VARIABLE DAMPING CONTROL FOR NUCLEAR MAGNETIC RESONANCE DATA ACQUISITION SYSTEM TO ENHANCE OILFIELD LOGGING PERFORMANCE

Abstract

A method to calibrate a nuclear magnetic resonance tool is disclosed having steps of starting a nuclear magnetic resonance sequence from the nuclear magnetic resonance tool, disabling an active damping circuit in the nuclear magnetic resonance tool, collecting auxiliary calibration data for the nuclear magnetic resonance tool, estimating a natural Q value for the nuclear magnetic resonance tool, determining an optimal active damping setting for the tool, deploying the optimal active damping setting for the tool, collecting nuclear magnetic resonance response data generated from the nuclear magnetic resonance sequence and calibrating the nuclear magnetic resonance data.

Claims

1. A method for processing nuclear magnetic resonance data, comprising: placing a nuclear magnetic resonance tool in a wellbore; activating the nuclear magnetic resonance tool to generate a signal to a geological formation; active damping the nuclear magnetic resonance tool; and receiving a response signal from the geological formation.

2. The method according to claim 1, wherein the active damping of the nuclear magnetic resonance tool is at least one of through a differential multiplexer, an arrangement of switched resistors, and an arrangement having a variable gate array.

3. A method to calibrate a nuclear magnetic resonance tool, comprising: starting a nuclear magnetic resonance sequence from the nuclear magnetic resonance tool; disabling an active damping circuit in the nuclear magnetic resonance tool; collecting auxiliary calibration data for the nuclear magnetic resonance tool; estimating a natural Q value for the nuclear magnetic resonance tool; determining an optimal active damping setting for the tool; deploying the optimal active damping setting for the tool; collecting nuclear magnetic resonance response data generated from the nuclear magnetic resonance sequence; and calibrating the nuclear magnetic resonance data.

4. The method according to claim 3, further comprising: determining when the nuclear magnetic sequence is completed.

5. The method according to claim 4, wherein when the nuclear magnetic sequence is not completed, a next nuclear magnetic resonance segment is gathered.

6. The method according to claim 4, wherein when the nuclear magnetic sequence is completed, the method returns to the determining the optimal active damping setting.

7. The method according to claim 4, further comprising: performing a multi-dimensional master calibration after the determining the optimal active damping setting.

8. The method according to claim 7, wherein after the performing the multi-dimensional master calibration after the determining the optimal active damping setting, the nuclear magnetic data are calibrated.

9. A method for processing nuclear magnetic resonance data, comprising: placing a nuclear magnetic resonance tool in a wellbore; starting a nuclear magnetic resonance sequence from the nuclear magnetic resonance tool; disabling an active damping circuit in the nuclear magnetic resonance tool; collecting auxiliary calibration data for the nuclear magnetic resonance tool; estimating a natural Q value for the nuclear magnetic resonance tool; determining an optimal active damping setting for the tool; performing a multi-dimensional master calibration master calibration based on the natural system Q and a variable active damping setting; deploying the optimal active damping setting for the tool; collecting nuclear magnetic resonance response data generated from the nuclear magnetic resonance sequence; and calibrating the nuclear magnetic resonance data based upon the Q value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In the drawings, sizes, shapes, and relative positions of elements are not drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements may have been arbitrarily enlarged and positioned to improve drawing legibility.

[0014] FIG. 1 is a diagrammatic representation of a nuclear magnetic resonance simplified sequence.

[0015] FIG. 2 is a change in noise to signal ratio with Q-Q variations that arise from sample salinity.

[0016] FIG. 3 is a change in noise to signal ratio and ringing with Q-Q variations that are due to circuit parameter changes.

[0017] FIG. 4 is an example embodiment of a non-limiting configuration used for variable damping control of nuclear magnetic resonance data.

[0018] FIG. 5 is a graph of the effect of changes in the value of capacitance at the output of the active damper on the system.

[0019] FIG. 6 is a second example embodiment of a non-limiting configuration used for variable damping control of nuclear magnetic resonance data.

[0020] FIG. 7 is a third example embodiment of a non-limiting configuration used for variable damping control of nuclear magnetic resonance data.

[0021] FIG. 8 is a method for selection of an optimal Q value.

DETAILED DESCRIPTION

[0022] The main events in a generic NMR pulse sequence are illustrated in FIG. 1. During the TX ON pulse, a transmitter applies a sinusoidal voltage to the antenna at a specified frequency and phase. From this voltage, the antenna generates a resonant frequency pulse towards the formation being logged. After transmission stops, the formation will generate a very small response known as an “echo” and illustrated in red in FIG. 1. Unfortunately, this echo response is typically small (around 1 microvolt) such that a receiver must amplify the response significantly to improve the signal and so the echo can be sampled into the tool's digital signal processing chain without significant quantization error. Any noise or distortion received will also be amplified, so it is necessary that no additional noise is added by the receiver circuitry.

[0023] After transmission, the capacitive and inductive elements in the antenna contain significant amounts of residual energy. Due to low natural damping in the antenna, this energy is dissipated very slowly and there may be substantial residual “ringing” as shown in blue in FIG. 1. The bulk of this residual energy can be dissipated quickly by introducing high resistive damping into the circuit using a device commonly known as a Q-switch; however, this Q-switch has to be disabled (turned-off) before the echo arrives, otherwise the echo signal is also dissipated and the formation response is lost. The Q-switch turn-off transient is known to cause the circuit to “ring anew”. In practice, this ringing can be large enough to corrupt the echo signal to the point of making it unusable unless something is done to mitigate it. One of the best methods to do so is through an active damping circuit. Active damping circuits introduce additional damping, causing the ringing injection from the Q-switch turn-off to decay faster, without adding significant noise.

[0024] Experimental data such as that illustrated in FIG. 2 and FIG. 3 indicates that there is a strong correlation between the overall tool noise, represented as the noise to signal ratio (NSR) performance metric in the figures, and the amount of damping in the system, represented as the quality factor (Q) in the figures. Note there is an inverse relation between Q and damping (high Q->low damping, low Q->high damping). As Q increases, either due to changes in the sample's salinity or due to changes in the electronic circuit, the noise to signal ratio decreases. Also, as Q decreases, the noise to signal ratio increases. As one would expect, the opposite is true for the ringing performance metric as shown in FIG. 3. As illustrated in FIG. 2, different salinity earth formations may be evaluated, providing a significant benefit to evaluators. The methods and apparatus described, therefore, can be modified, as described herein to adapt to environmental conditions, therefore the configurations disclosed should not be considered limiting.

[0025] The following facts are worth noting: [0026] 1—The salinity of the sample and the resulting Q cannot be controlled and will change depending on logging conditions. [0027] 2—Low noise to signal ratio and low ringing are desired for all conditions, so the optimization of the tool's electronics results in a tradeoff between ringing and noise to signal ratio. [0028] 3—The weight of importance between ringing and noise to signal ratio depends on the configuration of the measurement being made. For measurements with long echo spacings, ringing is less important than noise to signal ratio. For measurements with short echo spacings, ringing is more important than noise to signal ratio. [0029] 4—The parameters for the electronics are fixed and optimized for a “nominal” logging condition and measurement configuration. Once the condition or configuration changes, the tuning is no longer optimal and the performance of the measurement degrades.

[0030] The amount of damping introduced by the active damping circuit is determined by the electrical parameters of the circuit. In non-limiting embodiments, the aspects disclosed make those parameters variable and controllable. By doing so, the resulting system Q can be manipulated in real-time to adjust for the logging conditions or the measurement configuration. The added capabilities allow for: [0031] 1—Compensating for the effect of salinity such that the system Q remains constant regardless of logging conditions. [0032] 2—Pursuing the optimal compromise between ringing and noise to signal ratio based on the measurement being made. For low echo spacing, TE measurements, the system Q is intentionally lowered. For high echo spacing, TE measurements, the system Q is intentionally increased.

[0033] A simplified view of one example embodiment is illustrated in FIG. 4. The parameter being varied is the output capacitance of an active damping circuit using a differential multiplexer. The signal controlling the multiplexer is produced by an external digital control system. This digital control system has the appropriate information regarding the logging conditions and measurement configuration to select the optimal Q at any given time. Referring to FIG. 5, the effects of changes in the value of capacitance at the output of the active damper on the system Q is illustrated. The value of Q is graphed along an “x” axis of active damping output capacitance in pF. Referring to FIG. 6, the left side of the FIGS. shows a variable active damping circuit using switched resistors. The configuration illustrated in FIG. 6 uses switched resistors, as compared to FIG. 4 which uses a differential multiplexer. The right side of FIG. 6 illustrates the effects that the system of FIG. 6 has on different values. The value of Q is graphed versus resistance in ohms. As evident from the data, the value of Q rapidly decreases up to a value of resistance of 1 ohm and then gradually decreases thereafter. Referring to FIG. 7 left side, this configuration uses a variable gate array arrangement for variable active damping as another non-limiting embodiment. This configuration may be used as an alternative to FIG. 4 and FIG. 6. The right side of FIG. 7 shows the effect on values such as Q and variable active damping over a range of values. As Q decreases, the variable active damping gain decreases as illustrated.

[0034] The control system to select the optimal hardware setting follows the steps illustrated in FIG. 8. First, the active damper is disabled to avoid any interference with the auxiliary calibration measurement. The auxiliary calibration measurement is taken to determine the logging condition. This condition is characterized by the natural Q (QNAT). The echo spacing (TE) of the measurement about to be made is collected as well. Once QNAT and TE are known, it is possible to use calibration data collected a-priory to select the variable active damping setting that achieves the optimal compromise between noise and ringing. This setting is then implemented and the NMR measurement made. The calibration data is again used to calibrate the measurement, allowing the elimination of unwanted variability in the products caused by the variable active damping circuit.

[0035] A comprehensive master-calibration database is needed for proper control of the variable active damping circuit. The first dataset in the database captures the effect of the variable active damping setting to the system Q for a given set of logging conditions.

TABLE-US-00001 TABLE 1 System Q With Respect to Logging Conditions and Variable Active Damping Setting Natural System Q Q.sub.NAT.sub.—.sub.1 Q.sub.NAT.sub.—.sub.2 . . . Q.sub.NAT.sub.—.sub.N Variable Active AD.sub.SET.sub.—.sub.1 Q.sub.SYS.sub.—.sub.1,1 Q.sub.SYS.sub.—.sub.1,2 . . . Q.sub.SYS.sub.—.sub.1,N Damping Setting AD.sub.SET.sub.—.sub.2 Q.sub.SYS.sub.—.sub.2,1 Q.sub.SYS.sub.—.sub.2,2 . . . . . . . . . . . . . . . . . . . . . AD.sub.SET.sub.—.sub.M Q.sub.SYS.sub.—.sub.M,1 Q.sub.SYS.sub.—.sub.M,2 . . . Q.sub.SYS.sub.—.sub.M,N

[0036] The result is an N×M matrix provided in Table 1. N is the number of Natural System Qs (Q.sub.NAT) tested during calibration. The number of Q.sub.NAT values that will be encountered in practice is virtually infinite, however, if the calibration dataset covers the full range of possible Q.sub.NAT values with enough granularity, any Q.sub.NAT value found in practice can be approximated using the nearest value in the calibration table. M is the number of possible settings implemented in the variable active damping circuit. This is circuit dependent and can be scaled up easily.

[0037] The second dataset, provided in Table 2, relates the system Q to the most critical performance metrics for the tool: ringing and noise to signal ratio. These are measured over the expected range of echo spacing levels, which can be segregated into high and low to reduce the dimensions of the calibration database.

TABLE-US-00002 TABLE 2 System Q Q.sub.SYS.sub.—.sub.1,X Q.sub.SYS.sub.—.sub.2,X . . . Q.sub.SYS.sub.—.sub.M,X Ringing With Respect to System Q and Echo Spacing for Q.sub.NAT.sub.—.sub.X Echo TE.sub.HIGH RING.sub.HIGH,1,X RING.sub.HIGH,2,X . . . RING.sub.HIGH,M,X Spac- TE.sub.LOW RING.sub.LOW,1,X . . . . . . RING.sub.LOW,M,X ing NSR With Respect to System Q and Echo Spacing for Q.sub.NAT.sub.—.sub.X Echo TE.sub.HIGH NSR.sub.HIGH,1,X NSR.sub.HIGH,2,X . . . NSR.sub.HIGH,M,X Spac- TE.sub.LOW NSR.sub.LOW,1,X . . . . . . NSR.sub.LOW,M,X ing

[0038] With this data, it is possible to find the optimal system Q level for a given logging condition. The optimal Q (and the optimal active damper setting) is that which minimizes the function below.

[00001]$\begin{array}{cc}J\left(\mathrm{TE}\right)=C_{\mathrm{NSR}}\left(\mathrm{TE}\right).Math.\frac{\mathrm{NSR}\left(\mathrm{TE}\right)}{{\mathrm{NSR}}_{\mathrm{NOMINAL}}}+C_{\mathrm{RING}}\left(\mathrm{TE}\right).Math.\frac{\mathrm{RING}\left(\mathrm{TE}\right)}{{\mathrm{RING}}_{\mathrm{NOMINAL}}}& \mathrm{Equation}.Math..Math.1\end{array}$

[0039] The NSR.sub.NOMINAL and RING.sub.NOMINAL constants are used to normalize the performance metrics. The weights C.sub.NSR and C.sub.RING depend on the echo spacing of the segment to be executed. They can be defined as shown below in Table 3 to reflect that ringing is more important at low TE, and noise to signal ratio is more important at high TE. These can be re-adjusted as needed, but are expected to remain unchanged once the best set is found.

TABLE-US-00003 TABLE 3 Echo Spacing (TE) C.sub.NSR C.sub.RING HIGH 0.9 0.1 LOW 0.1 0.9

[0040] The calibration datasets can be approximated using smooth mathematical functions and curve fitting, if needed. The resulting parametric equations can improve computational efficiency and simplify the optimization effort, but so far this has been unnecessary due to the size of the matrices at play. As illustrated, the data in Table 3 may be used in FIG. 8 at 816.

[0041] The effects of variable active damping on the tool's final output may also be eliminated. This can be done using a variable calibration parameter, which depends on the operating condition and the active damping setting. The data needed to calculate this parameter is already available from the data gathered to fill the tables in the previous section. The calibration data is tabulated as shown below in Table 4.

TABLE-US-00004 TABLE 4 Master-Calibration Parameter With Respect to Logging Conditions and Variable Active Damping Setting Natural System Q Q.sub.NAT.sub.—.sub.1 Q.sub.NAT.sub.—.sub.2 . . . Q.sub.NAT.sub.—.sub.N Variable Active AD.sub.SET.sub.—.sub.1 CAL.sub.1,1 CAL.sub.1,2 . . . CAL.sub.1,N Damping Setting AD.sub.SET.sub.—.sub.2 CAL.sub.2,1 CAL.sub.2,2 . . . . . . . . . . . . . . . . . . . . . AD.sub.SET.sub.—.sub.M CAL.sub.M,1 CAL.sub.M,2 . . . CAL.sub.M,N

[0042] Table 4 is indexed using the variable active damping setting from the digital control algorithm and the natural damping data from the calibration algorithm. The corresponding calibration parameter is used to “normalize” the measurement prior to reporting it to higher level data processing processes. This master calibration parameter may be used, for example, in 826 and 824, as necessary, according to FIG. 8.

[0043] Pre-processing and optimization of the calibration data can be completed off-line to obtain a direct mapping from Q.sub.NAT.sub._.sub.X to AD.sub.SET.sub._.sub.Y and CAL.sub.Y,X. In this case, the optimal configuration and its corresponding calibration parameter become known as soon as Q.sub.NAT.sub._.sub.X is estimated, bypassing the need for real-time optimization. This can greatly simplify the software implementation.

[0044] Referring to FIG. 8, a method 800 for calibration of a nuclear magnetic resonance tool using variable damping control for a nuclear magnetic resonance data acquisition system is illustrated. At 802, a nuclear magnetic resonance sequence is started to activate a geological formation. At 804, active damping is disabled. At 806, auxiliary calibration data is collected. At 808, system Q is estimated. At 810, an optimal active damping setting is determined. At 812, optimal active damping setting is deployed for the system. Nuclear magnetic resonance data is then collected at 814. Nuclear magnetic resonance data is then calibrated at 816. At 818, if the nuclear magnetic resonance sequence is not completed, then the next nuclear magnetic resonance segment is proceeded to, where segment information is gathered and the process continues from 810. If the nuclear magnetic resonance sequence is completed as determined at 818, then the process returns to step 810 by going to the next segment 820 by gathering NMR segment information 821.

[0045] At step 810, multi-dimensional master calibration may be accomplished, as necessary in 826. After multi-dimensional master calibration is completed, the method may return to determine optimal active damping settings 810. After the estimation of the natural Q for the system 808, multi-dimensional master calibration 824 may also be accomplished. Similarly, multi-dimensional master calibration 824 may be performed after determination of optimal active damping setting 810.

[0046] Certain embodiments and features may have been described using a set of numerical upper limits and a set of numerical lower limits. It should be appreciated that ranges including the combination of any two values, e.g., the combination of any lower value with any upper value, the combination of any two lower values, or the combination of any two upper values are contemplated. Certain lower limits, upper limits and ranges may appear in one or more claims below. Numerical values are “about” or “approximately” the indicated value, and take into account experimental error, tolerances in manufacturing or operational processes, and other variations that would be expected by a person having ordinary skill in the art.

[0047] The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include other possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.