Wayne B. Cloud
Mobil Research & Development Corp.
Dallas
An inexpensive temperature tool provides accurate temperature measurements during drilling operations for better design of cement jobs, workovers, well stimulation, and well bore hydraulics.
Valid temperature data during specific well bore operations can improve initial job design, fluid testing, and slurry placement, ultimately enhancing well bore performance.
This improvement applies to cement slurries, breaker activation for stimulation and profile control, and fluid Theological properties for all downhole operations.
The temperature tool has been run standalone mounted inside drill pipe, on slick wire line and braided cable, and as a free-fall tool. It has also been run piggyback on both directional surveys (slick line and free-fall) and standard logging runs.
This temperature measuring system has been used extensively in field well bores to depths of 20,000 ft. The temperature tool is completely reusable in the field, very similar to the standard directional survey tools used on many drilling rigs.
The system includes a small, rugged, programmable temperature sensor, a standard body housing, various adapters for specific applications, and a personal computer (PC) interface.
The sensor is calibrated up to 300 F., and readings of up to 324 F. have been made in the field. The sensor can be programmed to take readings at a selected time interval, and it can take up to a maximum 1,000 readings per run. The nominal sensor operating life, before recalibration and changing of the battery, is a total run time of 800 hr.
The standard body housing is 1.75 in. OD, and the overall tool length can extend from 1 ft, depending upon the application and the adapters used.
The tool has been used to obtain low-cost well bore temperature data during drilling, logging, and casing operations on welts throughout Texas, eastern Oklahoma, western Colorado, and in the Arun field in Indonesia. In general, the use of this tool does not interfere with normal operations.
BACKGROUND
The need for better temperature information for oil field cementing operations drove the development of this tool; however, its application is not limited to cementing.
In the 1980s, almost 800,000 wells were drilled worldwide, excluding eastern Europe and the former Soviet Union. 1 Many of these wells were successfully cemented with practices developed by the oil industry since the first use of cement in an oil well just after the turn of the century. 2
Detailed examinations of cement job failures, however, quite often show that correct procedures were used throughout the job, except that there was not an adequate knowledge of the temperature regime present in the well bore during the cementing operation. During cementing operations, errors of 10-20 F. in the well bore temperature regime can cause failures because of the temperature impact on cement slurry performance, Accurate temperature data can greatly improve cement slurry design and testing.
Also, the inaccurate temperature data affect the engineering process throughout the life of the well.
The American Petroleum Institute's (API) Committee on Standardization of Well Cements has actively pursued cementing temperature data since the early 1970s. 3 The current API Specification 10A, Specification on Well Cements, contains testing schedules developed from the ongoing data collection.
A majority of these data comes from single-point maximum recording thermometers on logging runs (the logging temperature) or from circulating data on wiper trips.
It is widely recognized, however, that more temperature data are needed to improve cementing operations. 4 These additional data need to be collected under conditions as representative as possible of actual job conditions. On site measurement of operating temperatures provides an opportunity to validate anticipated temperatures and make changes to slurry designs and job procedures, if required.
Another major use of on site temperature data is for historical matching and validation of output from computer simulation of well bore temperatures.
DEVELOPMENT
The programmable temperature sensor used in developing the tool is a spinoff of a sensor from the aerospace and food-processing industries. For oil field applications, the sensor has been hardened to withstand the shocks and vibrations of the drilling operation.
The PC interface works with software for programming and reading the sensor in either a DOS or Macintosh format. Portable computers in field vehicles and field office PCs have been used with the sensors.
In addition to programming and reading the sensor, the software includes diagnostic testing for the sensor, data saving to diskette, and graphics plotting of the temperature-vs.-time data. The software includes provisions for insertion of depth-vs.-time data for correlating temperature vs. depth.
APPLICATIONS
After a sensor is programmed to take readings at the desired time interval, it is then assembled in the standard body or a special housing body (Fig. 1).
A thermal compound is first applied to the tip of the sensor to enhance tool heat transfer. The sensor, two rubber compression rings, and a stainless steel washer are inserted into the standard body and held in place by a snap retaining ring.
This assembly is then attached to an appropriate adapter for the application. After retrieval, the assembly process is reversed and the sensor is inserted in the PC interface which reads the data.
Typical uses include data acquisition during drilling, logging, cementing, work overs, completions, and well stimulation.
INSIDE DRILLSTRING
Fig. 2 shows the adapter that attaches the standard temperature tool body to a directional survey baffle plate inside a drillstring.
This configuration adds no detectable pressure drop, even if placed on the bit and inside 3-1/2 in. drill pipe. The flow area through the baffle plate in this setup is identical to the flow area with a directional survey tool sitting on the baffle.
Just like the baffle plate, the top of the adapter has a circular ring for centering a survey tool. This configuration can accommodate different size baffle plates to fit the drill pipe size in use.
The baffle-plate assembly can be run on the bit or anywhere up the string with sufficient string internal diameter. To avoid interference with a float located over the bit, the assembly is often run just above the near-bit stabilizer.
The use of the temperature tool in the drillstring has several advantages:
- It can determine the temperature profile of the well bore on the trip in the hole.
- Before circulation is started, it can measure the pseudo-undisturbed temperature at the bottom of the hole.
- After circulation is started, it can obtain the temperature falloff.
- It can then measure thermal gradients of formations drilled (from the temperatures while drilling).
- It can obtain other important temperature profiles, including the temperature buildups during circulation without drilling and the temperature profile of the entire well bore on the trip out of the hole.
The use of two sensors in a bit sub allows the acquisition of temperature data on fluids both going down the pipe and in the return annulus (Fig. 3).
When used at the bottom of a drillstring running a liner, it provides circulating data during cementing and displacement. This use also gives the opportunity to monitor static temperatures near the top of the liner after the liner is released and circulation is cleared.
This configuration also provides the ability to gather annular data up the hole at selected locations.
WIRE LINE
The temperature tool can be run on slick wire line or braided cable as a single, double, or triple unit. The use of tandem sensors on a wire line or in free-fall operation improves data acquisition in critical operations because individual tools can fail for any number of reasons (normal operating hazards).
Multiple sensors can also cover uncertainty in operational timing; each sensor can be programmed to start at staggered times.
Fig. 4 shows a double wire line unit with no weight bars, which can be easily added as needed to get into the hole. The expanded view on the left shows the two sensors with special body and adapter in between.
The tool has also been adapted to run piggyback below directional survey tools that are run on wire line periodically to satisfy regulatory requirements or to track directional control. Many directional companies can provide various centering devices which adapt to the temperature tool when run with a survey tool.
Run on wire line, the temperature sensor can obtain the temperature profile of the well bore on the trip in the hole, the initial disturbed temperature at the bottom of the hole, temperature buildup on bottom, and the temperature profile of the well bore on the trip out of the hole.
The use of a wire line lubricator allows measurements during circulation.
Many logging companies have a tail piece on most of their logging tools to accommodate a hole guide plug. Attachment to the tail piece may be by a through bolt, by studs that match into a J-latch, or by special threads.
A common adapter for commercial logging tools has threads to make up on the standard temperature tool body and flares out to a 3-1/2 in. OD stud set for J-latching, with a lock-down bolt.
The piggybacking of the tool below a logging tool permits acquisition of temperature data on the trip in the hole, while any logging tool adjustments are made near bottom hole, during any short trips to check the logging tool performance, and while tripping out of the hole.
FREE-FALL
The wire line version of the temperature tool may also be run in a free-fall mode if the drill pipe is full of liquid. Experience has shown that pipe with a less-than-full liquid level is undesirable.
A wire line configuration free-fall tool may be recovered by a wire line overshot to avoid the necessity of tripping the pipe.
Operational experience has also shown the tool can be adapted to a variety of hardware, including commercially available shock subs. Multiple sensor units may also be run in this mode.
Several operations have used sinker bars on the tool both to help it down the hole and to space out the tool length for easy retrieval from the drillstring.
This upside down method, with the sensor on top, is not amenable to fishing the tool (Fig. 5). It is best applicable when no directional survey is planned before the pipe is tripped or when it is piggybacked on top of a free-fall directional survey tool before a bit trip.
The free-fall tool can provide the initial disturbed temperature at the bottom of the hole, the temperature buildup while on bottom, any circulating temperature data, and the temperature profile of the well bore on the trip out.
SURFACE
Mounted in an inexpensive holder, the sensors provide time-vs.-temperature data on surface operations. Proper selection of start times and of intervals between readings simplifies the correlation of surface and downhole data.
This tool configuration can be used in the possum belly for return temperatures, in the various mud pits, including the suction pit for mud inlet temperature, in the water tanks, and in the makeup mud tanks. Other applications have included the monitoring of the cementing mix tanks and special units such as mud coolers.
FIELD DATA
Fig. 6 shows temperature data from a 143-ft interval drilled over a 34-hr period from a wildcat in western Colorado. For these measurements, the sensor was placed on the bit.
Straight lines have been placed on some of the data to indicate the values of a measured thermal gradient (MTG) across various depth intervals. The MTG values range from 0.0 F./100 ft to 36.5 F./100 ft.
In the drilling literature, one paper cited an 8 F./100 ft value (misstated in the paper as 8 F./ft) for some measurement-while-drilling data as "obviously incorrect" because of "the use of heated mud in the well and heat generation within the MWD tool." 4
Substantial MTG values do exist in well bores upon initial penetration by the bit. These distinct MTG values are quickly smeared out by the overall heat transfer process in the well bore and are generally not detected by other means.
Fig. 7 shows temperature-vs.-time data measured at a total depth of 20,070 ft in a wildcat in eastern Oklahoma. The bit had been drilling ahead, and the fluid circulation rate was 294 gpm.
When drilling stopped, the well was circulated for 3 hr at 340 gpm before the temperature tool was dropped in the hole.
The survey tool gland in this drillstring was 10 ft above the bit, and some sinker bars were used on the sensor which placed it about 17 ft above the bit. The tool was on bottom 22 min before it was started out of the hole.
The temperature built up from 259 F. to 264 F. during this time. Also note that the 270.5 F. "hot spot" was measured up the well bore, 26 stands off bottom.
The temperature tool was then run piggyback below the logging tool in the same Oklahoma wildcat (Fig. 8). The data were correlated to depth going in and coming out of the hole. The maximum measured bottom hole temperature was 324 F. at 25 hr after circulation stopped. A 318 F. logging temperature was reported for this well. The temperature tool has a patent pending, and Mobil Corp. is currently negotiating with a supplier to make the tool available to the industry.
ACKNOWLEDGMENT
The author would like to thank Barry Gilmore, Charles Islas, D. G. Calvert, and Mobil's drilling supervisors and engineers for technical support, guidance, and field assistance.
REFERENCES
- Smith, D.K., "Worldwide Cementing Practices," American Petroleum Institute Special Book Project 88-59 sponsored by Committee 10, Committee on Standardization of Well Cements, American Petroleum Institute, 1991, P. 233.
- Smith, D.K., "Cementing," SPE monograph Vol. 4, rev. ed., Dallas, 1990, P. 1.
- Minutes of the meeting of the API Task Group on Bottomhole Cementing Temperatures, New Orleans, June 19, 1973.
- Tilghman, S.E., Benge, O.G., and George, C.R., "Temperature Data for Optimizing Cementing Operations," SPE Drilling Engineering, June 1991, pp. 95-99.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
