. Consequently, it has been shown that
there is a minimum A.C magnetic field strength neces-
sary to produce saturation which is related to the
relaxation properties of the oil bearing formation.
Ano~her separate condition for the observation of
saturation is that the frequency must be swept
"slowly". This is known as "adiabatic passage". In
adiabatic passage, the change in the angular sweep
rate per unit time (dw/dt) must satisfy the following
condition (A. Abragam, The Principles of Nuclear
312.~
-13-
Magnetism, Clarendon Press, O~ford, 1961, p. 35):
dw/dt <~ Y B1 Equation 7.
This equation shows that if ~1 has a magnitude of 11.7
milligauss, the maximum frequency sweep rate is
approximately 100 HZ/sec. The maximum sweep rate
depends on ~1 and this quantity in turn depends on the
relaxation properties of the oil formation. Conse-
quently, under the conditions of adiabatic passage,
there is a maximum permissible frequency sweep rate
applicable to a given oil bearing formation.
If the above sweep rate is exceeded, other phenomena
happen which are classified as types of "fast
passage". In these cases, the magnetization of the
sample cannot follow the net magnetic field and none
of the conditions applicable to adiabatic passage are
satisfied. However, the magnetization in the z
direction, Mz, does in fact decrease or disappear if
the following equation is satisfied (T.C. ~arrar, E.D.
Becker, op. cit., p. lO-lS):
1 < y B1 t ~ r Bl T1 Equation 8.
In Eq. 8 the transit time through resonance t must
certainly be less than the longitudinal relaxation
time T1~ From Eq. 5, the minimum longitudinal time is
.1 sec and therefore Eq. 8 specifies that in this
worst case, the minimum magnitude of the required
magnetic field is approximately .37 milligauss. This
is a very small magnetic field. For example, if an
A.C. current of 10 amps (peak-to-peak) were passed
through a localized area of the petroleum reservoir,
the A.C. magnetic ~ield produced by this current ~ould
exceed .37 gauss (peak-to-peak) inside a radius of 54
-14-
meters. Consequently, the invention provides a
practical means to sweep significant portions of an
oil bearing formation through the condition of mag~
netic r~sonance.
The invention provides a practical method of detecting
the effects of saturation and fast passage phenomena.
The paramagnetism of the oil formation gives rise to a
small increase in the magnetic field above the oil
reservoir ~B(Z). For simplicity only, it is assumed
that the resonance is due only to "unpaired protons"
(also called "hydrogen-like" or "unbound"). Near the
reservoir:
~B(Z 0) ~1 pue-[2UBo/kT] ~ 1 x 10 9 Gauss EquatiOn 9-
The quantities used in this M.K.S. equation include
the following: uO (permeability of space); p (number
of unpaired protons/M ); U (magnetic moment of a
proton); Bo (earth's magnetic field); k (~oltzman's
constant); and T (absolute temperature).
Ideally, this small magnetic field is driven to zero
during a sweep through resonanceO The resulting
induced voltage in the induction magnetometer shown in
Fig. 1 may be estimated. The core labeled as 40 in
Fig. 1 is made of permalloy with a lcm2 area and a
length of 100 cm which therefore has an "effective
permeability" of 20,000 (G.V. Keller and F.C.
Frischknecht, Electrical Methods__in Geophysical
, Pergamon Press, N.Y., p. 237, 1966). The
coil has 100,000 turns and the sweep time is approxi-
mately one second. Consequently, an induced voltage
of 24 nanovolts appears across the induction coil.
This is five times larger than the input noise voltage
density of the OP-27A/E operational amplifier. ~hus,
-15-
the lnduced voltage due to the saturation o~ the oil
bearing formation may be easily detected over the
noise present in the circuitry.
Figures 3, 4 and 5 show the time dependence of ~he
experimental signals expected from the apparatus
embodied in Fig. 1. Fig. 3 shows that the requency,
F, of the A.C. magnetic field is swept from a lower
frequency E'l through the resonant frequency Fo~ the
formation is in the conditlon of nuclear magnetic
resonance. Fig. 4 shows the corresponding decrease in
the earth's magnetic field, Bo~ at the time To~ And
the voltage appearing across the induction coil, V, is
shown in Fig. 5.
The precise shape of the signal in Fig. 5 depends on
the longitudinal and transverse times Tl and T2 among
other parameters such as the sweep time, etc. The
oil/water ratio may be deduced under certain circum-
stances as has been done in another experimental
situation (J.D. Robinson, et al, op. cit.). Oil and
water are separated by periodically sweeping the
formation through resonance repetitiously and simul-
taneously measuring the amplitude modulation of the
earth's magnetic field for various different repeti-
tion rates. Furthermore, since the longitudinal
relaxation times Tl are much longer for solids, only
the paramagnetism of liquids in formation are observed
(E.R~ Andrew, op. cit., p. 151).
The total fluid volume, average fluid density, and the
dimensions of the petroleum reservoir may be found
from the following procedure. The variation in the
earth's magnetic field during resonance ~B(Z) is
measured for various distances Z above the oil field.
Please refer to ~ig. 6 which shows a typical plot of
-16-
~B(Z) vs. Z. Near the excitation zone of the oil
formation, measurement of ~B(Z-O) yields the quantity
p in Eq. 9 which is the average number of unpaired
protonstM3 in the formation. Since the si.ynal is
primarily from liquids, this immediately yields the
average fluid density within the forrnation. In region
A in Fig. 6 which is near the excitation zone of the
oil reservoir, the magnetic field decreases as l/z2
(R. Benumof, Concepts in~ Electri ty and Maynetism,
Holt, Rinehart and Winston, N.Y., p. 196, 1961). An
inflection at point B in Fig. 6 demonstrates that the
distance Z has reached some characteristic dimension
of the oil field such as the thickness or the average
diameter of the oil deposit. Region C shows the
behavior of the magnetic field for large Z when the
paramagnetic reservoir behaves as if it is a large
single magnetic moment UT. UT is of course the sum of
all the magnetic moments in the excitation zone of the
formation. Equation 10 sho~Js that ~B(Z) decreases as
1/Z3 for large Z. (D. Halliday and R. Resnick,
Physics For Students of Science and Engineering, John
Wiley ~ Sons, N.Y., p. 772, 1963).
uO UT -[UBo/kT]
~BIZ) = 4 z3 e Equation 10
Once UT is obtained by fitting the data, the total
number of unpaired protons in the formation is given
UT/U. From the known chemical composition, nuclear
properties and densities of crude oil, the free fluid
volume of the excitation zone of the formation may be
calculated.
There are a very large number of other embodiments of
the invention. ~ny means may be used to cause the oil
bearing formation to pass through the condition of
nuclear magnetic resonance. Consequently, any means
-17-
may be used to apply an A.C. magnetic field to the oil
bearing formation near the Larmor ~requency.
Different methods of applying this A.C. magn~tic field
to the oil strata include but are not limited to the
~ollowing: (1) passing A.C. current through the forma-
tion from one borehole to one or more surface elec-
trodes; (2) passiny A.C. current between two or more
boreholes; (3) passing A.C. current throuyh the earth
between two or more electrodes placed on the earth's
surface; (4) using one or more A.C. current carrying
circular loops on the surface of the earth; (5) using
one or more rectangular shaped A.C. current carrying
coils on the surface of the earth; (6) inducing an
A.C. magnetic field in the pipe surrounding a borehole
with A.C. current carrying coils on the surface of the
earth; (7) inducing an A.C. magnetic field in the pipe
surrounding a borehole using an A.C. current carrying
coil inside the borehole; (8) using any borehole tool
which generates an A.C. magr.etic field by any means;
(9) passing A.C. current from the pipe surrounding a
borehole into the earth; etc. Many of these surface
methods would require large A.C. power sources since
the statistical median resistivity of the surface
overburden is 143 ohm-meters in the United States
which results in an electromagnetic skin ~epth of only
134 meters at 2.1 kHz (G.V. ~eller and F.C.
Frischknecht, op. cit., p. 40). Furthermore, all
varieties of pulse methods may ~e used provided the
methods produce the condition of resonance in the oil
bearing formation.
Furthermore, any means with sufficient sensitivity may
be used to measure the variation of the earth's
magnetic field as the oil formation is swept through
resonance. There are numerous types of magnetometers
currently in use (~.M. Telford, Applied Geoph~sics,
2~
-18-
Cambridge University Press, Cambridge, p. 123, 176).
The sensitivity of these instruments may be suhstan-
tially increased by using thern in conjunction with
high permeability magnetic materials. Diferential
magnetometers or magnetic field gradiometers as are
widely used in the geophysical industry may also be
used. Superconducting quantum interference devices
may also be used as sensitive magnetometers (J.E.
Zimmerman and W.H. Campbell, Geophysics, 40, No. 2,
p. 269, 1975). A very large area induction coil on
the surface of the earth with many turns and a dia-
meter of one mile also has the required sensitivity.
And finally, the invention explicitly exploits the
properties of nuclear magnetic moments such as
unpaired protons. However, it is obvious that the
method and apparatus can be applied to other nuclear,
electronic, atomic or molecular properties of petro-
leum which have similar gyromagnetic features.
While the above description contains many specifici-
ties, these should not be construed as limitations on
the scope of the invention, but rather as an exempli
fication of one preferred embodiment thereof. As has
been briefly described, there are many possible
variations~ Accordingly, the scope of the invention
should be determined not only by the embodiment
illustrated, but by the appended claims and their
legal equivalents.
