Application of Sodium Silicate Prepad Fluid in Cementing Heavy Oil Zones in Venezuela

Viscous oil in the heavy oil zones in Venezuela is buried in shallow depths and the shale caps are unconsolidated.The reservoir formations have low pressure bearing capacity and are full of fractures.Conventional prepad fluids used in previous cementing jobs often resulted in lost circulations, making it difficult for the cement slurries to return to the designed depth.Cleaning of the annular spacing were generally poor during cementing, and cementation of the two interfaces(interface between casing string and cement sheath and the interface between cement sheath and borehole wall) is weak, resulting in annular channeling in thermal recovery and corrosion of uncemented casing string by acid gases such as H₂S and CO₂.The life of the production well is thus shortened.In field practices, it was found that by adding 25%sodium silicate solution into prepad fluid, lost circulation and discontinuity of cementing were prevented from occurring, the cement slurries in the annular space returned to the designed depth and the cement sheath was intact.Bond strengths of the two interfaces were enhanced.All these contributed to the prevention of casing damage by acid corrosion.The key point in cementing with sodium silicate prepad fluid is to prevent the sodium silicate from contacting with cement slurry, because the contact of the two will inevitably shorten the thickening time of the cement slurry and lead to cementing disasters.