Foamed Cement: A Second Generation

Abstract

Abstract Advanced technology in design and implementation of stable foamed cement systems now offers new solutions to critical oil-field cementing problems well beyond the original utility of lightweight slurries in reducing hydrostatic pressure across fracture-sensitive zones. Implications of new high-performance foamed cement capabilities are discussed for specialized applications such as thermal recovery, deep cementing in a narrow annular gap, lost-circulation control and stimulation across fractured or washed-out zones, as well as other unique advantages in primary cementing. Specialized foamed cement studies reveal improved prevention of strength and permeability retrogression prevention of strength and permeability retrogression and enhanced thermal isolation for steamflood applications, all at significantly reduced fracture stress as compared to conventional extended systems. Because of their unique rheology, high-performance foam slurries can be designed to exhibit moderate thixotropy but maintain excellent fluidity at extremely low shear. Cement foams further aid zonal isolation due to improved shear bonding and enhanced pipe/cement/ formation contact as a result of the expansive nature of the slurry during setting. A number of successful jobs are documented in which lightweight nitrified systems placed across weak and washed-out zones have been successfully perforated and fractured. Several new potential applications for moderate to high-density, stable, nitrified slurries are also discussed based on their unique performance. Introduction Primary oil-field cementing is necessary in order to serve several major objectives. To effect zonal isolation of gas and oil from other undesirable fluids. To securely bond the casing in the wellbore. To protect the pipe from corrosion. To provide a firm anchor and seal for wellhead equipment. Prior to initial set, the fluid cement column Prior to initial set, the fluid cement column placed in the well annulus exerts a hydrostatic placed in the well annulus exerts a hydrostatic pressure consistent with the height and density of the pressure consistent with the height and density of the supported slurry. If this pressure exceeds the fracture limits of the surrounding formation, damage can occur and cement may be diverted into the damaged zone, resulting in the loss of one or more of the critical objectives. Several options to reduce downhole pressures i.e., reduced slurry density and/or stage cementing, are currently practiced in oil-field cementing. Reduced slurry density can be accomplished by the use of the following. . Water extension of slurries. Ultralight weight additives. Foamed cements. The minimum practical density limitation with water-extended systems (i.e., bentonite, gilsonite, diatomaceous earth, etc.) is acknowledged to be about 11 lb/gal. Further reduction to as low as eight to nine pounds per gallon is obtainable using special tiny glass "balloons" which utilize encapsulated air as a density-reducing agent. Although effective in principle, two major drawbacks have been reported: "a principle, two major drawbacks have been reported: "a definite pressure limitation before crushing, and second, high relative cost due to the large amounts of the material added to the cement." High-pressure foamed cement was introduced into the oil field in the late 1970s based almost solely upon its cost-effective, low-density utility with fracture-sensitive formations. P. 153