Kars Water Reseowces (Proceedings of the Ankara - Antalya Symposium, July 1985). IAHS Publ. no. 161

USE OF A MICROCOMPUTER TO ANALYZE GEOPHYSICAL LOGS OF A WATER WELL IN A CARBONATE AQUIFER

W. Scott Keys U.S. Geological Survey Denver, Colorado, USA

Abstract

At the present time much of the analysis of geophysical logs of oil wells is accomplished using computers, but the method is used for very few water wells because the hardware and software needed to do analysis and plotting have been too expensive. The advent of the microcomputer or personel computer has made both hardware and software available to most groundwater « organizations. This report describes long - analysis techniques that were accomplished using a personal computer and peripherals that cost less than $ 4,000 (U.S.) and a spreadsheet program that cost less than $ 500 (U.S.).

The set of logs used to demonstrate computer - analysis methods is from a well drilled to a depth of 1,328 meters in the State of Wyoming, United States of America, to test a carbonate aquifer that produces significant quantities of water from solution openings and fractures. Page - sized computer plots of the logs greatly facilitated distinguishing the important lithologie units and correlating responce among the various logs. Cross - plots of two different types of logs were used for interpreting lithology and developing corrected values for porosity. Computer analysis of the logs aided in the location of permeable depth intervals characterized by secondary porosity.

Using a microcomputer, a large quantity of data from water wells can be reduced to a more readily understood format and the results can be shown to others through graphic output that is suitable for publication.

Introduction

The analysis of geophysical logs of water wells can provide data useful for developing water supplies at minimal cost. The analysis of these data commonly is difficult because of the quantity of information in the logs, hydraulic tests, and samples that all need to be combined. Determining the relation between these data sets will provide the maximum benefit. Because of space limitations in this report, the principles of geophysical logging and log analysis cannot be described, so the reader is referred to Keys and MacCary, 1971 for further information. For a number of years computers have been used extensively in the petroleum industry for the analysis of geophysical logs, but use for groundwater applications has been limited. In the past, most log analysis has been done on mainframe computers or large minicomputers that are expensive and require some programming experience for operation. The availability of relatively inexpensive microcomputers and a large variety of easy - to - use software has made computer analysis of logs and other hydrologie data

249

250 W. S. Keys

economically justified for most groundwater projects. This report briefly describes the use of a microcomputer and a readily available software package to analyze a suite of geophysical logs of a test well in a dolomite and limestone aquifer.

The Advantages of Using a Computer - A major advantage of using computers for log analysis is related to diminishing scale problems and processing the large mass of data available. Geophysical logs commonly are digitized on a depth interval of 0.15 m, which provides the detail needed for most types of analyses. For the well described in this report, each log recorded at an interval of 0.15 m produced a total of more than 17,000 data points, including depth and the corresponding log value. At least 14 different kinds of geophysical logs were made for the example well, and some of the logs were recorded several times. Thus, the large mass of quantitative data available for analysis is difficult to collate and interpret so that relations can be used in a predictive sense. The analog records of water wells usually are recorded at vertical scales of about 2.4 m of hole per centimeter of chart paper. For deep wells, each log is many meters long so the interdependence between logs and other data from a well are difficult to identify and to compile in a form that can be presented to others.

The plots of logs generated by the computer for this report represent a reduction in vertical scale of about 7.5 times yet the spreadsheet plotting options enabled the horizontal amplitude of some of the anomalies to be increased more than 10 times that of the original log. These changes in vertical and horizontal scales had the effect of improving the signal -to - noise ratio after smoothing techniques were applied so that the changes in logs due to geohydrologic factors were much more obvious. The computer also enables correction to be made in logs with relative ease, and statistical analyses can be done and the results plotted.

Hardware and Software Used — The log analysis was done on a popular brand micro­computer or personal computer (PC) with 256 kilobytes of random access memory : dual 360 - kilobyte, floppy - disk drives; and a dot - matrix printer. Both this report and all of the plots included were printed using this equipment. The total cost of this hardware was less than $4,000 (U.S.) and PC's that can use the same software are available for less money. The equipment fits on a small desk and many hundreds of programs are available at small cost.

The spreadsheet program used to process and plot the logs and other data is used worldwide for financial analysis and accounting and can be purchased for less than $500 (U.S.). This program permits the manipulation of any kind of data in a spreadsheet that can be as large as 256 columns by 2,048 rows depending on available computer memory. The spreadsheet was used by entering depth in one column and corresponding geophysical log values or sample data in subsequent columns. Some of the logs were digitized onsite simultaneously with analog recording, and others were digitized later from the analog records. The digitized logs were transmitted from a large minicomputer to the PC using a telephone and first edited with the word processor used to write this report. The logs men were transfered to the spreadsheet program from the word processor and were stored as separate, readily accessible files on floppy disks.

Microcomputer to analyze geophysical logs 251

Test Well

Madison Limestone Test Well No. 1 was drilled by the U.S. Geological Survey as part of a project to determine the quantity and quality of water available from the Madison Limestone of Mississippian age and adjacent rocks. The purpose of the project was to determine water availability for the onsite generation of power from extensive near - surface coal deposits or for transporting the coal by slurry pipelines. The well was drilled to a depth of 1,328 meters in the northeast corner of Wyoming, near the center of the study area. A number of geophysical logs were made in the well by a private company and the U.S. Geological Survey; core - analyses and packer - test data are available (Blankennagel and others, 1977). The relation between porosity and permeability determined by analyses of core from this well and from two others drilled in the study area was reported by Thayer, 1983. Although the well was drilled through rocks overlying the Madison Limestone and into the basement rocks, only the depth interval from about 700 to 1,035 m is described herein because most of the water was produced from this interval. The Madison Limestone in this area consists of argillaceous to cherty dolomite and limestone with some beds of anhydrite and halite (Thayer, 1983). The Mississipian, Madison Limestone is underlain by the Red River Formation, of Late Ordovician age, which is mostly dolomite with minor limestone. Secondary porosity due to both brecciation, and solution along fractures and channels is common in both types of carbonate rocks.

Data Processing and plotting

Each file consisted of depth and one or two logs so they could be entered in the spreadsheet or worksheet separately, then combined and compared quantitatively with other logs or with core data. If the depths of recognizable features on individual logs did not agree, columns of data were moved up or down in the worksheet to make depth corrections. Equations for making corrections in the log data or smoothing prior to plotting are written into one cell in the worksheet and checked; then a single command does all of the corrections.

Each of the logs then were plotted on the computer monitor and modified until the combination of vertical and horizontal scales produced a log that best showed the changes in lithology and porosity and reduced noise to a minimum. As many as six logs or other data sets can be plotted in one figure, but the result tends to be confusing. Because several thousand data points were being plotted at page size the log trace usually appeared noisy, and smoothing routines were used. When the plot appeared to be correct on the monitor it was saved on a disk. The printer easily can be programmed to automatically print a number of plots in sequence with many options on size and lettering. All of these operations can be done with no programming experience and very little reference to the user's manual. Available commands are listed in a menu on the monitor screen and can be selected by moving a cursor or by typing the first letter of the command. At any point in the data processing, numerous help menus can be called up with keystroke to explain the options available.

252 W. S. Keys

Geohydrologic Data Obtained From Logs

Plots of several of the more useful logs through the most productive interval of the Madison Limestone penetrated by Test Well No. 1 are shown in figures 1 through 6. Lithology, as interpreted from logs and core data, has been added to several of these figures. The gamma log, figure 1. indicates that some of the dolomite is more radioactive than limestone and other dolomite intervals, which probably is due to a greater content of clay. More radioactive dolomite occurs at depths of 745 to 775 m, 830 to 865 m and 925 to 935 m. Although some limestone is interbedded, dolomite is the predominant carbonate rock in this well. Long - and short - normal resistivity logs are shown in figure 2. The long normal log indicates greater resistivity values than the short normal possibily because of invasion by more saline fluids or hole diameter - effects. The intervals of greatest resistivity are mostly due to dense dolomite or limestone that do not produce much water.

Secondary porosity due to solution has produced vugs and open channels along fractures and bedding planes and the logs indicate that these intervals have the greatest permeability. A high resolution caliper log is plotted in figure 3 and relatively large solution openings are clearly shown at depths of 773, 890, 910, 925 and 935 m. More important, however, is the small scale roughness or hole rugosity shown by the caliper log in the interval from 940 to 1,035 m. This roughness was repeated on several different types of caliper logs, so it is not due to instrumental noise. The flowmeter log shown in figure 4 indicates that part of this rough interval on the caliper log produced more than three - fifths of the water flowing at the surface at the time the flowmeter log was made. An acoustic -televiewer log clearly demonstrates that this roughness is caused by small solution openings in the carbonate rocks. The flowmeter log shown in figure 4 was plotted from stationary measurements made with an impeller flowmeter while the well was flowing at a rate of about 31.5 l/s.

Neutron and acoustic - velocity logs plotted using a similar porosity scale are shown in figures 5 and 6. The absolute porosity values are not correct for several reasons. These two logs were recorded using a matrix value for limestone, and subsequent information showed that dolomite was predominant in this well. Clay and hole - diameter corrections were made to the neutron log based on the gamma and caliper logs, and correlation between core - and log - porosity values were greatly improved. Note that the difference between the apparant porosity on the neutron and acoustic - velocity logs is much greater at depths of more than 920 m than at shallower depths. The neutron log indicates a greater porosity than does the acoustic - velocity log in the deeper interval that had numerous solution openings because acoustic - velocity probes do not detect most secondary porosity. Thus, the difference between porosity under some conditions. A crossplot of the neutron log versus the acoustic - velocity log with the limestone and dolomite lithology lines super­imposed is shown in figure 7 Crossplots of two different logs enable the determination of lithology and of porosity corrected for matrix effects. Crossplots also can be used to identify intervals of substantial secondary porosity (Pickett, 1969). The interval with substantial secondary porosity is indicated in figure 7 by the group of points to the right and below the dolomite line where the neutron log indicates a greater apparent porosity than does the acoustic - velocity log.

Microcomputer to analyze geophysical logs 253

P u l s e s p e r s e c o n d 20 4 0 6 0

7 5 0 - f

8 0 0 -

8 5 0 -|

9 0 0 - 1

9 5 0 -

Mostly dolomite

10 0 0

8 0 i

1 0 0

=— Dolomite

Dolomite

Arg i l laceous

d o l o m i t e

10 3 5

Fig 1. - Gamma log, Madison Limestone Test Well No. 1.

254 W. S. Keys

Resis t iv i ty (Ohm-meters ) IOOO 2 0 0 0

?oo

7 5 0

8 0 0

in

« 8 5 0

Q. 0) O

9 0 0 -

9 5 0

I O O O

Long normal

3000 3500 !

Dense

l i m e s t o n e

10 3 5

Fig 2. —Short - and long - normal resistivity logs, Madison Limestone Test Well No. 1.

Microcomputer to analyze geophysical logs 255

Hole diameter (Cen t imete rs ) 30

i 35 4 0

S e v e r a l t i gh t f r a c t u r e s

Large so lu t ion open ing

S c a t t e r e d s o l u t i o n open ings

10 0 0 -

1 0 3 5 -

B e d d i n g p l a n e s ? Seve ra l t i g h t f r a c t u r e s

Numerous so lu t i on openings

<__ F r a c t u r e s w idened by s o l u t i o n

C l o s e l y s p a c e d sma l l s o l u t i o n

o p e n i n g s Qlong b e d d i n g p l a n e s

N u m e r o u s sma l l s o l u t i o n o p e n i n g s

45 i

Fig 3. - Caliper log, Madison Limestone Test Well No. 1 , with structure interpreted

from an acoustic televiewer log.

256 W. S. Keys

Flow ( L i t e r s per second) 10 15 2 0 2 5 30 35

7 0 0 -j »-

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8 0 0

in

a>

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CD

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J

9 0 0 -

9 5 0 - 1

,A^

10 0 0 -

10 3 5 T

X

Fig4. - Flowmeter log, Madison Limestone Test Well No. 1.

Microcom

puter to analyze geophysical logs 257

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ros

ity (P

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70

0

tos

s

Fig 5. — N

eutron log, Madison Lim

estone Test Well N

o. 1.

258 W

. S. Keys

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15 10

5

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Acoustic - velocity log, M

adison Lim

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ell No. 1.

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260 W. S. Keys

Conclusions

A microcomputer and relatively inexpensive and easy - to - use software packages can be used to analyze geophysical logs of water wells and to collate the findings for public­ation, The geophysical, logs in the example presented in this report enabled the identific­ation of permeable intervals that produced water because of the presence of secondary porosity. The specific log - analysis techniques briefly summarized in this report may be applicable in other areas where secondary porosity in carbonate rocks is important to water production. The general computer techniques may be used in any analysis of geophysical logs or other hydrological data to aid in the development and protection of water supplies in many parts of the world.

REFERENCES

Blankennagel, R.K. , Miller, W.R., Brown, D.L. , and Gushing, E.M., 1977. Report on Pre­liminary Data for Madison Limestone Test Well No. 1, NE1/4SE1/4 Sec. 15. T. 57 N., R. 65 W., Crook Co. Wyoming: U.S. Geol. Survey Open - File Report 77-164,97 p.

Keys, W.S. and MacCary, L.M., 1971. Application of Borehole Geophysics to Water Resources Investigations : Techniques of Water Resources Investigations of the U.S. Geol. Suwey. Chapter E1, Book 2,126 p.

Pickett, G.R. and Reynolds, E.B., 1969. Evaluation of Fractured Reservoirs : Soc. Pet. Eng. |our., V.9. No. 1, p. 28-38.

Thayer, P.A., 1983. Relationship of Porosity and Permeability to Petrology of the Madison Limestone in Rock Cores from Three Test Wells in Montana and Wyoming VS. Geol. Survey professional Paper 1273-C, 29 p.