A Critique of Filler Cements

Abstract

Abstract A critique is presented of filler cement compositions as related to primary cementing to establish a comparative evaluation of composition properties and effectiveness. Physical properties of filler cements are outlined and discussed. The minimum requirements of filler cements considered are the economics of the slurry due to the quantity of cement considered, sufficient strength to form a good sheath around the pipe yet light enough in weight to reduce excessive hydrostatic head on the formation and sufficient bond to support the casing weight and help prevent migration of fluids and gases from one section to another. In addition, the flow properties of filler cement compositions are discussed considering their ability to be placed into turbulent rates and their effectiveness for mud removal in the wellbore. A study is made of current cement compositions using cements manufactured with bentonite, prehydrated bentonite, diatomaceous earth, perlite, gilsonite, pozzolans and special lightweight compositions. All compositions studied are compared based on strength, weight, cost and flow properties. Physical properties of each of these compositions are outlined as to their ability to meet certain field applications or conditions. Introduction Much well founded concern has been exhibited about the type and character of cement to be applied to a potentially productive section of a formation. This application requires a thorough knowledge of cements and cement additives. Cement is used in the wellbore for various reasons; but primarily, it is to help prevent the migration of fluids or gases from one section to another. of secondary importance is its use as a casing support. This support may be in the form of slacked off weight in the case of an oil well, or casing tension in the case of an extremely high-temperature formation or gas well. The cement may have the single purpose of protecting casing against corrosion. In any case, a cement that does not have to withstand the more rigorous requirements of a primary cement is referred to as a filler cement. Filler Cement Requirements Filler cements are very controversial; therefore, the minimum requirements vary a great deal between well operators. Primary requirements for these cements are that they be low in density, economical and of a reasonable compressive strength. The density of a filler-type cement is not always important, but in many instances it is critical when a long cement column is necessary to cover long sections. This is especially true when a production string or deep conductor pipe is to be circulated. If a filler cement can not be designed to have a pressure gradient less than the fracturing gradient of the exposed formation, the cement must be placed in multiple stages. The cost of filler cements is especially important because of the volumes normally required. Most filler cement blends will save from 30 to 70 cents per cubic foot of slurry, depending on the composition selected. Strength is probably the requirement of greatest concern, and basically is dependent upon the amount of mixing water. Generally, the lower the density and cost, the lower the compressive strength. The minimum acceptable compressive strength of a filler cement is probably a great deal less than is generally specified. This compressive strength must be sufficient to support at least a portion of the casing string. Even though the primary cement on bottom will support the weight of the string, the filler cement must support any reversals in length (such as slacked off weight or tension) so that the bond may be maintained. For many years it has been recognized that cements need very little strength to support a string of casing. Tests by Farris et al., demonstrated that as little as 8 psi tensile strength may be adequate. These data have been widely accepted and have been the basis for many regulatory rules governing WOC time. A comparison of tensile and compressive strengths of Class A cement with varying quantities of bentonite revealed that the average compressive strength is 5 to 10 times greater than the tensile strength. Since cement strengths are normally reported as compressive strength, the minimum compressive strength to support casing is approximately 50 psi.