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Geothermal Resources Council Transactions, Vol. 26, September 22-25, 2002

Evaluating Light-Weight Cement in the Geothermal Environment

Gary W. Batcheller

GWB Consultants, Yukon, OK 73099

Keywords Lightweight cement, foam cement, acoustic impedance, casing, ultrasonic logs, acoustic logs, micro annulus

ABSTRACT The use of lightweight cements has become more prevalent

in high temperature wells due to its improved elasticity. Addi- tionally, the low density of cement adds benefits for geothermal wells where the fracture gradient is low and the incident of natu- ral fractures may be large. These cements have historically been difficult if not impossible to detect with conventional means of log interpretation. New techniques are now available that dra- matically improve the chances for determining the integrity with these cements.

However, the geothermal environment offers many unique challenges that are not generally present in the oil industry. Much higher temperatures and larger casing sizes generally add to the degree of difficulty to gather and interpret data. Several ex- amples illustrate these problems and offer guidelines for im- proving interpretation of these cements.

Quality control of the data is important and particularly with large casing sizes and because of the higher probability of a micro annulus being created. Both extreme temperature changes and the pressure testing of casings can create a micro annulus. These extreme temperature changes and large annulus spaces could change the cement properties (particularly with nitrified cement). The results would make it even more difficult to un- derstand the expected acoustic impedance from ultrasonic logs.

Introduction

Conventional acoustic logs have been used for over 30 years to evaluate cement integrity. These logs depend upon the ce- ment to attenuate the sound. Conventional cement will attenu- ate sound more significantly than will lightweight cement. The attenuation also depends on the density, thickness and compres-

sive strength of cement. Because lightweight cements attenu- ate the sound much less and there is a loss of signal transmitted in larger size casings, it may be very difficult to evaluate ce- ment in this environment with conventional logs. Yet, without sufficient cement around the casing, the dramatic changes in casing due to temperature cycling may cause a string to fail. Another mode of failure is caused any liquid left in places void of cement. The casing could collapse when the liquid flashes to steam under operating temperatures.

The introduction of ultrasonic logs such as the Cement Evalu- ation Tool (CET) and Pulsed Echo Tool (PET) around 10 years ago brought new techniques for evaluating cement in place as well as the distribution of cement around the casing using 8 radial measurements. These technologies have been improved through new algorithms, new electronics and rotating transduc- ers. Schlumberger’s Ultrasonic Imaging Tool (USIT) offers 72 radial measurements and Halliburton’s Circumferential Acous- tic Scanning Tool (CAST-V) tool offers 100 radial measure- ments.

The physics of this measurement compares the acoustic impedance (density x velocity) of cement and the fluid inside the casing and depends upon the value for cement being greater than that of mud. For conventional cements this is true, how- ever, lower density cements have lower acoustic impedance. Therefore, the interpretation of fluid behind the casing may be a liquid when it could be cement.

Figure 1. Transducer size offered by Schlurnberger for USIT (Courtesy Schlurnberger).

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Recently Halliburton and Schlumberger have developed new interpretation techniques to determine if the fluid is a solid rather than a liquid even when the acoustic impedance may be equiva- lent to water. These techniques evaluate the changes in acous- tic impedance that occur rather than the specific value of acous- tic impedance. When these changes are of sufficient magni- tude, the results are an interpretation of a solid rather than a liquid. Even more variability in the acoustic measurements is caused by the addition of gas or lightweight rigid beads to lower the density of the cement.

The diameter and thickness of the casing used in geother- mal wells requires even more attention be paid to quality con- trol, tool selection, standoff and frequencies used in the data gathering. Finally the proper interpretation of the data is af- fected by the near well bore environment and of particular im- portance are the fluids in the well bore. All of these points will be discussed in the paper along with geothermal well examples demonstrating their importance.

Equipment Selection

When gathering ultrasonic data from either the USIT or the CAST it is important to have the greatest signal to noise ratio. This helps assure quality data that can be evaluated. Two im- portant parameters are the distance from the transducer to the casing (standoff) and the frequency of the ultrasonic pulse.

In addition the inner surface of the casing must have a good reflective surface. In other words be a virtually flat surface for the reflection to return properly to the transducer. Thus, mate- rials such as rust, corrosion or even cement on the inside of the casing could cause a signal to be poorly reflected to the trans- ducer. It should be common practice, to run a bit and scraper prior to logging in order to improve this surface.

Standoff is determined by the size of the transducer as com- pared to casing size. Each company has various sizes of trans- ducers available and charts to determine which is best for a given set of conditions. However, neither company has tools that will log effectively in sizes larger than 13 3/8 inch casing. Figure 1 shows the choice of transducers available from Schlumberger. Centralization of the tool will also affect the standoff. The cen- tralization can be monitored by a curve that represents the dif- ference between the opposing radii measured by the tool. This curve is often referred to as the eccentricity curve. If the stand- off varies more than .1 inches it may be affecting the data, and it should not exceed .4 of an inch.

Other quality control checks include the calculations of the thickness of the casing and the internal diameter as compared to known values. In Schlumberger’s case there is an internal plate used for calculating fluid properties in the casing. The thickness and distance of this plate are well known and these calculations can be checked by tool measurements.

conjunction with the ultrasonic logs. As a result of the differ- ences in physics in measurement the main purpose of running both CBL and ultrasonic logs is precisely to determine where they read differently and why? If there is a solid (cement) at any portion of the formation-casing annulus the sound wave from the CBL will be transmitted to the formation and return to the re- ceiver. The VDL will then show a formation signal indicating acoustic coupling though the acoustic impedance may be too low to sufficiently attenuate the sound. Figure 2 illustrates the com- bination of both acoustic and ultrasonic data from Halliburton.

In addition, Halliburton provides a new interpretation tech- nique, Statistical Variance Processing, for the variable density. Free pipe has very little vertical change on the VDL (railroad tracks) while more of a change will occur when there is cement behind the casing. The CBL variance in Figure 3 illustrate the difference in the vertical variations where light colors are little change and dark colors illustrate areas of greater change. There is a change from the free casing near the top the channeled ce- ment near the middle and the good cemented casing near the bottom of the log.

However, in geothermal wells, we are often dealing with larger casing sizes than in oilfield wells. Casing sizes can vary from 8 5/8 inches to as large as 20 inches. Therefore, the loss of signal due to the acoustic attenuation of the fluid inside the cas- ing and the casing size can be significant. Larger diameter tools should be used since they have stronger transmitted signals.

Fa

Use of Acoustic Measurements in Lightweight Cements

Since the acoustic impedance of foam cement may be low, it is important to run an acoustic tool such as a CBL and VDL in

Figure 2. Acoustic and ultrasonic measurements (Courtesy Ha I I i burton).

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Micro Annulus Effect

Free Pipe

Channeled

Good Cement

Figure 3. Halliburton’s Statistical Variance Processing for MSG or VDL. (Courtesy Halliburton).

Segmented Bond Log from Baker Atlas Pads Against the Casing

I

4

Sound Transmission from Pad to Pad

Figure 4. Segmented Bond Log has less attenuation in larger casings since the pads are pushed up against the casing. There are also very

little affects due to centralization or fluid in the hole.

One service company, Baker Atlas, offers a VDL tool that will reinforce the strength of the signal and, therefore, work better in larger casing sizes. Baker can combine this VDL with their Segmented Bond Tool (SBT) with transmitters and receiv- ers which push up against the casing. This configuration with signal reinforcement increases the signal strength in larger cas- ings. Figure 4 illustrates the tool design used for the SBT.

In his 1992 paper Ken Goodwin of Mobil said, “failure to re-create the intimate casingkement acoustic coupling will cre- ate a relatively meaningless bond log.” The effects of a micro annulus on acoustic devices are well known and documented in the literature. The loss of contact between the cement and the casing can affect ultrasonic measurements also, particularly when filled with gas. The source of that gas could either be nitrogen from foam cement, or natural COz, or hydrocarbons from the formation.

The potential causes of a micro annulus for an oil field well or a geothermal well are similar. These causes may be cement shrinkage, a change in hydro static pressure, thermal expansion and contraction of the casing, or mechanical movement of drill pipe or casing. However, in geothermal the fluctuations are much more extreme. Therefore, all of the logs should be run with sufficient pressure to re-establish the contact between cas- ing and cement. Pressure is applied in increments until quality sonic and ultrasonic is ensured.

Inputs Needed for Data Gathering

Both service companies require inputs for the casing and fluids inside the casing, which affect the resultant acoustic im- pedance calculation. These parameters, if chosen improperly, can make the calculations incorrect and even cause the inter- pretation to be meaningless. Figure 5 illustrates the changes of the acoustic impedance values as the acoustic impedance of the

Improper Inputs Proper Inputs Mud weight = 8.4 PPG Mud weight = 10.3 PPG Interpretation LIQUID Interpretation CEMENT

Darker Tones represent blue (water) lighter tones represent yellow or brown (cement) on the cement maps.

Figure 5. Mud weight input can have a larger impact on acoustic impedance interpretation.

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casing fluid is changed. Obviously, one of these interpretations is not correct, since one says the annulus fluid is a liquid and the other interprets a solid. Figure 6 (left side) shows acoustic impedance values that are “spiking” and invalid because of an incorrect value for casing acoustic impedance. Compare these curves to the right hand side of Figure 6 with correct inputs.

There are several inputs used by Schlumberger to describe their model based processing. Halliburton has fewer inputs since their calculation is more of a hands-off approach. Tables 1 and 2 respectively list values casing and fluid parameters. There are additional calculations made from these inputs used in the processing also described in Tables 1 and 2.

Most of Halliburton’s calculations are automated with fewer inputs and less flexibility when necessary. For example, imped- ance parameters cannot be changed for unconventional casings such as titanium used in some geothermal wells. Also no cor- rection is available for very thin cement sheaths between two casing strings. A recording of the entire waveform, however could allow for reprocessing of the data if necessary. An ex- amination of the waveform would provide a better understand- ing of non-conventional log responses sometimes encountered in a geothermal environment.

Schlumberger offers a good deal of flexibility with calcula- tion inputs, which means each input will have to be chosen care- fully for valid results. Some of these inputs can be changed and corrected in a playback. However, some inputs set at the wellsite cannot be changed later and recovery is not possible. Table 3 shows the non-recoverable inputs from Schlumberger, some of which are set by the proper value of other input parameters. Results from an ultrasonic log will be accurate only if these inputs are correct.

New Lightweight Interpretation Techniques

In order to determine zonal iso- lation, we must realize that tradi- tional interpretation methods will not suffice for all situations. Com- plex cements require additional considerations and new methods must be utilized, particularly when non-standard cement slurries have an acoustic impedance value simi- lar to those of the wellbore fluids. New methods do not rely on abso- lute acoustic impedance or cement compressive strength values. These methods have been developed to determine the difference between a liquid, which may be displaced from the cement annulus and a solid that cannot be displaced during a remedial cementing operation.

Ultrasonic tools emit a signal from a transducer that encounters an acoustic interface between the casing and the annular material out-

Table 1. Mnemonics with Descriptions for Casing Parameters

Schlum berger Logging Inputs: Description Casing outer diameter Casing weight in pounds per foot

Used with CDIA to compute nominal thickness

Velocity of sound in casing Acoustical impedance of casing Current casing size

Values calculated from Inputs: Casing Density

Nominal thickness of casing based

Ha lliburton Logging Inputs: Casing outer diameter Casing Weight Thickness Calculation Method Compute thickness using resonance Compute thickness using OD-ID

(ID computed from velocity through mud)

Calculated:

Mnemonic CDIA

CWEI (Steel) VCAS (Assumes steel) ZCAS (Assumes steel) CSIZ (For header informa-

tion only)

CSDE (Assumes steel

THNO (Uses internal table) on density)

CASEOD CASEWT THMD RES0 (Assumes steel only) RANG

Casing velocity fixed for steel only (sheer and compressional) Casing acoustic impedance (steel only) is calculated using

velocity and density

Improper Input - Casing Impedance = 46.3 Proper Input - Casing Impedance = 27.5

Impedance scale 0 - 9 for each track

Figure 6. Casing impedance values can change impedance calculations.

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side the casing. The resultant signal energy is reflected at the interface while some is transmitted across the interface. The fractional amount of the reflected and transmitted energy de- pends on the acoustic impedances of the materials at the inter- face. Acoustic impedance, Z, is a function of the bulk density,

. p, and the compressional wave velocity of the material, V as show in Equation 1:

Liauid in the annulus

z = p x v (1)

Lightweight cements come in three varieties. Water extended cements, foam cements and cements extended with hollow high

Table 2. Mnemonics with Descriptions for Fluid Parameters.

Schlum berger Logging Inputs: Drilling fluid density

Mud type

Mud weight

Acoustic impedance of mud

Default fluid velocity (Actually slowness of pulse through fluid)

Halliburton Logging Inputs:

Acoustic impedance offset

Slowness of pulse through fluid

Fluid density inside the casing

DFD

TMUC (Brine or Mud)

MW (For header only)

ZMUD (Check measurement/expected)

DFWL (Measured values normally)

ZREF (Shift., all values)

FLUIDTT (Input is from mud cell if USEIT is no and input by engineer if USETT is changed to yes)

MUD Density

Table 3. Mnemonics with Descriptions for Non-Recoverable Inputs from Sch lum berecer.

1.

2.

3.

4.

5.

v

UPAT - Emission pattern a function of casing size (CDIA) and weight (CASEWT) well bore fluid type and weight. The engineer may overwrite this value and the manual should refer to the correct value. Determines operating frequency, data compression and flat plate response.

UWKM - High frequency for thinner casing (4.5 - 7 mm) and low frequency for thicker casing (7- 15 mm). The titanium liners are on the order of ‘/2 inch or a little less which would be around 12 mm. WINB -Beginning of the time window for detecting the return signal amplitude. The engineer may overwrite this value. It is a function of casing geometry and the borehole fluid. This is particularly important with the unusually large casing/liner sizes in this environment.

WINE - End of time window for detecting the return signal amplitude. Also a function of casing geometry and the borehole fluid.

WLEN - TA3 processing length NEEDS TO BE LONG ENOUGH. This value may be shortened in playback later to reduce effects from the third interface reflection. Calculate this value based upon the casing 7(2*casing thickness (mm))/5.93.

Gas-cut cement in the annulus

Figure 7. Impedance responses for different media behind the casing. (Courtesy Schlumberger).

strength ceramic beads. These cements can exhibit acoustic impedance values similar to drilling mud which would be mis- interpreted as a liquid. Cements with even lower acoustic im- pedance could be interpreted as gas.

Liquids containing little or no solids exhibit a consistent or steady activity level of acoustic impedance. When solids are mixed with either liquid or gas, the resultant mixture will have an irregular activity level. Lightweight cements and particu- larly those with hollow ceramic spheres or gas, as well as ce- ment that have been contaminated with gas from the reservoir will experience a high degree of variability of acoustic imped- ance values. Figure 7 shows the different log responses of indi- vidual acoustic impedance curves from the USIT. This particu- lar log presentation has 9 tracks with 4 acoustic impedance curves in each track on a scale of 0 to 10 MRayl. While the liquid and conventional cement are showing different acoustic impedance values, they both have less variability than the gas cut cement. Gas contaminated and lightweight cement is char- acterized by a high variance from 0 to 6 MRayl.

The new methods to determine a solid from a liquid assumes a fluid of any type will exhibit homogenous impedance values where solids will exhibit dynamic readings. The rate of change, or statistical variation, of impedance values are derived as a function of those measurements surrounding a given point mea- sured. Halliburton applies a statistical variation process (SPV). Thus the derivative of impedance will have a low value in free pipe sections and a higher value in cemented sections. Experi-

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Batcheller

ence has shown that cut-off values used for the interpretation of a solid are derivatives greater than .15 to .45 while a liquid will have a derivative of less than .15 to depending upon the tool and service company.

Halliburton then combines a graphic with both values that have exceeded the acoustic impedance of cement and those ver- tical variances that are great enough to be considered a solid as a brown color in the far right hand track of Figure 8. Table 4 shows the colors used for various ranges of impedance and de- rivatives used in their presentation. Figure 8 illustrates Halliburton’s analysis of a low-density cement environment. Notice the consistent cement in the variance track at M070. All five images are used to provide sufficient information to deter- mine zonal isolation. The three right hand tracks represent the acoustic impedance analysis. The third track from the right represents impedances greater than liquid with colors other than blue. The second track from the right represents a large enough variance for a solid with colors other than blue. Jn all three right hand tracks blue represents a liquid and is the lighter color on the black and white graphic.

Schlumberger performs a statistical analysis of a group of points around a specific point in the vertical, horizontal, and two diagonal directions. This allows an analysis based upon a known log response. The standard deviation is calculated for a

Darker Colors Cement

Lighter Colors Liquid -

I I L eUTU, 5l I I I I I AMP.

Table 4. Image Color Definitions for Halliburton. Impedance Image Plotted from 0 to 6.15

Color Range

Light Blue Red 0 - 0.38

0.38 - 1. I5 Blue 1.15 - 2.30 Yellow 2.30 - 2.70 Light Brown 2.70 - 3.85 Dark Brown 3.85 - 5.00 Black > 5.00

Definition Gas Gas-Cut Liquid Liquid Solid-Liquid Transition Low-Impedance Cement Medium-Impedance Cement High-Impedance Cement

Variance Image Plotted from 0 to 4 times upper fluid level

Color Range Definition Light Blue 0 - 0.15 Minor Variation (Single Phase) Light Brown 0. I5 - 0.30 Low Variation Brown 0.30 - 0.45 Medium Variation Black 0.45 - 0.60 High Variation

Cement Image Plotted from 0 to 1 (Data is either 0 or 7)

Color Range Definition Blue 0-.5 Fluids Brown .5 - 1 Cement

CBL MSC Normally plotted -20 to 20; however range of data is determining factor

Color Range Definition Light colors are negative amplitude Dark colors are high amplitude

CBL Variance Normally plotted 0 to ‘/z of the upper range for CBL MSC

Color Range Definition Light colors show little change

Dark colors show high changes or free pipe

and collar responses CBL Total Normally plotted 0 to the upper range for CBL MSC This is a clipped CBL MSC + CBL Variance

Color Range Definition Light colors show little change

Dark colors show high changes and low amplitude

and high amplitude

group of points. A high standard deviation is all directions would indicate the central point is gas-cut or low-density cement. A low standard deviation in any direction indicates the point is considered part of a channel. When a 10 degree radial is made with 6 inch vertical resolution thresholds are typically S0.5 for the diagonal and horizontal directions (SDTHOR) and 0.3 for the vertical direction (SDTVER).

Geothermal Example Wells

Figure 9 show two Schlumberger interpretations of a well in Imperial Valley, California. The importance of the new tech- nique is obvious when compared to the old technique. Data points that indicate gas (red) or liquid (light blue) in the annu- lus are analyzed for standard deviation. The data point is left

Figure 8. Halliburton’s analysis for lightweight cement. Advanced Cement Evaluation (ACE). (Courtesy Hal I i burton).

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Cement Eva lua tion without Cement Evaluation with Lightweight Technique Lightweight Technique

Interpretation Gas Interpretation Cement (Darker shades) (lightest & Lighter)

Plus Liquid (Darkest) Plus Cement (Lightest)

Figure 9. Difference with using new processes to determine solid from liquid. Interpretation is mostly gas or liquid without new technique.

-a-b Foam Cement - - Neatcement -

Figure 10. Minimum, Maximum, and Average acoustic impedances in 9 sections around the casing.

with its original coding of gas or liquid if the standard deviation is less than the threshold selected. If the threshold values are met or exceeded the point is considered a solid and is coded green. The older processing interprets the annulus as mostly a gas. While the current lightweight cement processing interprets the annulus as a solid. An unnecessary squeeze on the well is therefore eliminated.

Figure 10 is a Halliburton log on a Nevada well cemented with foam cement of 11.4 PPG density with the last portion cemented with 15.6 PPG unfoamed cement. The presentation includes the minimum, maximum and average acoustic imped- ance in 9 segments around the casing. The denser cement be- low 1,500 feet shows less difference between the minimum and maximum curves as would be expected for heavier and non- foamed slurry. Above 1,500 feet the log indicates a response from a mix of foamed and non-foamed cement around the cas- ing. About 2/3 of the sections are indicating little difference between the minimum and maximum values (non-foamed re- sponse) and about 1/3 are indicating a larger difference as expected for foamed cement. At a depth around 1,300 feet all sections are indicating a foamed cement response.

The difference between the minimum and maximum values and the average acoustic impedance should respond to the den- sity of foamed cement. However, above 1,300 feet the mixture of foamed and non-foamed cement responses becomes about 50%. This difference in values may be utilized to indicate a change of cement density or foam quality response. Why some of the re-

3 P -5

Figure 11. Changing log response due to linerkasing overlap.

TOP

Previous Casing Shoe

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Batcheller

sponses indicate non-foamed cement when it shouldn't have yet to be answered. Very little research has been done on the behav- ior of foamed cements once they have reached downhole condi- tions. The log responses indicate there may be some movement of the nitrogen within the cement after its placement downhole.

Figure 1 l(previous page) is from the same Nevada well cov- ering an interval where there is an overlap between the 13 3/8 inch casing and the 9 9 8 inch liner. The overlap occurs at 606 feet and there is a corresponding noticeable acoustic imped- ance change at that depth. Whenever there is an overlap of casing there is the potential for either destructive or construc- tive interference of the ultrasonic reflections. Reflections from the interface of the outer casing string and cement can lower or increase calculated values of impedance. In other words, the reflective energy returning to the transducer may be higher or lower as a result of these additional reflections. Unfortunately, the results of this response are unknown and it impossible to accurately determine the impedance of cement behind the liner. This problem, however, could be minimized or eliminated in some cases by changing the wave form time interval used in determining acoustic impedance.

Figure 12 is a Schlumberger log on a well cemented with 11.0 PPG foam cement in 13 3/8 inch titanium casing. This suggested presentation is to help the operator with a better visu- alization of the interpretation. It shows only the interpretation,

Track 1 Track 2 Track 3 Track 4 Track5 Track6 Track7

Corre- Casing Csing Impedance Impedance Type Mapping With lation Diameler Thickness Raw Values Ranges % Classifications

Acoustic Fluid Impedance

Figure 12. New suggested Schlumberger presentation to include mostly interpretation data.

correlation and casing data with a minimum amount of quality control data. This interpretation has all of the correct inputs and uses the Micro DeBonding information from Schlumberger. Some of the quality control data from this well is presented in Figure 13. In addition to the suggested presentation, there will be a typical presentation for help in determining the data qual- ity. Finally a presentation of the raw acoustic impedance as in Figure 6 is helpful in evaluating possible data input problems.

The alternate presentation suggested in Figure 12 has 7 Tracks of data. Track 1 consists of a gamma ray and collar locator for correlation, and tool eccentricity as a quality indica- tor. Track 2 has a calculated casing minimum, maximum, aver- age internal radius and an average external radius. The external radius is a combination of the calculated thickness plus the in- ternal diameter. The thickness of the casing is shaded a gray color. Track 3 is the remainder of the casing related informa- tion, and includes 3 curves representing the minimum, maxi- mum and average calculated thickness. When compared to known casing values, log casing calculations can indicate prob- lems with input data. The graphic of Track 4 represents the calculated acoustic impedances and their distribution around the casing. Darker colors or shades represent larger acoustic impedance values. The three curves in Track 5 represent the minimum, maximum and average acoustic impedance values of all those recorded. The graphic of Track 6 illustrates the

Invalid Valid - Inputs - - Inputs - No Micro Debonding Quality Flags Micro Debonding No Quality Flags

Quality Flags are Micro DeBonding Darker Shading is Darkest Shading

In Left Half

Figure 13. Incorrect data gives incorrect results compare to Figure 12 with correct data. Quality control flags indicate a problem.

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percent of each interpreted fluid type. Red represents gas (dark- est shade), blue a liquid (medium shade), yellow (lightest shade) is considered as cement (bonded), and green (Micro DeBonding) represents a solid using the new interpretation technique. The graphic of Track 7 is representative of the distribution of acous- tic impedance values with classifications. The determination of a gas, liquid or solid is from chosen impedance values (inputs) when acoustic impedance exceeds that chosen for cement. Darker colors again represent higher values of impedance.

Data Qualify Control Examples

The Schlumberger presentations on the left side of Figure 13 use steel inputs when the casing is titanium. The presentations on the right hand side of Figure 13 is the same log, but with tita- nium inputs. The left hand interpretation has no Micro DeBonding, where on the right it does. The wrong inputs are resulting in an invalid interpretation because the velocity and acoustic impedance of casing are both used in the interpretation process. The original interpretation (Figure 13 left side) with steel inputs indicates liquid behind the casing where the correct inputs (Figure 12 and Figure 13 right side) indicate cement. Ob- viously, the correct inputs are critical for a proper interpretation.

The second and fourth presentations from the left, take a look at some quality control issues related to the incorrect inputs. The modeling used by Schlumberger may have to go through several iterations until finding an appropriate fit of the data. When the number of iterations exceeds a certain amount, the data may not ever reach a good fit of the model. The number of iterations is shown by the colors for the processing flags indicated in these tracks. The maximum number is indicated by blue flags. There are number blue flags in this left Figure indicating too much it- eration when steel rather than titanium inputs are used. In con- trast, when the correct inputs are used (on the right hand side) there is an absence of flags. Therefore, the quality control pre- sentation will often indicate an initial problem with the data. In this case, fortunately, it could be corrected by changing the in- puts prior to logging.

The inputs of casing and fluid impedance or density are also used in the interpretation process. Figure 5 illustrates a misin- terpretation based on the wrong fluid impedance. Another indi- cator of invalid input is the thickness calculation (casing input) and casing radius calculation (fluid input). Another example of improper inputs is in Figure 6. In this instance the actual acoustic impedance curves are indicating a problem. When the casing inputs are invalid not only are casing thickness calculations in- correct, but so are the acoustic impedance calculations. The use of an improper input in the left side of Figure 6 shows acous- tic impedances that are unrealistically high in value and show- ing several spikes in the curves not seen with the proper input on the right hand side of the figure.

Service Companies Interpretation Comparison

While Halliburton and Schlumberger use a different math- ematical approach in interpreting data €or lightweight cements. In addition, Halliburton uses only vertical variations in acous-

tic impedances, while Schlumberger investigates variations for vertical, horizontal and 2 diagonal directions. Both compa- nies, however, are considering acoustic impedance variations in these complex cements to determine if the material behind the casing is a solid or a liquid.

Both Schlumberger and Halliburton interpreted a Schlumberger log on a geothermal well with similar results as show in Figure 14. Both service companies have the capability of making these interpretations in the field. Halliburton has the additional capability of presenting the acoustic impedance curves with their interpretation and analyzing the VDL using their SPV processing. Currently Schlumberger does not have the capability of presenting all acoustic impedance curves ex- cept through addition processing at a computing center.

Schlum berger Halliburton Interpretation Interpretation

Figure 14. Comparison of Schlumberger and Halliburton of a USlT log on a geothermal well.

Conclusions

1 .

2.

3.

4.

5.

181

The correct standoff and frequency used with ultrasonic devices is important in geothermal wells. Larger size casing may reduce an acoustic signal signifi- cantly. The Segmented Bond Log from Baker Atlas is an alternative in that case. Micro annulus effects may be more significant due to heat- ing and cooling in geothermal wells. Therefore, cement evaluation logs should be ran under pressure. Proper inputs for fluid, casing and cement values are criti- cal to a valid interpretation of ultrasonic logs. New lightweight cement evaluation techniques are neces- sary for determining a solid from a liquid when cement acoustic impedance is too low.

Batchel ler

6. ’ h o additional presentations from Schlumberger can help with both the quality and understanding of the interpreta- tion.

7. While both Schlumberger and Halliburton use different methods of interpretation the results are consistent.

Acknowledgements

The author wishes to acknowledge and thank several orga- nizations and people whose information and assistance added significant assistance with this paper: CalEnergy; Dennis Kaspereit, Fred Pulka, and Todd Van de Putte. Schlumberger Well Services; Mark MacGlashan, Jerry Rushing and Lisa Silipigno. Halliburton Energy Services; Gary Frisch and Jim Bray. Finally a special thanks to Alan Sattler at Sandia Na- tional Labs for giving me the time and the encouragement needed for this project.

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Frisch G, Graham L and Griffith J: “A Novel and Economical Processing Technique using Bond Logs and Ultrasonic Tools for Enhanced Ce- ment Evaluation” presented at SPWLA 2000 Annual Logging Sympo- sium, June 4-7.

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