scope:

1 Scope

Temperature is the single most important variable affecting cement hydration [1]. Therefore, accurate information about temperatures in a well is critical to cementing operations. Many variables affect wellbore temperature, and measurement of temperature during an actual cementing operation is difficult. However, since the early 1940s temperature data has been gathered, analyzed and converted into estimates of wellbore temperatures with depth. These estimates of temperature versus depth have been incorporated into American Petroleum Institute (API) guides for testing cement formulations since the late 1940s. The API well-simulation test schedules have been valuable tools for the petroleum industry as guides for testing cement formulations in the laboratory and as estimations of wellbore temperatures for cementing operations.

Work performed by the 1984-91 API Task Group on Cementing Temperature Schedules to update the temperatures in API well-simulation test schedules is summarized in this report. The Task Group reviewed the largest set of temperature data available to the industry to date. Changes were proposed to temperatures in the well-simulation test schedules in API Specification 10, 5th, Edition, based upon review of the data.

This report documents:

a. The proposed changes for temperatures.

b. The process used to develop the proposed changes.

c. The process used to develop temperatures in API Specification (Spec) 10, 5th Edition well-simulation test schedules.

d. The comparison of temperature data sets used to develop the proposed changes to temperatures in API Spec 10, 5th Edition, well-simulation test schedules.

Additionally, this report compiles most of the temperature data collected by the industry to date into a single document that may serve others involved in similar efforts in the future.

1.1 HISTORY OF API THICKENING TIME TEST SCHEDULES

The API has served as the focal point for gathering temperature information useful in cementing operations for over fifty years. Standardized cementing well-simulation test schedules containing temperatures for primary and remedial (squeeze) cementing operations have been prepared from data sets of temperatures measured in wells. The first test schedules were published in API Code 32, 1st Edition, issued February 1948. A second edition was published in June 1950. These documents were issued as tentative codes.

Both editions of API Code 32 contained well-simulation test schedules for casing-cementing operations for depths down to and including 18,000 feet. The depth range for schedules was the same as the format in API Spec 10, 5th Edition. Test schedules start at a depth of 1,000 feet and are incriminated on even numbered depths from 2,000 feet to the deepest depth of the schedules.

Temperatures for the well-simulation test schedules in API Code 32 were based upon a correlation developed from data collected by Farris in 1941[2]. Eight data points were collected from wells along the Gulf Coast of Texas and Louisiana. Geothermal temperature gradients for these wells ranged from 1.05°F/100 feet to 1.63°F/100 feet. The average temperature gradient was about 1.35°F/100 feet. A single correlation for circulating temperature with depth was developed from this data.

API Code 32 was replaced by API Recommended Practice (RP) 10B for the 3rd Edition of the testing procedures and well-simulation test schedules, which was issued in April 1953. Squeeze-cementing well-simulation test schedules for depths down to and including 16,000 feet first appeared in this document.

Temperatures in the casing and squeeze-cementing test schedules did not change until the 19th Edition of API RP 10B issued in January 1974. A single temperature depth correlation was used for casing-cementing well-simulation test schedules from February 1948 until January 1974. A separate, single temperature and depth correlation was used for squeeze-cementing well simulation test schedules from April 1953 until changes were proposed this Task Group in June 1991.

Although the temperatures did not change for over twenty years, there were other changes in the schedules. The 3rd and 4th Editions of API RP 10B were issued as tentative standards. The tentative classification was removed for the 5th Edition issued in May 1956. Also, a 9,000-foot well-simulation schedule for both casing and squeeze-cementing appeared in the 5th Edition and remained through the 10th Edition issued in March 1961.

A casing-cementing well-simulation schedule for 20,000 feet and a squeeze-cementing well-simulation schedule for 18,000 feet first appeared in the 11th Edition of API RP 10B issued in March 1962. In March 1964, the 13th Edition of API RP 10B was published and the squeeze-cementing well-simulation schedules were extended for use as plug-back cementing well-simulation schedules. This was indicated by a change of title for Table 9.2 of that document which is shown as "Basis for Squeeze and Plug Back Cementing Well-Simulation Test Schedules".

Liner cementing well-simulation test schedules first appeared in the 14th Edition of RP 10B issued in March 1965. Liner schedules were based upon temperatures and pressures from the casing schedules; and times to temperature and pressure from the squeeze and plug-back cementing schedules.

Hesitation squeeze-cementing well-simulation schedules were included as alternate schedules and classified as tentative in the 17th Edition issued in April 1971. The hesitation squeeze schedules were an extension of the squeeze and plug-back cementing schedules from previous editions. Hesitation squeeze schedules increased the temperature of the slurry from the squeeze temperature to the static temperature at a rate of about 0.2°F/min after reaching the squeeze pressure for the schedule. The static temperature for these alternate schedules was calculated using a geothermal temperature gradient of 1.5°F/100 ft. Cycling of the stirring of the slurry was also performed during this second temperature ramp to simulate intermittent pumping during the squeeze operation.

Significant changes to the test schedules occurred in the 19th Edition of RP 10B issued in January 1974. Six casing schedules corresponding to depths of 1,000 ft, 6,000 ft, 8,000 ft, 10,000 ft, 14,000 ft and 16,000 ft were designated as casing cement specification test schedules. These schedules were the same as those originally published in API Code 32 up through the 18th Edition of API RP 10B. New casing-cementing well-simulation test schedules were issued with a tentative classification. These schedules contained temperatures correlated on depth and geothermal temperature gradient. Temperatures for these test schedules were higher than temperatures for previous schedules at some depths. No changes to liner, squeeze or hesitation squeeze schedules occurred in this edition.

Temperatures for liner cementing well-simulation schedules were correlated to temperature gradients in April 1977 when the 20th Edition of API RP 10B was issued. A 20,000-ft liner schedule was added. The tentative classification for the casing-cementing schedules was dropped for this edition.

In April 1979, the 20th Edition of API Spec 10A, was issued but did not contain test schedules. Spec 10A was a specification for oil well cements and cement additives. The 21st Edition of API RP 10B was published in December 1979, with the following equation shown for calculation of temperature gradient:

where

Temperature Gradient = °F/100 ft,

BHLT = bottom-hole log temperature, °F,

Depth = well depth, ft,

80°F = Surface temperature.

The API Spec 10, 1st Edition, replaced API RP 10B when it issued in January 1982. Equation 1 shown above was modified to the following:

where

Temperature Gradient = °F/100 ft,

BHT = bottom-hole temperature, °F,

Depth = well depth, ft,

80°F = Surface temperature.

A footnote was added to clarify the source of bottom-hole temperature used to calculate the temperature gradients in the casing and liner cementing well-simulation test schedules.

Four other editions of API Spec 10 have been issued. The 5th Edition was issued on July 1, 1990, and contains the well-simulation test schedules as described to this point. Changes proposed by this Task Group are based upon the schedules in API Spec 10, 5th Edition.

Note: As of the publication date of this Technical Report, 10TR3, the 5th Edition of Spec 10 has been replaced by API Spec 10A, 22nd Edition January 1995, and API RP 10B, 22nd Edition, December 1997.

1.2 MEASUREMENT AND COLLECTION OF TEMPERATURE DATA

Temperature data used to prepare correlations for well-simulation test schedules were not collected during actual cementing operations. All of the data used has come from mechanical temperature sensors run on drill pipe prior to running the casing to be cemented into the well. Ideally, the drilling fluid (mud) is circulated for a sufficiently long period of time to allow the temperature in the wellbore around the temperature sensor to reach a nearly steady-state condition as illustrated in Figure 1.

Temperatures in the well-simulation test schedules are predicted values based upon correlations of measured temperature versus depth, developed from sets of temperature data. They should be considered as estimates and serve as general reference information. The actual temperatures during a cementing operation or during circulation of the mud with casing run into the wellbore may vary significantly from the temperatures in the API tables.

However, it is important to note that temperatures in the well-simulation test schedules have served the industry well. Temperatures in the original schedules published in 1948 were used for 26 years by the industry worldwide. Temperatures in the present test schedules have been used for nearly 20 years.

1.3 1984-91 API TASK GROUP ON CEMENTING TEMPERATURE SCHEDULES

The 1984-91 API Task Group on Cementing Temperature Schedules started as a Work Group in API Committee 10 in the latter half of 1984. The Work Group was assigned to review a new set of measured temperature data, consider present industry practices in primary and remedial cementing operations, and make recommendations to Committee 10 regarding the cementing well simulation test schedules in API Spec 10, 5th Edition.

Based upon review of the temperature information, the Work Group recommended that temperatures in the cementing test schedules be revised. Additionally, the Work Group recommended industry surveys of field cementing operations for both primary and remedial cementing to determine if other variables in the thickening time test schedules, such as pressures and times to reach temperature and pressure, needed to be updated as well. Surveys of field cementing operations were last conducted in the late 1950s and the pressures and times to temperature and pressure in the API Spec 10, 5th Edition cementing schedules are based upon data from those surveys.

The Work Group's recommendations were accepted by the Steering Committee of API Committee 10 and a Task Group was formed to conduct the work. The following general charges were given to the Task Group:

a. Prepare new temperature correlations based upon the body of new temperature data.

b. Compare the predicted temperatures from the new correlation(s) with temperatures in the API Spec 10, 5th Edition, well-simulation test schedules

c. Conduct surveys of field operations for primary and remedial cementing.

d. Prepare new time and pressure schedules for primary and remedial cementing operations from the survey data.

e. Compare the new time and pressure schedules to the times and pressures in the API Spec 10, 5th Edition well-simulation test schedules.

f. Propose new well-simulation test schedules or changes to the API Spec 10, 5th Edition, thickening time test schedules.

All of these assignments were completed. The Task Group reached agreement on the most difficult assignment--new temperatures for the well-simulation test schedules. Surveys of field operations are completed. The results of the survey of squeeze operations were documented in 1987 [3]. New time and pressure schedules have been prepared for squeeze cementing operations. Documentation of other surveys of field cementing operations is in progress.

A document titled "Proposed Replacement for Appendix E (API Spec 10) Operating Thickening Time Tests" was accepted by API Committee 10 for letter ballot at the June, 1991 Annual Standardization Conference held in San Diego, California. This document contained the changes to the well-simulation test schedules recommended by the Task Group. The recommended changes were accepted by API Committee 10 after passing the letter ballot. This report documents the data, methods and process used to develop the new temperature correlations proposed by the Task Group to API Committee 10.