Electricity: measuring and testing – Of geophysical surface or subsurface in situ – With radiant energy or nonconductive-type transmitter
Reexamination Certificate
2004-01-08
2004-09-14
LeDynh, Bot (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
With radiant energy or nonconductive-type transmitter
C324S346000
Reexamination Certificate
active
06791331
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to well logging using electromagnetic measurements. More particularly, the invention relates to determining subsurface formation properties using electromagnetic induction tomography in a borehole lined with a conductive tubular or casing.
BACKGROUND OF THE INVENTION
Geological formations forming a reservoir for the accumulation of hydrocarbons in the subsurface of the earth contain a network of interconnected paths in which fluids are disposed that may ingress or egress from the reservoir. To determine the behavior of the fluids in the aforementioned network, knowledge of both the porosity and permeability of the geological formations is desired. From this information, efficient development and management of hydrocarbon reservoirs may be achieved. For example, the resistivity of geological formations is a function of both porosity and permeability. Considering that hydrocarbons are electrically insulative and most water contains salts, which are highly conductive, resistivity measurements are a valuable tool to determine the presence of hydrocarbon reservoir in geological formations.
To that end, there have been many prior art attempts to model geological formations. In two articles, “Crosshole Electromagnetic Tomography: A New Technology for Oil Field Characterization,” The Leading Edge, March 1995, by Wilt et al. and “Crosshole Electromagnetic Tomography: System Design Considerations and Field Results,” Society of Exploration Geophysics, Vol. 60, No. 3 1995, by Wilt et al., measurement of geological formation resistivity is described employing a low frequency electromagnetic system.
FIG. 1
shows typical equipment used in the measurement of geological formation
10
resistivity between two drill holes
12
a
and
12
b
using electromagnetic induction. A transmitter T is located in one borehole, while a receiver R is placed in another borehole. The transmitter T typically consists of a coil (not shown) having a multi-turn loop (which consists of N
T
turns of wire) wrapped around a magnetically permeable core (mu-metal or ferrite) with a cross section, A
T
. The transmitter T may further comprise a capacitor (not shown) for tuning the frequency of the coil. When an alternating current, I
T
, at a frequency of f
0
Hz passes through this multi-turn loop, a time varying magnetic moment, M
T
, is produced in the transmitter. This magnetic moment is defined as follows:
M
T
=N
T
I
T
A
T
(1)
The magnetic moment M
T
can be detected by the receiver R as a magnetic field, B
0
. The transmitter T, receiver R, or both are typically disposed in boreholes (e.g.,
12
a
and
12
b
) in the earth formation
10
. In this case, the detected magnetic field, B
0
, is proportional to the magnetic moment of the transmitter, M
T
, and to a geological factor, k
1
, as follows:
B
0
=k
1
M
T
(2)
The geological factor, k
1
, is a function of the spatial location and orientation of a field component of the magnetic field, B
0
, with respect to the magnetic moment of the transmitter, M
T
.
The receiver R typically includes one or more antennas (not shown). Each antenna includes a multi-turn loop of wire wound around a core of magnetically permeable metal or ferrite. The changing magnetic field sensed by the receiver R creates an induced voltage in the receiver coil (not shown). This induced voltage (V
R
) is a function of the detected magnetic field (B
R
), the frequency (f
0
), the number of turns (N
R
) of wire in the receiver coil, the effective cross-sectional area of the coil (A
R
), and the effective permeability (&rgr;
R
) of the coil. Thus, V
R
can be defined as follows:
V
R
=&pgr;f
0
B
R
N
R
A
R
&rgr;
R
(3)
While f
0
and N
R
are known, the product, A
R
&rgr;
R
, is difficult to calculate. In practice, these constants may be grouped together as k
R
and equation (3) may be simplified as:
V
R
=k
R
B
R
(4)
where k
R
=&pgr;f
0
N
R
A
R
&rgr;
R
. Thus, instead of determining the product A
R
&rgr;
R
, it is more convenient to determine k
R
according to the following procedures. First, the receiver coil is calibrated in a known field, at a known frequency. Then, the exact value for k
R
is derived from the magnetic field (B
R
) and the measured voltage (V
R
) according to the following equation:
k
R
=B
R
/V
R
(5)
When this system is placed in a conducting geological formation, the time-varying magnetic field, B
0
, which is produced by the transmitter magnetic moment, produces a voltage in the geological formation, which in turn drives a current therein, L
1
. The current, L
1
, is proportional to the conductivity of the geological formation and is generally concentric about the longitudinal axis of the borehole. The magnetic field proximate to the borehole results from a free space field, called the primary magnetic field, while the field resulting from current L
1
is called the secondary magnetic field.
The current, L
1
, is typically out of phase with respect to the transmitter current, I
T
. At very low frequencies, where the inductive reactance is small, the current, L
1
, is proportional to dB/dt and is 90° out of phase with respect to I
T
. As the frequency increases, the inductive reactance increases and the phase of the induced current, L
1
, increases to be greater than 90°. The secondary magnetic field induced by current L
1
also has a phase shift relative to the induced current L
1
and so the total magnetic field as detected by receiver R is complex.
The complex magnetic field detected by receiver R may be separated into two components: a real component, I
R
, which is in-phase with the transmitter current, I
T
, and an imaginary (or quadrature) component, I
1
, which is phase-shifted by 90°. The values of the real component, I
R
, and the quadrature component, I
1
, of the magnetic field at a given frequency and geometrical configuration uniquely specify the electrical resistivity of a homogeneous formation pierced by the drill holes. In an inhomogeneous geological formation, however, the complex field is measured at a succession of points along the longitudinal axis of the receiver borehole for each of a succession of transmitter locations. The multiplicity of measurements thus obtained can then be used to determine the inhomogeneous resistivity between the holes.
In both cases, i.e., measuring homogeneous geological formation resistivity or measuring inhomogeneous geological formation resistivity, the measurements are typically made before extraction of hydrocarbons takes place. This is because the boreholes typically are cased with conductive liners (e.g., metallic casing; see
16
a
and
16
b
in
FIG. 1
) in order to preserve the physical integrity of the borehole during hydrocarbon extraction. The conductive tubular liners interfere with resistivity measurements and are difficult and costly to remove from the borehole once they are installed. As a result, prior art systems such as that shown in
FIG. 1
are not suitable for analyzing hydrocarbon reservoirs once extraction of the hydrocarbons begins.
The problems presented by conductive liners (
16
a
and
16
b
in
FIG. 1
) are described by Augustin et al., in “A Theoretical Study of Surface-to-Borehole Electromagnetic Logging in Cased Holes,” Geophysics, Vol. 54, No. 1 (1989); Uchida et al., in “Effect of a Steel Casing on Crosshole EM Measurements,” SEG Annual Meeting, Texas (1991); and Wu et al., in “Influence of Steel Casing on Electromagnetic Signals,” Geophysics, Vol. 59, No. 3 (1994). These prior art references show that coupling between a transmitter and a conductive liner is independent of the surrounding geological formation conductivity for a wide range of practical formation resistivities encountered in the field and that the magnetic field produced inside the conductive liner at a distance of a few meters or less from the transmitter depends only on the conductive liner properties and not on the formation properties.
The net or effective moment, M
eff
, of a
Echols Brigitte L.
LeDynh Bot
McEnaney Kevin P.
Ryberg John
Schlumberger Technology Corporation
LandOfFree
Method and apparatus for measuring characteristics of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for measuring characteristics of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for measuring characteristics of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3246614