Porosity of oil well cement slurries during setting

The phenomenon of ‘gas migration’ during oil well cementing is believed to occur during the transition state between initial and final set of the cement. In order to evaluate the importance of pore openings and total porosity in the critical time gap, a suitable experimental technique was tested on some neat oil well cement slurries. The hydration was effectively stopped every 30 min at 20°C and every 20 min at 60°C by dropping plastic tubes containing the cement slurry into liquid nitrogen (i.e. quenching), cracking the tubes open and letting the frozen bits of paste thaw in ethanol. The change in porosity and pore size distribution was determined by helium pycnometry and mercury intrusion porosimetry as a function of time in the setting period for a plain API class G cement slurry (w/c = 0·50) at both 20°C and 60°C. These data were compared with the amount of chemically hound water in the same samples, and used to predict the total porosity at a given degree of hydration. The excellent correspondance of experimental and theoretical porosities validates the experimental procedure, which can also be used in explaining the variation in gas migration between commercial oil well cement slurries.