New Ductile Cement System for ISC Wells in the Orinoco Oil Belt, Venezuela

摘要

For a new project in the heavy oil fields of the Orinoco oil belt(OOB)in Venezuela,thermally stable slurry has been designed to retain strength and permeability during long exposure to temperatures up to 1200°F in the process called in situ combustion(ISC).In this thermal recovery technique,heat is produced within the oil reservoir to reduce the viscosity off the heavy oil to improve recovery. Cementation of in situ combustion wells is challenging for several reasons:The wells are often associated with weak and unconsolidated formations;and the cement sheath must withstand extreme stress due to temperature cycling during in situ combustion process.Conventional Portland cement slurries,even with silica flour additives to prevent strength retrogression,tend to fail with long exposure at temperatures above 350°F. Different compositions of cement slurries have been made for decades with a calcium aluminate phosphate(CAP)system for the casing strings,but cementing additives commonly used for Portland cement-based systems do not work for CAP cement.Other compositions based on class G cement, Metakaolin and carbon fibers can create a high-temperature thermally stable and less permeable cement sheath as required for in situ combustion process;however,mechanical properties of the formation and set cement will determine the chances of failure of the cement sheath due to induced stress by temperatures and pressures changes.