Wednesday, March 30, 2011

Did you know that Well Casing Cement is NOT just poured and left to set?

Having attended many forums in the county, I must say that the type of information shared is nominal; there is just not enough time to explain certain processes. Occasionally, in-depth information will be posted here about some of the complex issues behind what it takes to develop a natural gas well to the production phase.
Since protecting the aquifers reigns high in environmental stewardship, it is important to develop an understanding on what is done to protect the aquifers.  Besides the steel casings, cementing the wellbore is another protective mechanism employed. But how can well operators tell if the cement has bonded? SImply, by obtaining a cement bond log.
Cementing Wellbores
There is a lot of speculation about the cement that is used in natural gas wells. People have expressed doubts on the ability of cement to isolate drinking water from the processes within the wellbore. However, comparing wellbore cement jobs to buckling sidewalk is a grave injustice.  The cementing of wellbores is a complex process requiring bond logs to ensure that the job is well done. NYSDEC regs require bond logs. So exactly what is a bond log?
Oil field glossary: A representation of the integrity of the cement job, especially whether the cement is adhering solidly to the outside of the casing. The log is typically obtained from one of a variety of sonic-type tools. The newer versions, called cement evaluation logs, along with their processing software, can give detailed, 360-degree representations of the integrity of the cement job, whereas older versions may display a single line representing the integrated integrity around the casing.
See also bond log webinar
“Two kinds of cement-bond-log (CBL) tools are run as part of a standard cement-evaluation program. The effectiveness of these tools and their evaluations often are challenged, and they are not regarded as a replacement for reservoir interzonal-communication tests performed between producing reservoirs. The value of continuing to run these tools was questioned by management. In response, the reliability of these tools and their ability to determine the existence of poor behind-casing”    If you are interested in learning more about bonded cement here is an SPE article: Cement-Bond-Log Interpretation Reliability

If you can read through this and understand the above article, then maybe you can develop an appreciation for all that goes into ensuring that no communication occurs between the wellbore and the adjacent aquifer, and why this particular information is difficult to explain.

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