Effect of Hydrofluoric Acid on Well Cement during Acidizing

This paper covers the study of the effect of hydrofluoric acid (HF) on well cement
class G when wells are subjected to acidizing operation. It has been shown that well
cement, when exposed to acid attack, will show loss of integrity in providing zonal
isolation to the well. Its compressive strength is also affected. Cement samples are
cured at a range of pressure and temperature before subjected to acid attack. The
effect of acid on the cement samples is quantified in terms of mass loss in cement,
decrease in compressive strength and also compositional changes in the cement
sample. There are claims that acid attack is purely superficial, where it only attacks
the surface of the cement. Further attack is said to be hindered by the formation of a
white precipitate identified as fluorite. From the result of the experiment, white
precipitate is indeed observed on the surface of the cement cubes exposed to acid
attack. To determine whether the attack is confined only to the surface, the sample
was sliced open to include the middle portion for x-ray fluorescence (XRF) and
scanning electron microscope (SEM) testing. Results show that the middle portion of
the cement sample experience little damage to acid attack. For cement cubes cured at
higher pressure and temperature, the effect of acid on the well cement is less
damaging. We can deduce that pressure and temperature do play a role in
determining cement resistance to acid, and that HF attack on cement is superficial.