API TR 10TR2:1997(R2002) pdf download.Shrinkage and Expansion in Oilwell Cements
4.1.1 Some equipment measures the external volume change. Other equipment measures the volume of fluid intake or expulsion during the hydration. Measurement of V, and y must be performed at the same temperature. In the measure- ment of an external volume change, the external stress applied to the material has to be constant during the experi- ment. For a correct measurement of fluid intake or expulsion, the pore pressure at the end of the test has to be the same as at the beginning. a. Bulk shrinkage corresponds to an external volume decrease. b. Bulk expansion corresponds to an extemal volume increase. c. Inner shrinkage corresponds to an intake of fluid. d. Inner expansion corresponds to fluid expelled from the slurry. In a slurry made of cement and water only, one could observe bulk and inner shrinkage. In a slurry containing alu- minum powder, one might notice hydrogen production, which could lead, under appropriate boundary conditions, to an inner expansion. In a sluny containing carbon or magne- sium oxide, one could measure a bulk expansion. The magnitude of these possible effects depends on the cement powder, slurry design, and curing conditions (temper- ature, pressure). The slurry’s permeability, mechanical properties, and stress applied to the boundaries control the partition between these effects. If the cement is infinitely permeable, then a volume change in the pores simply requires a net flux of fluid into the sample and, thus, no bulk shrinkage is measured. The inner shrinkage is measured by the amount of fluid entering the cement. If the cement is impermeable, then a volume change in the pores results in a pore pressure change and, subsequently, a bulk shrinkage or expansion governed by the mechanical properties of the slurry.If the slurry is contained between fixed, impermeable boundaries, no bulk expansion can occur. Inner shrinkage results first in pore pressure reduction and then in creation of internal tensile stress that could lead either to debonding from the walls or to failures, and eventually both. In this situation, dramatic pore pressure decrease is observed down to the vapor pressure of the water at the temperature where the test is conducted. In turn, expansion creates compressional stresses. Depending on the mechanical properties of the slurry, the additional stress (tensile or compressive) can result in failures.