Apparatus and method for downhole transient resistivity measurement and inversion

Patent metadata

Field	Value
Publication number	US-9841527-B2
Application number	US-201313903665-A
Country	US
Kind code	B2
Filing date	May 28, 2013
Priority date	Nov 15, 2012
Publication date	Dec 12, 2017
Grant date	Dec 12, 2017

Abstract

A method of processing electromagnetic signal data includes: disposing a downhole tool in a borehole in an earth formation, the downhole tool including at least one electromagnetic transmitter; performing a downhole electromagnetic operation, the operation including transmitting an electromagnetic pulse from the transmitter into the formation and measuring a time domain transient electromagnetic (TEM) signal over a selected time interval following a transmitter turn-off time; transforming the measured time domain TEM signal into a frequency domain TEM signal measured; and applying an inversion technique to the transformed frequency domain TEM signal to estimate one or more formation parameters.

First claim

What is claimed is:

1. A method of processing electromagnetic signal data, the method comprising:
disposing a downhole measurement tool with a borehole string in a borehole in an earth formation, the downhole tool including at least one electromagnetic transmitter;
performing a downhole electromagnetic operation, the operation including transmitting an electromagnetic pulse from the at least one electromagnetic transmitter into the formation and measuring a time domain transient electromagnetic (TEM) signal over a selected time interval following a transmitter turn-off time;
calculating an apparent resistivity of the formation at a selected time within the time interval;
calculating a synthetic TEM signal based on a homogeneous medium having a resistivity equal to the apparent resistivity, the synthetic TEM signal corresponding to a primary magnetic field signal generated by the at least one electromagnetic transmitter;
generating a time domain secondary signal based on comparing the measured time domain TEM signal to the synthetic time domain TEM signal;
transforming the time domain secondary signal into a frequency domain TEM signal;
applying an inversion technique to the transformed frequency domain TEM signal to estimate one or more formation parameters;
estimating a distance from the measurement tool to a boundary or interface in the formation based on the one or more formation parameters; and
geosteering the borehole string to advance the borehole string in a selected direction based on the distance to the boundary or interface from the downhole measurement tool.

2. The method of claim 1 , wherein generating the time domain secondary signal includes subtracting the synthetic TEM signal from the measured time domain TEM signal.

3. The method of claim 2 , wherein transforming includes calculating a synthetic frequency domain TEM signal corresponding to the primary magnetic field, and adding the synthetic frequency domain signal to the frequency domain secondary signal to generate the transformed frequency domain TEM signal.

4. The method of claim 1 , wherein the selected time is an early time at which the primary magnetic field is equal to a measured magnetic field or is at least a selected portion of the magnitude of the measured magnetic field.

5. The method of claim 1 , wherein calculating the synthetic time domain TEM signal includes calculating a TEM signal that would be measured over the time interval by the downhole tool based on an assumption that the downhole tool is disposed in a homogeneous medium having the calculated apparent resistivity.

6. The method of claim 1 , wherein the measured time domain TEM signal is transformed via a Fourier transform.

7. The method of claim 6 , wherein the measured time domain TEM signal is transformed via the following:

H
→

*

⁡

(
f
)

=

-

1

μ
0

⁢

∫

t
1

t
n

⁢

d
⁢

B
→

⁡

(
t
)

dt

⁢

e

i2π
⁢

⁢
f
⁢

⁢
t

⁢

⁢
dt

,

(
1
)

wherein

d
⁢

B
→

⁡

(
t
)

dt

is the derivative of the magnetic field or the measured voltage resulting from a turn-off current impulse in a transmitter coil, [t 1 , t n ] is the measurement time interval, {right arrow over (H)}*(f) is the frequency-dependent magnetic field, and μ 0 is magnetic permeability.

8. The method of claim 6 , wherein the downhole tool includes a first receiver disposed at a first axial distance (L 1 ) from the transmitter, and a second receiver disposed at a second axial distance (L 2 ) from the transmitter that is less than the first axial distance.

9. The method of claim 8 , wherein the measured time domain TEM signal is transformed via the following:

F
→

⁡

(
f
)

=

H
→

1
*

⁡

Assignees

• Nikitenko Marina N
• Baker Hughes A Ge Co Llc

Inventors

• Nikitenko Marina N

Classifications

• G01V3/28 Primary

  using induction coils · CPC title