Permeability Damage Due to Precipitation of Insoluble Salts From Cement Slurry Filtrate
Authors: Krilov Zoran , Romic Leopold , Celap Stojan , Cabrajac Stevo 
Published: December 2007

Abstract
The phenomenon of precipitation of the insoluble salts particularly calcium carbonate and calcium sulfate, deposited from cement filtrate into a rock pore space as a potential source of formation permeability damage, was studied.
The series of flow tests were run in laboratory on sandstone rock examples using cement slurry filtrate containing various additives such as extenders, dispersants, retarders and fluid loss control additives.
The impact of precipitation of insoluble salts on rock permeability reduction has been evaluated on the basis of experimental results applying different analytical methods: chemical and SEM/EDAX analysis, permeability change measurement, as well as X-ray computerized tomography (CT) imaging technique.
All of the three types of cement filtrates, obtained from API class G cement used in experiments, caused sandstone permeability reduction, mainly due to deposition of coprecipitated insoluble CaCO3 and CaSO4 salt crystals in the pore space. The use of cellulose derivative, as fluid loss control additive caused the most severe formation damage. Lignin derivative as retarder and synthetic polymer as dispersants if used in cement slurry composition caused less damaging effects than in case of cement slurry filtrate without additives.
The permeability damage due to precipitation of CaCO3 and CaSO4 from cement slurry filtrate could be partially recovered flushing the rock with brine.

INTRODUCTIONS
During drilling and completion operations, the permeability of potentially productive rock formations can be significantly decreased by fluids which come in contact with the reservoir. As filtration process downhole proceeds, the filtrate, with its accompanying particles, creates an invaded zone around a wellbore. Within that zone the productivity may be reduced by physical and/or chemical change to the formation rock or fluids.
Drilling mud and cement slurry are the most frequently encountered as fluids generating a filtrate which penetrates into permeable formations under a differential pressure. Muds are forming an external filter cake on the face of formation exposed to it, and a relatively shallow internal cake is formed to a few centimeters from the wellbore. The chemical composition of the majority of convenient water base muds is such that mud filtrates do not produce insoluble particulate precipitates.
On the other hand, when cement slurry is pumped into a well and passes by permeable formations, the aqueous phase is squeezed into the rock pore space under relatively high differential pressure between the annular space and the formation. This cement slurry filtrate contains the various ions (Ca2+, K+, Na+, OH-, Al(OH)4-, SO42-, HCO3-) dissolved in aqueous solutions. The concentration of calcium carbonate and sulfate is often close to supersaturation which would result with deposition of insoluble calcium carbonate and/or calcium sulphate precipitate deep inside the pore space.
The cement slurry additives also play a role in the formation of precipitate during the filtration process downhole.
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