Cement Evaluation - Past, Present, and Future
Authors: Pilkington Paul E. 
Published: December 2007

Introduction

Cement evaluation began with the calculation of cement tops. This calculation assumed gauge holes and no channeling of the cement through the mud. Calipers were not available at that time. In the mid-1930's, the use of temperature surveys to determine in technical journals. Properly run temperatures journals. Properly run temperature surveys can identify the TOC, but distribution of cement-e.g., vertical isolation through zones of interest-is difficult to ascertain. Radioactive tracer surveys were run in the late 1930's to determine cement tops. Carnotite was mixed in the lead slurry and cement tops were determined with a gamma ray lot. Tracer surveys had the same limitations as temperature logs but were not time-sensitive. Ref. 1 includes an excellent bibliography. The acoustic cement bond log (CBL) appeared in the early 1960's and is still being run. The major limitation of the CBL is the difficult in identifying small channels. More recent developments in the field include attenuation-rate and pulseecho tools. A newer tool that is be coming more available in the field is the Segmented Bond Tool (SBT). This tool evaluates circumferential bond over the full 360deg. circumference in 60deg. segments. Another tool the Micro CBL, that would evaluate circumferential bond in eight 50deg. segments was under development, but has been discontinued because of the success of the SBT. Another new cement evaluation tool, the Ultrasonic Imager, is in the field-testing stage. Examples of these acoustic circumferential cement bond tools are presented later. Future developments may include more sophisticated tools, such as the cement volumetric scan tool. Each logging technique currently in use has limitations.

Temperature Logs

Temperature logs have been used to detect the TOC for many years. The most critical factor in the evaluation technique is the timing of the survey. Heat from the exothermic reaction as cement sets dissipates rapidly. Laboratory tests indicate a return to near normal temperatures in 24 hours. Early surveys also showed the effects of time, bottomhole temperature, circulating time, and thermal conductivity of the surrounding formations on the temperature profile. The temperature log remains an economical and effective way to determine the TOC and in tervals of larger cement accumulation. It is not a direct measure of vertical isolation across hydrocarbon-bearing zones; however, if a four-arm caliper is available, it is possible to infer good cement displacement if the TOC agrees closely with the calculated top (and full returns were present during pumping and displacement of cement). Fig. 1A shows a TOC temperature log. The local temperature profile must be known to avoid mistaking a change in the thermal conductivity of the formation for a cement top. Fig. 1B is a survey run to determine the TOC in a deepwater well. The base survey run at the end of the openhole log runs made interpretation possible. Fig. 1C is from a foam cement job. The temperature peaks marked "A" indicate fracture zones filled with cement. Note the hole enlargement indicated on the caliper curve in possible fracture intervals. Temperature logs, followed by a CBL are recommended on all foam cement evaluations. Data in the literature indicate that the optimum time to run the temperature log may precede the optimum time for the CBL by several hours or more. Compressive strength development lags behind the exothermic reaction as cement sets. It should be possible to record a temperature log and the CBL on tape and play the temperature and amplitude curves back with an openhole caliper curve on a compressed depth scale. This procedure facilitates cement evaluation in foam cement jobs and in conventional cement jobs in fractured formation areas. The temperature log is still useful in cement evaluation, particularly in foam cement evaluation and in combination with other tools. The tool cannot accurately delineate vertical isolation when run alone. Other tools were developed to fulfill this need.

JPT

P. 132⁁