A Comparison of the Measured Properties of Annular Cement with Ultrasonic Cement Evaluation Logs

Different measurement methods have been utilized to investigate the quality of the cement sheath inside two cemented sandwich sections including high-resolution ultrasonic cement evaluation logs, analysis of samples, mechanical loading, and fluid seepage measurements. The sections were recovered from a North Sea well during a permanent plug and abandonment operation. The measurements have been analyzed with an aim to describe in detail the spatial variations in the cement properties and relate them to the logs. Ultrasonic cement evaluation logs were recorded to map the acoustic properties of the annular cement in the casing sections. Logging passes were recorded using different annular fluids and with different internal casing pressures to investigate the potential effect on the casing to cement bond response. The casing annulus was pressure tested using water and gas, and seepage rates were recorded whilst varying the annulus and the inner casing pressures. The sections tested were instrumented with an array of annulus pressure sensors. On one section, strain gauges were installed on the casing outer surface to record the transfer of strain through the cement sheath to the outer casing. The eccentricity of the inner casing was up to 70% compared with the outer casing which results in a substantial variation of the cement sheath thickness. Accordingly, the pressure sensors and strain gauge arrays were positioned to capture both axial and azimuthal variations of the cement sealing properties. Cement mechanical, chemical, and acoustic bulk properties were also measured on core plugs taken from the cement sheath. The log recordings and sensor measurements showed that the cement sheath properties vary considerably, both along the section length and from the narrow to the wide side of the annulus in the casing sandwich sections. The sealing quality of the cement sheath measured by pressure testing could be correlated with the log response. We observed a nearly linear reduction in seepage rates when increasing the inner casing pressure due to the reduction in size of the annular leakage path. Analysis of bulk properties confirm the presence of cement defects such as mud contamination and microannuli. The logs identified features related to the test cell construction that demonstrated the log spatial resolution and enabled an accurate spatial comparison to be made between the logs and cement sheath sealing properties. A comprehensive data set has been recorded on casing in casing-cemented sandwich sections with axial and azimuthal variations in the cement sheath quality. The data analysis has improved the understanding of the cement sheath mechanical properties, the seal quality, and the response of the ultrasonic cement evaluation logs.

Use of seepage meters in a groundwater – lake interaction study in a fractured rock basin — a case study

Applications of seepage meters in lakes with slight bottom sediments in fractured rock basins have not been reported. A study of the hydrogeology of Long Lake, at Halifax, Nova Scotia, used seepage meters in this situation. Fifteen meters were installed in the 1.7 km2 lake — seven in a line extending from the shore, the remaining eight on the lake periphery. Seepage measurements were made monthly over 1 year. Groundwater levels were measured in boreholes in the fractured granite of the watershed. A positive inflow of groundwater to the lake occurred through the year, seepage flux at the shoreline meters varied from 0.15 to 0.78 mL∙m−2∙s−1, with no consistent seasonal variations. Seepage decreased exponentially with offshore distance. A linear relationship was evident between water levels in a borehole near the shore and rate of seepage at the closest meter. Chloride applied to this borehole was reflected in elevated chloride concentrations in seepage collected by the meter. Key words: seepage meter, groundwater – lake system, fractured rock tracers.