Diagnostic Use of Thermal Anomalies in Wells $ AARON E. PIERCE,* J. B. COLBY,* AND BELDON A. PETERS^

INTRODUCTION

ABSTRACT

Since the 1930's, when subsurface measuring and

Temperature surveys offer a quick, economic means of diagnosing trouble in wells o r solving well-completion and operation problems. Fast-response thermometers a r e now available in both subsurface-recording and surface-recording types. These thermometers can meas- ure temperature anomalies of a s little a s 1 F, or less. In addition to conventional uses of temperature surveys

recording instruments were first developed, subsurface- recording thermometers have proved t h a t temperature surveys in wells a r e direct, economical, and sometimes

(locating cement top, leaks), the fast-response ther- mometers offer a way to evaluate results of hydraulic fractur ing and to locate zones actually taking fluids, whereas flowmeters show only the point of exit of in- jected fluids. To evaluate fractures, hot o r cold fluid is injected to create a thermal anomaly. A temperature profile then delineates the fractured zone.

the only practical means of solving several types of drilling and completion problems.'*? Also, surface- recording thermometers have proved to be very suc- cessful fo r many purpose^.'.^ Temperature surveys a r e now commonly used for :

1. Location of cement freshly placed outside the cas- ing.

2. Location of tubing and casing leaks. 3. Location of excessive gas entry in oil wells. 4. Location of zones receiving fluids in injection wells. 5. Location of unwanted flow outside casing. I n recent years, the location of zones receiving fluids

in injection wells has increased in importance a s more extensive use of secondary, and even tertiary, recovery methods is made. Also, experience has shown t h a t tem- perature surveys can be used to determine location of hydraulic fractures - and possibly the orientation of fractures.

Extensive experience with routine temperature sur- veys a s a means of early detection of possibly danger- ous well conditions has shown tha t highly sensitive thermometers can head off trouble. F o r example, recent calculations have shown t h a t small oil l eaks-of about 30 t o 40 bbl per day--can cause a thermal anomaly of about 1 F.

Two commonly used subsurface thermometers, the Amerada and the Humble type, can detect anomalies this small. Consequently, close attention to survey charts made in routine reconnaissance runs can result in early detection of leaks. A subsequent diagnostic run can pin-point the trouble. In many cases, detailed runs can lead to correct diagnosis of the trouble and indicate corrective action.

The purpose of this paper is to show examples of the use of sensitive, rapid-response thermometers in diag- nosing wells. Examples will also be given of more com- mon uses of subsurface-recording thermometers. "Esso Production Research Company. Houston, Texas ?Humble 011 & Refining Co.. Baytown. Texas $Presented at the spring meeting of the Mid-Continent District, API Division of Production, March 1966.

'References are at the end of the paper.

RAPID-RESPONSE THERMOMETERS Two subsurface-recording thermometers available to

the industry a r e the Amerada and Humble types. The Amerada thermometer consists of a bulb sealed to a helical Bourdon tube. The bulb and tube both contain a volatile fluid. Changes in the vapor pressure a r e a measure of changes in temperature. The improved Humble-type thermometer consists essentially of a mercury-filled cylinder equipped with a piston operating through a packing box. Changes in extension of the piston a r e caused by changes of the mercury volume, which a r e a measure of the changes in well temperature. Both types of instrument contain clock-driven charts on which extension is marked a s a function of time. Data points a r e read off the charts and correlated with a record of time and depth kept by the instrument oper- a tor a t the surface. Normal continuous logging speed f o r these tools is about 100 ft/min; for greater accuracy, stops of 30 sec should be made in intervals of interest.

Agreement between the two types of instruments is remarkable. Fig. 1 shows a comparison of Amerada and Humble temperature surveys in a well shut in f o r 7 years. The biggest difference between the measure- ments of the two instruments is about 1 F. Over most of the range, there is a difference of less than 1/2 F. These measurements were made with the Humble and Amerada instruments coupled during the same traverse.

TEMPERATURE O F 166 174 182 190 198 S P

6000- -HUMBLE S1A

6200- - - -AMERADA RT-7 [CURRENT)

6400- Y Y

n. 6800-

7000-

7200

PERF. lR Fig. 1 -Comparison of Amerada and Humble

Temperature Surveys

A point worth observing in this illustration is t h a t the well surveyed was formerly used for gas injection into the two zones a t about 6,600 f t and 7,100 f t . Al- though the well was shut in, dead, and practically full of water, the thermal depression a t the two injection zones is still clearly defined.

Instruments used in this survey were the Amerada RT-7 thermometer and a Humble model STA thermom- eter. The Humble instrument is 15/4G-in. in diameter and is provided with a n aluminum case. The smaller size and aluminum case considerably reduce the heat capac- i ty and increase the thermal conductivity of Humble- type thermometers a s compared to heat capacity and conductivity of the older Humble-type instruments.

Good response is obtained with surface-recording thermometers. These tools a r e run on a conductor cable. One type, which uses a resistance-wire heat sensor, is run a t rates up to 15 ft/min. A newer type uses a semiconductor element with much less mass than the resistance-wire type. The smaller mass permits faster response; logging speeds up to 100 ft/min can be used with good accuracy. Use of two semiconductor elements spaced a short distance apar t i n a down-hole tool gives another means of measuring temperature differences much smaller than 1 F. The two sensing elements a r e in a balanced electrical circuit. When one element senses a small temperature change, a large deflection is re- c0rded.l

SURVEYS IN WATER-INJECTION WELLS Small Temperature Difference

Usefulness of rapid-response thermometers is shown in surveys made in water-injection wells. Fig. 2 shows

TEMPERATURE O F 76 77 78 79 80

- \\ PERF. 1;:. CUMULATIVE INJ.-%

Fig. 2 -Temperature and Flowmeter Surveys in Water-injection Well with Small (2 F) Anomaly

TEMPERATURE O F

20% HRS AFTER I k c ,,,,,,,

(FLOW ~5 , , S!JR,VE,Y 0 100 CUMULATIVE INJ.- %

Fig. 3 -Temperature and Flowmeter Surveys in Completed Injection Well

results f o r a well in which only a small temperature difference exists.

I n this well, reservoir temperature was less than 90 F, and injection water temperature was about 65 F. As Fig. 2 shows, the temperature anomaly found was about 2 F. The plot shows clearly t h a t the perforations from about 2,460 to 2,485 f t were taking water t h a t was cooling the formation. A subsequent flownleter survey, also plotted in Fig. 2, showed tha t 88 percent (440 B/D) of the injected water left the casing a t 2,485 f t and 12 percent (60 B/D) left a t 2,460 ft . Temperature o r Flowmeter Survey?

Whether to use a temperature o r a flowmeter survey, o r both, to make water injection profiles is a good ques- tion. Generally, a flowmeter survey costs more, but it gives quantitative answers. It tells how much fluid is leaving the casing and where. A flowmeter survey, how- ever, does not always tell where the fluids enter the formation. A temperature survey, on the other hand, shows intervals being invaded by injection fluids. It does not tell how much.

Often, a temperature survey gives a good diagnosis of a n injection well, but sometimes temperature and flowmeter surveys complement each other, each con- tributing needed information to the diagnosis. An ex- ample in which a temperature survey does a good job of showing a water-injection profile is given in Fig. 3. This survey was made in a dual, tubingless-completed well. About 35 B/D water were being injected through each string. The temperature survey was r u n only in the lower s t r ing of casing, but i t gave good profiles fo r both strings.

Note tha t a normal temperature gradient is followed until the thermometer approached the perforation in the upper s t r ing a t about 2,330 f t . Water injected through perforations in the upper string cooled the formation

188 AARON E. PIERCE, J. B. COLBY, AND BELDON A. PETERS

TEMPERATURE O F

I 41-1/2 HRS AFTER SHUT IN

STRING 2 (FLOW SURVEY)

CUM. INJ., % Fig. 4 - Temperature and Flowmeter Surveys

Showing Zones Taking Injected Water a t this depth, and the temperature survey made a clear picture of the cooling. As the survey went on down, the gradient went back up to normal until the per- forations in the lower s t r ing were approached. The plot clearly shows the injection zone a t about 2,450 f t . A later flowmeter survey confirmed the temperature sur- vey finding tha t water was being injected essentially through one formation stringer in both the upper and lower zones, a s shown by the arrows.

A good example of how flowmeter and temperature data complement each other is shown in Fig. 4. This well also was a dual con~pletion and was being injected with water a t about 35 B/D through perforations in each string. The temperature survey showed water was being taken by the upper formation a t about 2,190 f t and a t 2,330 f t . The lower completion was taking water a t about 3,420 f t . A flowmeter survey then showed tha t water was leaving the upper s t r ing mostly through perforations a few feet away from the formations actually taking the water (in the upper completion). The interpretation was t h a t water leaving the casing through perforation a t 2,250-52 f t was communicating upward to the formation opposite perforations a t 2,188- 90 f t . Water leaving the casing through perforations a t 2,302-04 f t was migrating to the formation opposite perforations a t 2,332-34 f t and 2,339-41 f t .

I n these surveys, temperature measurements were made by stopping the instrument every 2 f t in the in- tervals opposite perforations. Taking readings this often is important in describing injection profiles. I n routine reconnaissance surveys, runs can be made moving the instrument continuously at approximately 300 ft/min.

EVALUATING FRACTURING TREATMENTS Oil production from multiple, low-permeability zones

is common to many fields in West Texas. F o r good oil production, well stinlulation by fractur ing and water flooding is often required. When multiple tubingless

completions a r e used, flow often occurs between zones close to the well bore. Diagnostic use of thermal anom- alies has been very helpful in getting a measure of what zones have been fractured and where injected fluid goes.'

Procedure for Making Anomalies The procedure used is to inject hot or cold fluid dur-

ing the fractur ing operation. The f rac fluid used must be a t a temperature several degrees above o r below the temperature of the formation. Heat t ransfers to the formation and f rac faces, mainly by conduction. Before the formation is fractured, a "base" temperature survey i s made; a f te r fracturing, a detailed temperature survey is made through the zone of interest.'

Fig. 5 shows results of a "hot frac" 19 hours a f te r the fractur ing job. In this case, water a t 130 F was injected through perforations in sand A. The survey made a f te r f ractur ing shows near-normal temperatures right down to the upper par t of the sand. Through the sand, a n anomaly of about 2 F indicates tha t the sand was fractured, but the fracture did not seem to pene- t ra te the dense zones above and below it.

These results were obtained with a sensitive, surface- recording thermometer. Much more extensive work has been done by Agnew? who summarizes results of 344 jobs i n which surface-recording thermometers were used. Agnew suggests t h a t surface-recording thermom- eters a r e more convenient, because with them repeated traverses of the same interval can be made to check results o r make detailed surveys of intervals in which unexpected preliminary temperature readings were ob- tained.

CONVENTIONAL USES O F TEMPERATURE LOGGING

Locating Top of Cement Among the older uses of temperature logging is locat-

ing the top of cement. The amount of heat evolved by

TEMPERATURE O F 25001 8 4 I 85 I 86 87 88 8 9

'I \ ' 0 DENSE ZONE 1 JORMAL GRADIENT

Fig. 5 -Temperature Profile 19 Hours after Injection of Hot Fracturing Oil

the setting of oil-well cement placed behind casing is sufficient to be measured inside the casing. In general, the temperature increases in an interval of the well bore above the cement during the usual time allotted for waiting on cement to set. Fig. 6 shows an example. In this well, 5%-in. casing was set a t 12,460 f t in 9%-in. hole and cemented with 800 sacks. The temperature log, run 13 hours after cementing, located the top of the cement a t about 11,250 ft. The dotted line in this graph shows normal temperature gradient for the formation; the depression in the borehole is due to circulation of drilling fluid. The sharp rise a t the top of cement is from the heat evolved during setting. Usefulness of a temperature survey is shown by the fact that the height of cement in this job was found to be only about 50 percent of the calculated height. The difference resulted from large-diameter washouts in the hole.

TEMPERATURE O F 180 220 260

9000k I 1 I I I,- A P P R O X \ NORMAL

\\TEMPERATURE

10 J

Fig. 6 - Locating Top of Cement

Locating Tubing Leak Another conventional use of temperature surveys is

shown by Fig. 7. This illustrates leaks in two gas wells equipped with tubing packers. Both wells had been pro- duced a t moderate rates for some time, then shut in. Gas expanding through the leaks reduced the temper- ature several degrees a s shown. The leaks were approxi- mately located in reconnaissance surveys, then pin- pointed by stops made a t 25 to 50-ft intervals, a s shown. In subsequent tubing repair work, the leaks were found to be very close to the points indicated.

Gas Entry through Small Leak in Casing Detection of a small leak in the casing of a well is

shown in Fig. 8. This was a low-productivity oil well that had been killed prior to installing lifting equip- ment. After i t had been filled with water, i t was dead

T E M P E R A T U R E O F 60 100 140 180

\

TUBING LEAK,

APPROX FORMATION TEMPERATURE

70001 I I I I I Fig. 7 -Locating Small Gas Leaks in Tubing

on tubing but would flow gas from the oil string. Two temperature surveys were made. The first was made with the well completely shut in a t the surface. The second was made the next day while flowing gas a t the rate of 0.5 MMcf/D from the oil string. As is shown, a small casing leak was indicated near surface. Repair work showed the gas entered from a pressured forma- tion near the bottom of the surface casing, flowed up the outside of the oil string, and entered the oil string a t a depth of about 205 ft. Estimated pressure differ- ential for this leak was about 170 psi.

T E M P E R A T U R E O F 60 80 100 120

FLOWING 0 5MMCF/D FROM OIL STRING

1000 -7 - a - - a . . - - 1 Sl. 2 DAYS A

I I I I I I I Fig. 8 -Locating Small Gas Leaks in Casing

190 AARON E. PIERCE, J. B. COLBY, AND BELDON A. PETERS

SAND A A ' B C

TEMPERATURE " F

Fig. 9 - Temperature Logs Showing Communication Outside Casing (Gas Well)

Communication Outside the Casing Faul ty primary cementing can result in pressure

communication between zones. Fig. 9 shows such a condition. In this well, a high-pressure gas sand (B) proved to be flowing into two upper zones (A and A') and a lower zone (C). Because the well had been nro-

. ,

ducing a t a ra te of 8 MMcf/D f o r some time, tempera- tures were considerably higher than the normal gra- dient. But a survey made just before a workover showed a considerable temperature depression in the interval from about 6,600 to 6,900 f t . This depression was inter- preted to mean gas flow behind the casing. The casing was perforated and squeeze-cemented. A temperature survey made a month a f te r the workover showed tha t the temperature depression had been raised consider-

ably, indicating the repair work was effective in stop- ping the communication.

CONCLUSIONS Use of temperature surveys in wells can provide con-

siderable diagnostic infoimation. When coupled with other types of data (flowmeter surveys, pressure tests), temperature surveys can help locate points of fluid entry into the well bore in producing wells or entry into the formation in injection wells.

F a s t - r e s p o n s e t h e r m o m e t e r s , e i t h e r subsur face- recording or surface-recording, can be used to help lo- cate induced fractures. Routine reconnaissance surveys, whlch can be made cheaply and quickly, can spot trouble. Detailed surveys can then be made to measure temperature anomalies of a s little a s a degree or so, to pin-point the trouble in wells.

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REFERENCES IPeacock, D. R: What You Can Learn from Tempera-

ture Logs, Petrolet~w~ E n g ~ . , 37 [I01 96, Sept. (1965). Willikan, C. V: Temperature Surveys in Wells,

Trans. Am. Inst. Mining Met. Engrs. (Petrolez~m De- velopment and Technology) 142, 15 (1941).

3Riordan, M. B: Surface Indicating Pressure, Tem- perature, and Flow Equipment, Trc~ns. Ant. Imt . Min- ing Met. Engrs. pet role?^?)^ Development and Tech- nology) 192, 857 (1951).

4Agnew, B. G: Evaluation of Fracture Treatments with Temperature Surveys, T,rc~ns. Am. Inst. Mining Met. E?~grs. (Petrolezcnz. Dcvelopinezt cund Tecl~nology) 892 (1965) ; also J. Petv. Tech., XVIII l71 892, July (1966).