Directional electromagnetic measurements insensitive to dip and anisotropy

• US 20030085707A1
• Filed: 03/29/2002
• Published: 05/08/2003
• Est. Priority Date: 09/26/2001
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Rosthal, Richard A., Minerbo, Gerald N., Omeragic, Dzevat
• Granted Patent: US 6,969,994 B2
• Time in Patent Office: Days
• US Class Current: 324/343
• CPC Class Codes: G01V 3/28 using induction coils

Abstract

Systems and methods are provided for directional propagation-type logging measurements not sensitive to dip and anisotropy over a wide frequency range. Estimates of the distance of the logging tool to a bed boundary are obtained using “up-down” measurements with antenna configurations having tilted magnetic dipoles. These estimates are insensitive to anistropy and dip, undesirable complications of directional measurements.

Claims

1. A method for determining a property of a subsurface formation traversed by a borehole, comprising:

a) disposing a logging instrument having a longitudinal axis and equipped with multiple antennas within the borehole, a first transmitter antenna having its magnetic moment oriented in a first direction with respect to the instrument axis, a first receiver antenna having its magnetic moment oriented in said first direction with respect to the instrument axis, a second transmitter antenna having its magnetic moment oriented in a second direction with respect to the instrument axis, a second receiver antenna having its magnetic moment oriented in said second direction with respect to the instrument axis;

b) activating the first transmitter antenna to transmit electromagnetic energy;

c) measuring a signal associated with the transmitted energy at the second receiver antenna;

d) deactivating the first transmitter antenna;

f) activating the second transmitter antenna to transmit electromagnetic energy;

g) measuring a signal associated with the transmitted energy at the first receiver antenna; and

h) calculating the difference between the measured signals of step (c) and step (g) to determine the formation property.

2. The method of claim 1 wherein the first and second directions are different and each direction is between 0 and 180 degrees to the instrument axis, excluding 90 degrees to the instrument axis.

3. The method of claim 2, wherein the method comprises an induction type measurement.

4. The method of claim 2, wherein the formation property is resistivity or a distance to a layer boundary.

5. The method as defined in claim 1, wherein the first and second directions are identical and between 0 and 180 degrees to the instrument axis.

6. The method of claim 5, wherein the measured signals comprise a voltage amplitude or a phase shift.

7. The method of claim 5, wherein the formation property is resistivity or a distance to a layer boundary.

8. A method for determining a property of a subsurface formation traversed by a borehole, comprising:

a) disposing a logging instrument having a longitudinal axis and equipped with multiple antennas within the borehole, a first transmitter antenna having its magnetic moment substantially aligned with the instrument axis, a first receiver antenna having its magnetic moment substantially aligned with the instrument axis, a second transmitter antenna having its magnetic moment substantially perpendicular to the instrument axis, a second receiver antenna having its magnetic moment substantially perpendicular to the instrument axis;

b) activating the first transmitter antenna to transmit electromagnetic energy;

c) measuring a signal associated with the transmitted energy at the second receiver antenna;

d) deactivating the first transmitter antenna;

f) activating the second transmitter to transmit electromagnetic energy;

g) measuring a signal associated with the transmitted energy at the first receiver antenna; and

h) calculating the difference between the measured signals of step (c) and step (g) to determine the formation property.

9. The method of claim 8, wherein the method comprises an induction type measurement.

10. The method of claim 8, wherein the formation property is resistivity or a distance to a layer boundary.

11. A method for determining a property of a subsurface formation traversing a borehole, comprising:

a) disposing a logging instrument having a longitudinal axis and equipped with multiple antennas within the borehole, a transmitter antenna having its magnetic moment oriented in a first direction with respect to the instrument axis, a receiver antenna having its magnetic moment oriented in said second direction with respect to the instrument axis;

b) activating the transmitter antenna to transmit electromagnetic energy;

c) measuring a signal associated with the transmitted energy at the receiver antenna;

e) rotating the instrument by 180 degrees about its longitudinal axis;

f) activating the transmitter antenna to transmit electromagnetic energy;

g) measuring a signal associated with the transmitted energy at the receiver antenna; and

h) calculating a difference between the measured signals of step (c) and step (g) to determine the formation property.

12. The method of claim 11, wherein the first direction consists of an angle θ

₁ to the instrument axis and the second direction consists of an angle θ

₂=180°

−

θ

₁ to the instrument axis, θ

₁ excluding 90 degrees to the instrument axis.

13. The method of claim 12, wherein the method comprises an induction type measurement.

14. The method of claim 12, wherein the formation property is resistivity or a distance to a layer boundary.

15. A method for determining a property of a subsurface formation traversing a borehole, comprising:

a) disposing a logging instrument having a longitudinal axis and equipped with multiple antennas within the borehole, a transmitter antenna having its magnetic moment oriented in a first direction with respect to the instrument axis, a receiver antenna having its magnetic moment oriented in said second direction with respect to the instrument axis;

b) activating the transmitter antenna to transmit electromagnetic energy;

c) measuring a signal associated with the transmitted energy at the receiver antenna;

e) rotating the instrument by 180 degrees about its longitudinal axis;

f) activating the transmitter antenna to transmit electromagnetic energy;

g) measuring a signal associated with the transmitted energy at the receiver antenna; and

h) calculating a ratio between the measured signals of step (c) and step (g) to determine the formation property.

16. The method of claim 15, wherein the first direction consists of an angle θ

₁ to the instrument axis and the second direction consists of an angle θ

₂=180°

−

θ

₁ to the instrument axis, θ

₁ excluding 90 degrees to the instrument axis.

17. The method of claim 16, wherein the formation property is resistivity or a distance to a layer boundary.

18. A method for determining a property of a subsurface formation traversed by a borehole, comprising:

a) disposing a logging instrument having a longitudinal axis and equipped with multiple antennas within the borehole, a first transmitter antenna having its magnetic moment oriented in a first direction with respect to the instrument axis, a second transmitter antenna having its magnetic moment oriented in said first direction with respect to the instrument axis, a first receiver antenna having its magnetic moment oriented in a second direction with respect to the instrument axis;

b) activating the first transmitter antenna to transmit electromagnetic energy;

c) measuring a signal associated with the transmitted energy at the receiver antenna;

d) deactivating the first transmitter antenna;

e) displacing the instrument along the borehole;

f) activating the second transmitter to transmit electromagnetic energy;

i) measuring a signal associated with the transmitted energy at the receiver antenna; and

j) calculating a difference between the measured signals of step (c) and step (i) to determine the formation property.

19. The method of claim 18 wherein the first and second directions are different and each direction is between 0 and 180 degrees to the instrument axis, excluding 90 degrees to the instrument axis.

20. The method of claim 18, wherein the method comprises an induction type measurement.

21. The method of claim 19, wherein the measured signals comprise a voltage amplitude or a phase shift.

22. The method of claim 19, wherein the formation property is resistivity or a distance to a layer boundary.

23. A method for determining a property of a subsurface formation traversed by a borehole, comprising:

a) disposing a logging instrument having a longitudinal axis and equipped with multiple antennas within the borehole, a first transmitter antenna having its magnetic moment substantially aligned with respect to the instrument axis, a second transmitter antenna having its magnetic moment substantially aligned with respect to the instrument axis, a first receiver antenna having its magnetic moment oriented in a first direction with respect to the instrument axis, a second receiver antenna having its magnetic moment oriented in a second direction with respect to the instrument axis, a third receiver antenna having its magnetic moment oriented in a third direction with respect to the instrument axis, a fourth receiver antenna having its magnetic moment oriented in a fourth direction with respect to the instrument axis;

b) activating the first transmitter antenna to transmit electromagnetic energy;

c) measuring a signal associated with the transmitted energy at the first receiver antenna;

d) measuring a signal associated with the transmitted energy at the second receiver antenna;

e) deactivating the first transmitter antenna;

f) activating the second transmitter to transmit electromagnetic energy;

g) measuring a signal associated with the transmitted energy at the first receiver antenna;

h) measuring a signal associated with the transmitted energy at the second receiver antenna;

i) calculating a ratio between the measured signals of step (b) and step (c)

j) calculating a ratio between the measured signals of step (k) and step (l)

k) calculating a difference between the values of step (m) and step (n) to determine the formation property.

24. The method of claim 23, wherein the calculations of step (i) and step (j) include deriving a logarithmic value.

25. The method of claim 24, wherein the measured signals comprise a voltage amplitude or a phase shift.

26. The method of claim 24, wherein the formation property is resistivity or a distance to a layer boundary.

27. A method for determining a property of a subsurface formation traversed by a borehole, comprising:

a) disposing a logging instrument having a longitudinal axis and equipped with multiple antennas within the borehole, a first transmitter antenna having its magnetic moment oriented in a first direction with respect to the instrument axis, a second transmitter antenna having its magnetic moment oriented in a second direction with respect to the instrument axis, a first receiver antenna having its magnetic moment oriented in a third direction with respect to the instrument axis, a second receiver antenna having its magnetic moment oriented in a fourth direction with respect to the instrument axis, a third receiver antenna having its magnetic moment oriented in a fifth direction with respect to the instrument axis, a fourth receiver antenna having its magnetic moment oriented in a sixth direction with respect to the instrument axis;

b) activating the first transmitter antenna to transmit electromagnetic energy;

c) measuring a signal associated with the transmitted energy at the first receiver antenna;

d) measuring a signal associated with the transmitted energy at the second receiver antenna;

e) measuring a signal associated with the transmitted energy at the third receiver antenna;

f) measuring a signal associated with the transmitted energy at the fourth receiver antenna

g) deactivating the first transmitter antenna

h) rotating the instrument by 180 degrees about its longitudinal axis;

i) activating the second transmitter to transmit electromagnetic energy;

j) measuring a signal associated with the transmitted energy at the first receiver antenna;

k) measuring a signal associated with the transmitted energy at the second receiver antenna

l) measuring a signal associated with the transmitted energy at the third receiver antenna

m) measuring a signal associated with the transmitted energy at the fourth receiver antenna; and

n) calculating a series of ratios and differences between the measured signals to determine the formation property.