A New Ultra-Lightweight Cement With Super Strength
Authors: Smith Robert C. , Powers Charles A. , Dobkins Terrell A. 
Published: December 2007

By using cement slurries having densities as light as fresh water, casing can be cemented in wells extending through formations with the lowest of fracture gradients. These slurries use hollow glass spheres to lower density to the desired level. Case histories of two successful field tests of a 9.5 lbm/gal (1138 kg/m3) cement slurry are presented.

Introduction
This paper presents a cementing innovation developed by Amoco Production Co. primarily for casing cementing, but it also has application in plugback cementing, grouting, and other operations. plugback cementing, grouting, and other operations. The newly developed ultra-lightweight cement slurry uses hollow glass bubbles to reduce density to as low as that for water and even less. Although the patent literature reports the idea of glass bubbles in cement, the laboratory process never had been reduced to field practice until now. In addition, the cement slurries developed by Amoco are different from those contained in patent literature. Pertinent laboratory data are presented on the current slurries and cover a wide range of density, temperature, depth, and additive concentrations. Also described are case histories of two successful field tests of a 9.5-lbm/gal bubble cement slurry in an offshore Gulf of Mexico well.Lightweight or low-density cement slurries are used to reduce the hydrostatic pressure exerted by the cement column in wells extending through weak subterranean formations. Conventional low-density cements (as low as 11.5 to 12 lbm/gal) also are used widely as filler cements because they are more economical than neat cements. However, slurries made with glass bubbles are not filler-type slurries. The bubble slurries are applicable mainly where density control to less than 11.5 to 12 lbm/gal is required or where higher strength development is needed at densities comparable to conventional low-density cements.Examples of weak formations where density control is required are the unconsolidated, geologically young formations in the U.S. gulf coast region, shallow coal seams in Wyoming, Muskeg formations in Canada, and fractured formations worldwide. In some of these areas the equivalent formation breakdown density approaches that of water. The lightest-weight cement previously available was about 11 lbm/gal, which often exceeded the formation fracture gradient. Exceeding the fracture gradient during cementing prevents obtaining a competent and complete cement sheath around the entire casing. This sometimes prevents adequate control of the well during deeper drilling operations, particularly offshore where remedial cementing of the shallow casing strings is not possible. In many onshore wells, remedial cementing possible. In many onshore wells, remedial cementing is required to cover all freshwater sands when incomplete fillup results from primary cementing with conventional lightweight slurries. This extra remedial cementing is a significant expense. The problems in well control and the remedial cementing costs led to Amoco's development of an ultra-lightweight glass bubble cement slurry.
JPT
P. 1438