Hydraulic or zonal isolation behind pipe plays a vital role in the life of a well. It guarantees well control, production from hydrocarbon zones and preventing the production of unwanted fluids. Perforation zone’s selection and remedial jobs are based on cement integrity. Common challenges faced in cementing wells in Pakistan are presence of thief zones, water/gas invasion in cement and improper mud displacement resulting in cement channelling.
Cement evaluation is as crucial as the cementing job itself. It is done using conventional sonic tools (CBL-VDL) which provides information about cement integrity. Sonic tools provide a qualitative uni-directional measurement. Then we have ultrasonic technology which measures "Acoustic Impedance" (AI) of annulus material; gives 100% azimuthal coverage with much better vertical and radial resolution than sonic tools. However, for light cements; the above conventional cement evaluation techniques cannot be used due to its limitations like:
Effect of liquid and gas micro-annuli and dual casing
Inability to resolve micro-debonded/contaminated class G cement due to its threshold principle
Low acoustic impedance contrast between liquid and cement i.e. for liquid 1.4–2.6 MRayls and for light cement 1.5-3.6MRayls
To address the above limitations a novel measurement of "Flexural Attenuation" (FA) is combined with AI that classifies the annulus material into three distinct states Solid, Liquid and Gas (SLG). In flexure mode casing is vibrated at an angle and combined with AI (vibration in thickness mode); giving us a unique output of SLG map. AI stand alone cannot evaluate light cements and contaminated class G cements with certainty.
A case study is presented; where light cement slurry was used. FA and AI measurements technology was combined to evaluate for zonal isolation. First time in Pakistan an ICE "Integrated Cement Evaluation" processing was applied; specifically designed for light cements and confident answers were obtained successfully. The AI showed 40–50% liquid contamination whereas SLG map confirmed solidified light cement behind pipe. FA and AI together confirmed zonal isolation and in turn provided aid for successful well control and completion of the well.