Method for Determining Minimum Waiting-on-cement Time
- R. Floyd Farris (Stanolind Oil and Gas Co.)
- Document ID
- Society of Petroleum Engineers
- Transactions of the AIME
- Publication Date
- December 1946
- Document Type
- Journal Paper
- 175 - 188
- 1946. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 1.6 Drilling Operations, 4.3.1 Hydrates, 1.14 Casing and Cementing, 1.14.3 Cement Formulation (Chemistry, Properties), 2 Well Completion
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A method is presented for determining minimum waiting-on-cement time, whichtakes into account the differences that exist between types and brands ofcements and such individual well conditions as depth, temperature, andpressure.
The basis for the method was determined by laboratory tests. Being a laboratorydevelopment, several steps were required to prove its merit. The first stepconsisted of laboratory tests designed to determine the minimum cement strengthrequired in wells. Basis was found for setting a minimum value of 8 lb. per sq.in. tensile strength. Next, it was shown by laboratory tests that the time to 8lb. per sq. in. tensile strength may be expressed as a function ofconsistometer stirring time to 100 "poises," the approximate relationbeing "the time to 8 lb. per sq. in. tensile strength equals the time to100 'poises' times three." Next, it was shown that the time of maximumtemperature development in cement slurries, due to heat of hydration, is alsorelated to consistometer stirring time to 100 poises, but only by a factor ofapproximately two. It was shown also that the shut-in casing pressure willbuild up after cement is placed and register a maximum pressure atapproximately the same time the slurry down the hole attains maximumtemperature. From this and the relationships listed above, the general rule wasestablished that minimum waiting-on-cement time (time to 8 lb. per sq. in.)after casing cement jobs in any well is equal to the time when the shut-incasing pressure reaches a maximum, as measured from the initial mixing ofcement, times a factor of 1.5.
Cement plugs drilled in the field at the time prescribed by this formula werefound to drill "firm to hard," thus confirming the laboratorytests.
These tests prove that many of the present regulations for waiting on cementrequire a longer time than is absolutely necessary. Use of the method hereinproposed offers the possibili ty of a saving of $ 1200 per well.
The length of time allowed for cement to set after casing is determined eitherby state-wide rules, field rules, or self-imposed rules written into drillingcontracts. In general, the time is dictated by experience and common practice.However, owing to differences in opinion and in experience of the variousgroups involved, waiting-on-cement time often varies from one area to the next.For example, an operator in an area where no rules exist may drill out ofsurface pipe at 24 to 36 hr., while another operator in another area may wait48 hr. or more to comply with state or field rules, although the depth of thewell, hole size, type of cement, and other data are identical. An even greaterdifference in practices will be found by making similar comparisons withrespect to oilstring cement jobs. Differences in waiting-on-cement times of 36to 48 hr. are common.
Further complicating the picture is the rather common practice of allowing morewaiting time for cement to set at the greater depths than is allowed at theshallow depths.
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