A procedure for determining the minimum waiting-on-cement time before running a cement bond log has been developed to minimize idle drilling rig time prior to performing completion operations. The prior to performing completion operations. The method accounts for both the set properties of cement slurries and the cement bond log tool response function. Using this method, valid cement bond logs have been obtained after waiting-on-cement times of less than 24 hours.
The Cement Bond Log (CBL) was introduced in the early 1960's as a method to determine the quality of primary cement.' Since its introduction, it has undergone a number of improvements in both the hardware used to make the measurements and the interpretation of the data. These refinements have improved its value as a diagnostic tool for analyzing primary cement problems.
A major concern when running a CBL is the waiting-on-cement (WOC) time required before logging. WOC times of 24, 36, and 72 hours have been recommended for various cement slurries and well conditions. The 72-hour WOC time is commonly used since most cement slurries should have developed 80-90% of their ultimate compressive strength by this time. These WOC times are, however, only estimates and do not account for either the setting characteristics of specific cement slurries or the response function of the CBL tool.
When a CBL is run without adequate WOC time after a primary cementing operation, the log will not be representative of the true quality of the cement sheath. If this log is interpreted and used to make engineering decisions, unnecessary remedial operations may be performed.
In situations where either a drilling rig is performing the completion operations or prompt performing the completion operations or prompt completion is desired, the WOC time required before running a CBL could affect the time required for well completion. The WOC time is particularly important for drilling rigs operating in isolated land locations and on offshore platforms. This paper demonstrates a technique to record a valid paper demonstrates a technique to record a valid CBL as soon as possible after cementing thereby minimizing idle rig time.
The minimum WOC time required before running a CBL is a function of two factors: cement slurry compressive strength development, and CBL tool response. These two factors are accounted for by the method described in this paper. The method does not presuppose any particular WOC time. The WOC time is strictly a function of the foregoing factors and may be greater than or less than the rule-of-thumb 72 hours, depending on the particular situation.
To be valid, a CBL must fulfill two criteria:
Response not a function of time
The requirements for a technically correct CBL are documented adequately in available literature and will not be discussed. For a CBL to be useful, the response that is observed in a log run soon after cementing should be the sane, within error, as that obtained when a log is run at a longer time, i.e., when the cement has reached its estimated ultimate compressive strength (EUS). When this criterion is fulfilled, the engineer may be reasonably assured that the interpretation applied to the CBL is valid within the limitations of the technique.