This paper documents recent field cases in which attempts weremade to mitigate casing vent flows (CVF’s) on producing andabandoned wells by incorporating permeability-blocking gelswith specialized cement blends. CVF’s are defined in this paperas sustained gas pressure on the annuli of producing and surfacecasings. The amount of gas flow rate can vary from a few bubblesto cubic meters per day. However, when the annuli are shut-in,the gas pressure can build to a significant amount. The proceduresdetailed in this paper are the result of lab studies and postjobreviews of failed remedial attempts. Specific attention isgiven to why squeeze-cementing procedures can fail to providelong-term seals against source-gas production from casing vents.In most cases, rework of the abandonment procedures cost theoperators over $200,000 CAD.The studies showed four possible causes of recurring CVF’s:1. Development of thaumasite in the setting cement2. Leaking isolation tools3. Incomplete long-term seal of source zones4. Incorrect source detection or squeeze intervalCement-source quality and bulk handling methods werealso investigated, but showed no evidence of probable cause.This paper explains the findings from laboratory studies andjob reviews that lead to improved procedures. These improvedsolutions were conducted on 23 wells in the later part of 2001.
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