Electricity: measuring and testing – Of geophysical surface or subsurface in situ – With radiant energy or nonconductive-type transmitter
Reexamination Certificate
2001-04-18
2003-03-18
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
With radiant energy or nonconductive-type transmitter
C324S346000, C166S250160, C340S854800
Reexamination Certificate
active
06534986
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates generally to subsurface characterization of geologic formations. More specifically, the invention relates to using sensors mounted outside wellbore casing in crosswell electromagnetic measurement techniques.
2. Background Art
Subsurface characterization of earth formations is an important aspect of drilling, for example, oil and gas wells. Subsurface characterization may help identify, among other factors, the structure and fluid content of geologic formations penetrated by a wellbore. The geologic formations surrounding the wellbore may contain, for example, hydrocarbon products that are the target of drilling operations. Knowledge of the formation characteristics is important to hydrocarbon recovery.
Geologic formations that form a hydrocarbon reservoir contain a network of interconnected fluid paths, or “pore spaces,” in which, for example, hydrocarbons, water, etc., are present in liquid and/or gaseous form. To determine the hydrocarbon content in the pore spaces, knowledge of characteristics such as the porosity and permeability of the geologic formations penetrated by the wellbore is desirable.
Information about the geologic formations and about reservoir characteristics promotes efficient development and management of hydrocarbon resources. Reservoir characteristics include, among others, resistivity of the geologic formation containing hydrocarbons. The resistivity of geologic formations is generally related to porosity, permeability, and fluid content of the reservoir. Because hydrocarbons are generally electrically insulating and most formation water is electrically conductive, formation resistivity (or conductivity) measurements are a valuable tool in determining the hydrocarbon content of reservoirs. Moreover, formation resistivity measurements may be used to monitor changes in reservoir hydrocarbon content during production of hydrocarbons.
Formation resistivity measurements are often made with wireline conveyed measurement while drilling (MWD) and logging while drilling (LWD) tools. However, wireline MWD and LWD resistivity tools typically only measure formation resistivity proximate the individual wellbore in which they are operated. As a result, there have been several attempts to determine the resistivity of geologic formations surrounding and between adjacent wellbores drilled into the geologic formations of interest. For example, measurement of formation resistivity between adjacent wellbores using a low frequency electromagnetic system is discussed 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.
FIG. 1
shows an example of a system used to measure formation resistivity between two wellbores. A transmitter T is located in one wellbore and consists of a coil C
T
having multi-turn horizontal loop (vertical solenoid) of N
1
turns and an effective cross section A
T
. The multi-turn horizontal loop carries an alternating current I
T
at a frequency of f
0
Hz. In free space, the multi-turn horizontal loop produces a time varying magnetic field B
0
. The magnetic field B
0
is proportional to a magnetic moment M
T
of the transmitter T and to a geometric factor k
1
. The magnetic moment M
T
of the transmitter T can be defined as follows:
M
T
=N
T
I
T
A
T
. (1)
In free space, the magnetic field B
0
can be defined as follows:
B
0
=k
1
M
T
. (2)
The geometric factor k
1
is a function of a spatial location and orientation of a component of the magnetic field B
0
measured by a receiver R.
The receiver R is located some distance from the transmitter T and is typically disposed in a different wellbore. The receiver R typically includes a loop of wire (e.g., a coil C
R
having N
R
turns wound about a core of high magnetic permeability metal such as ferrite). A time-varying magnetic field B
R
sensed by the receiver R, having a frequency f
0
, creates an induced voltage V
R
in the coil C
R
which is proportional to B
R
, the frequency f
0
, the number of turns of wire N
R
, an effective cross-sectional area of the coil A
R
, and an effective magnetic permeability &mgr;
R
of the coil C
R
. From the foregoing, V
R
can be defined as follows:
V
R
=f
0
B
R
N
R
A
R
&mgr;
R
. (3)
By simplifying equation (3), V
R
may be written as follows:
V
R
=k
R
B
R
. (4)
where k
R
=f
0
N
R
A
R
&mgr;
R
. The product of A
R
&mgr;
R
is difficult to calculate. To accurately determine A
R
&mgr;
R
, C
R
is calibrated in a known magnetic field and at a known frequency to determine an exact value for k
R
. Thereafter, the magnetic field B
R
sensed by the receiver R is related directly to the measured voltage V
R
by the following equation:
B
R
=
V
R
k
R
.
(
5
)
When a system such as this is placed in a conductive geologic formation, the time varying magnetic field B
0
produces an electromotive force (emf) in the geologic formation which in turn drives a current therein, shown schematically as L
1
in FIG.
1
. The current L
1
is proportional to the conductivity of the geologic formation and the flow of the current L
1
is generally concentric about the longitudinal axis of the wellbore. The magnetic field proximate the wellbore is a result of the free space field B
0
, called the primary magnetic field, and the field produced by the 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 of the surrounding formation is small, the induced current L
1
is proportional to dB/dt and is, consequently, 90° out of phase with respect to I
T
. As the frequency increases, the inductive reactance increases and the phase difference increases.
The secondary magnetic field detected by the receiver R is caused by the induced current L
1
and also has a phase shift so that the total magnetic field at the receiver R is complex in nature. The total magnetic field has a component B
R
in-phase with the transmitter current I
T
(referred to as the real component) and a component B
1
phase shifted by 90° (referred to as the imaginary or quadrature component). The values of the real B
R
and quadrature B
1
components of the magnetic field at a given frequency and geometric configuration uniquely specify the electrical resistivity of a homogenous formation penetrated by the wellbores. In a nonhomogeneous geologic formation, the complex magnetic field is generally measured at a succession of points along the longitudinal axis of the receiver wellbore for each of a succession of transmitter locations. The multiplicity of T-R locations suffices to determine the nonhomogeneous resistivity between the wellbores as described in the references listed below.
In general, nonhomogeneous distribution of electrical resistivity in a geologic formation is determined through a process called inversion, which is well described in
Audio
-
frequency electromagnetic tomography in
2-
D
, Geophysics, Vol. 58, No. 4, 1993, by Zhou et al.;
Electromagnetic conductivity imaging with an iterative born inversion
, IEEE Transactions on Geoscience and Remote Sensing, Vol. 31, No. 4, 1993, by Alumbaugh et al.;
An approach to nonlinear inversion with applications to cross
-
well EM tomogaphy
, 63rd Annual International Meeting, Society of Exploration Geophysics, Expanded Abstracts, 1993, by Torres-Verdin et al.; and
Crosswell electromagnetic inversion using integral and differential equations
, Geophysics, Vol. 60, No. 3,. 1995, by Newman. The inversion process has been used to determine resistivity in the vicinity of a single wellbore or between spaced-apart wellbores wells and is described in detail in
Crosswell electromagnetic tomography: System
Jeffery Brigitte L.
McEnaney Kevin P.
Ryberg John J.
Schlumberger Technology Corporation
Strecker Gerard R.
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