Determining Electrical Characteristics of an Electrical Cable

• US 20090322554A1
• Filed: 09/04/2009
• Published: 12/31/2009
• Est. Priority Date: 04/26/2007
• Status: Active Grant

• Current Assignee: Ramon Hernandez-Marti, Vinod Rajasekaran
• Original Assignee: Ramon Hernandez-Marti, Vinod Rajasekaran
• Inventors: Rajasekaran, Vinod, Hernandez-Marti, Ramon
• Granted Patent: US 8,058,882 B2
• Time in Patent Office: Days
• US Class Current: 340/854.9
• CPC Class Codes: G01R 31/58 Testing of lines, cables or conductors testing of electric windings G01R31/72
• CPC Class Codes: G01V 11/002 Details, e.g. power supply systems for logging instruments, transmitting or recording data, specially adapted for well logging, also if the prospecting method is irrelevant means for transmitting well survey signals E21B47/12; signal transmission systems in general G08C; transmission in general H04B
• CPC Class Codes: G10K 11/002 Devices for damping, suppressing, obstructing or conducting sound in acoustic devices G10K1/06 - G10K1/10 take precedence; for electro-mechanical transducers for communication H04R3/002

Abstract

To characterize an electrical cable that is deployed in a well, a voltage input is applied to the electrical cable at an earth surface location, where the well extends from the earth surface location. A current response resulting from the voltage input is measured at the earth surface location. At least one parameter of the electrical cable is computed according to the measured current response.

Claims

1-25. -25. (canceled)

26. A method of characterizing an electrical cable that is deployed in a well, comprising:

applying a voltage input to the electrical cable at an earth surface location, wherein the well extends from the earth surface location;

measuring a current response in the electrical cable resulting from the voltage input, wherein the current response is measured at the earth surface location;

computing at least one parameter of the electrical cable according to the measured current response; and

updating the at least one parameter to account for a change in a characteristic of the electrical cable as a result of operation of a load system connected to the electrical cable.

27. The method of claim 26 wherein the load system is connected to a distal end of the electrical cable.

28. The method of claim 26 wherein updating ensures a constant steady state downhole voltage at the load system.

29. The method of claim 26 wherein computing comprises computing the resistance of the electrical cable and updating comprises updating the resistance.

30. The method of claim 29 wherein computing the resistance comprises computing an exact resistance according to:

$V_{1,\mathrm{meas}}=V_{2,\mathrm{meas}}+I_1<br><br>R_{\mathrm{exact}}$ $V_{1,\mathrm{meas}}=V_{2,\mathrm{set}}+I_1<br><br>R_{\mathrm{est}}<br><br>\Rightarrow <br><br>R_{\mathrm{exact}}=R_{\mathrm{est}}-\left(\frac{V_{2,\mathrm{meas}}-V_{2,\mathrm{set}}}{I_1}\right).$

wherein, I₁ is the surface current, V_1,meas is the measured surface voltage, V_2,meas is the measured downhole voltage, V_2,set is the target downhole voltage, R_exact is the exact resistance of the cable, and R_est is the estimated resistance of the cable and wherein the I₁, V_1,meas, V_2,meas and, V_2,set are steady state voltages and current.

31. The method of claim 30 wherein an updated resistance value, R_m+1, is computed according to:

$R_{m+1}=R_m-\left(\frac{V_{2,m}-V_{2,\mathrm{set}}}{I_m}\right)$

wherein R_m+1 corresponds to the correct resistance value R_exact, R_m represents the current resistance value (corresponding to the estimated resistance value R_est), I_m is the current surface current, V_2,m is the measured downhole voltage (same as V_2,meas), and V_2,set is the target downhole voltage.

32. The method of claim 30 wherein an updated resistance value, R_m+1, is computed according to:

$R_{m+1}=R_m-\mathrm{\mu <br><br>}<br><br>\left(\frac{V_{2,m}-V_{2,\mathrm{set}}}{I_m}\right)$

wherein R_m+1 corresponds to the correct resistance value R_exact, R_m represents the current resistance value (corresponding to the estimated resistance value R_est), I_m is the current surface current, V_2,m is the measured downhole voltage (same as V_2,meas), and V_2,set is the target downhole voltage and wherein μ

is a correction factor to correct the updated resistance R_m+1.

33. The method of claim 30, wherein a difference between V_2,meas and the estimated voltage V_2,est. is computed and R_m+1, is computed according to:

R_m+1=R_m+Δ

R if V_2,est>

V_2,meas+Δ

V

R_m+1=R_m−

Δ

Rif V_2,est<

V_2,meas+Δ

V

R_m+1=R_m if |V_2,meas−

V_2,est<

Δ

V 

 

(Eq.

     31)

and wherein Δ

V is a predefined threshold.

34. The method of claim 30, computing the resistance comprises computing an exact resistance, R_exact, according to:

$R_{\mathrm{exact}}=R_{\mathrm{est}}+\frac{\left(V_{2,\mathrm{set}}-E<br><br>\left(V_2\right)\right)}{E<br><br>\left(I_1\right)}.$

where R_est is the estimated resistance and I₁ is the supply current, and where

E(V₂)=V_2,set+E(I₁)(R_est−

R_exact)

σ

²(V₂)=σ

²(I₁)(R_est−

R_exact)²

where E(V₂) represents the average of several downhole measured voltage samples, E(I₁) represents the average of several measured surface current samples, and σ

represents the variance (of the current or voltage).

35. The method of claim 34 wherein computing the resistance comprises computing an exact resistance, R_exact, according to

$R_{\mathrm{exact}}=R_{\mathrm{est}}+\frac{(V_{2,\mathrm{set}}-E}{}$

( V 2 ) + V offset ) E 

( I 1 )

wherein V_offSet is a DC offset.

36. The method of claim 34 wherein computing the resistance comprises obtain an estimate of the absolute value of the model error according to:

$<br><br>R_{\mathrm{est}}-R_{\mathrm{exact}}<br><br>=\frac{\mathrm{\sigma <br><br>}<br><br>\left(V_2\right)}{\mathrm{\sigma <br><br>}<br><br>\left(I_1\right)}.$

37. The method of claim 26, wherein the load system comprises a downhole tool.

38. The method of claim 26, wherein the load system comprises a transformer.

39. The method of claim 26, wherein the transformer allows coupling of power and telemetry.