Master of Science in Petroleum Engineering (MSPE)
Well cementing is one of the key processes performed during drilling and completion of wells. The main objective of primary cementing is to provide zonal isolation. Failure of cement to provide zonal isolation can lead to contamination of fresh water aquifers, sustained casing pressure, or blowout. For effective cementing, the cement slurry should completely displace the drilling fluid. In practice, this is hard to achieve and some mud is left in the wellbore where it contaminates the cement after cement placement.
This study investigates the effect of mud contamination of cement on the integrity of cement-formation interface. Flow-through experiments were conducted over 30-day periods using cement-sandstone composite cores and brine (salinity ~20,000 ppm) at 2100 psi (14.48 MPa) confining pressure, temperature of 72°F (22.22°C) and flow rate of 1 ml/min. Each cylindrical composite core was composed of half-cylinder Berea sandstone and half-cylinder Portland cement paste, with dimensions 12 in (30.48 cm) in length and 1 in (2.54 cm) in diameter. One composite core had no contaminated layer and each of the two other cores had a ~1.27 mm (0.05 in) thick layer of contaminated cement with 5% or 10% mud contamination by volume.
Image based techniques used to characterize the composite cores revealed the presence of large non-circular pores (with as much as ~750 µm length and ~150 µm thickness) in the mud contaminated cement at the end of the core-flood. The large pores were higher in number in the 10% than in the 5% mud contaminated cement. The pores did not appear to be interconnected at the end of 30-day core-flood although leaching of the cement surrounding the large pores was observed. The porosity of the 10% mud contaminated cement was found to have increased from 0.65% to 12.53%.
Based on the observations, we can conclude that large pores are created in cement due to the presence of mud contamination and the pores increase in number as level of mud contamination increases. Leaching of the cement surrounding the large pores may lead to interconnectivity of the large pores in the long run and result in loss of zonal isolation.
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Agbasimalo, Nnamdi Charles, "Experimental study of the effect of drilling fluid contamination on the integrity of cement-formation interface" (2012). LSU Master's Theses. 2403.