Additive for increasing the density of a fluid for casing annulus pressure control

A method of treating a wellbore, the method comprising:

• circulating a wellbore fluid in the wellbore, wherein the wellbore fluid comprises an base fluid and a wellbore additive colloidal solid material,
• wherein the wellbore additive colloidal solid material is formed by grinding a solid particulate material and a polymeric dispersing agent for a sufficient time so that the polymer dispersing agent is absorbed to the surface of the resulting colloidal solid particles,
• wherein the gringing is carried out in the presence of an aqueous base fluid and a dispersing agent comprises a homopolymer or copolymer of any monomers selected from the group consisting of acrylic acid, itaconic, maleic acid, maleic anhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamide 2-propane sulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether, vinyl acetate, and combinations thereof, and
• wherein the grinding is carried out that the wellbore additive colloidal solid material has a weight average particle diameter d90 less than 2.0 microns.