Ultralow-Density Cementing Operations

Abstract

Summary Attempts to improve ultralow-density cement slurries [9 to 12 Ibm/gal (1078 to 1438 kg/cm3)] suitable for oil and gas well cementing have accomplished little except to define a disappointingly low strength/density ratio and to confirm the low-density limit for useful compressive strengths. The use of high-strength hollow spheres as a lightweight additive has been under investigation for a number of years. Encouraging results with high-strength but high-cost glass bubbles were reported by Amoco Production Co. researchers 1 in 1980. This paper describes the use of a new relatively low-cost lightweight admixture that maintains low density, even at high pressures, and has a relatively low mixing water requirement. Studies using this new material have greatly improved strength compared with other types of low density slurry systems. Final compressive strengths for some hydraulic cement slurries with densities of only 8.8 Ibm/gal (1054 kg/m3) can exceed 500 psi (3.45 MPa). This new admixture consists of small diameter [10 to 100 microns (10 to 100 /Am)] inorganic high-strength microspheres (HSM's). This paper presents data on several types of cement systems prepared with this new admixture. Physical properties are given and compared with other types of low-density slurries. Suggested applications are offered and case histories summarizing more than 100 successful jobs are included. Introduction The control of water flow in mine shafts by grouting with cement slurries 2 dates back to the 1880's, while cementing casing pipes to the formation in oil and gas wells was a recommended procedure as early as 1911. However, the advantages of using low-density cementing slurries to achieve successful, competent cement jobs was not recognized generally until the 1940's. Since that time, lightweight slurries have been recommended for use in wells extending through weak subterranean formations that are sensitive to hydrostatic pressure exerted by the column of cement. Weak or unconsolidated formations are found worldwide, and with increased exploration activities, more and more incompetent formations are being encountered that will break down when the fluids that are slightly heavier than water fill the annular space. Thus, an increasing need for ultra-low- density cements has arisen. Many different materials have been used for lightweight cementing additives. Bentonite, attipulgite, diatomaceous earth, fly ash, gilsonite, ground plastics, walnut shells, and silicate extenders all have been well accepted within their density limitations. Asphalt, oil emulsions, and low-strength hollow spheres I were tried but not widely accepted. Common to all the older lightweight additives was a lower density limit below which useful strengths could not be obtained. For water extending additives, a water ratio of 180% water/cement (w/c) (20 gal/sack) is needed for a 11-lbm/gal (1320-kg/M3) slurry. This water ratio is near the upper limit for useful compressive strength except for fillertype cements where slow strength development can be tolerated. For a 10-lbm/gal (1200-kg/m3) slurry, a w/c of greater than 300% would be needed, which would result in no useful strength for any purpose. About 10 years ago, investigators began to look at high-strength microspheres as a lightweight additive. The first published reports of the successful use were in 1980. R. C. Smith et al. reported the use of glassbubbles by Amoco, and H.E. Ripley reported the use of specially processed microspheres by Halliburton Services Ltd. Canada. The two materials were similar except for cost and amount needed for a specified density. JPT P. 61^