Utilizing Cement Bond Logs to Evaluate Wellbore Integrity for Local and Regional Scales

Abstract

Abstract Wellbore integrity is a fundamental aspect of site characterization for storage in areas with existing or abandoned oil and gas wells. The condition of well cement and casing structures determine if a well can sustain optimal hydrostatic pressures and prevent leakage. This is a necessary but underappreciated aspect of assessing potential reservoirs when optimal and sustainable pressure conditions are critical for success. Cement bond logs (CBLs), a tool not generally used efficiently, can represent the cement quality around a wellbore and be used to identify intervals of poor cement integrity. A standardized CBL evaluation tool was created to remove inconsistencies in interpretations and used to evaluate 394 CBLs in Michigan and 306 CBLs in Ohio. 700 CBLs were analyzed for six site specific study areas and reviewed regionally across Michigan and Ohio to identify any trends in cement quality and highlight possible fluid migration pathways through wellbores with poor cement integrity. The CBL evaluation tool uses a quantitative bond index method to assign percent bond to wireline log readings based on user defined conditions. Once analyzed, the tool outputs a descriptive statistical dataset. Industry standards were used to categorize wells based on cement quality. Correlations between cement quality and the year and season the well was completed, well depth, cement column thickness, and formations cemented over were explored using basic statistics and graphing methods. As well, a standardized assessment of such a variety of wells and cement conditions allows for the incorporation of these wellbores into subsurface models to identify possible zones of weakness for ideal pressure conditions. There were no strong correlations on the regional scale between cement quality and year/season well was completed, depth of the well, and cement column thickness. The tool was very useful in providing red flags in the assessment of a site specific study area and in the evaluation of potential confining layers. Within a study area it was easy to identify wells with poor cement quality and determine if the wells posed a risk to a potential storage site. High risk areas for potential confining layers were also mapped and could be used as a guide to determine areas of poor cement bond within a specific formation.