Oil-Well Cement

• Oil-wells are drilled through stratified sedimentary rocks through a great depth in search of oil. It is likely that if oil is struck, oil or gas may escape through the space between the steel casing and rock formation. Cement slurry is used to seal off the annular space between steel casing and rock strata and also to seal off any other fissures or cavities in the sedimentary rock layer. The cement slurry has to be pumped into position, at considerable depth where the prevailing temperature may be upto 175°C. The pressure required may go upto 1300 kg/ cm2. The slurry should remain sufficiently mobile to be able to flow under these conditions for periods upto several hours and then hardened fairly rapidly. It may also have to resist corrosive conditions from sulphur gases or waters containing dissolved salts. The type of cement suitable for the above conditions is known as Oil-well cement.
• The desired properties of Oil-well cement can be obtained in two ways: by adjusting the compound composition of cement or by adding retarders to ordinary Portland cement. Many admixtures have been patented as retarders. The commonest agents are starches or cellulose products or acids. These retarding agents prevent quick setting and retains the slurry in mobile condition to facilitate penetration to all fissures and cavities. Sometimes workability agents are also added to this cement to increase the mobility.