Expanding Cements for Primary Cementing

Abstract

Abstract The expansion of cement and the effect of various expansive aids upon oil well cementing compositions have been investigated to determine the amount of expansion feasible and to observe the stability of cements displaying increased expansion. Linear expansion measurements of 1×1×10-in. cement specimens have revealed that admixtures - sodium sulfate, sodium chloride, possolan and combinations of these - will effectively increase the expansion of cement. Since sodium sulfate solutions have been known to be deleterious to fresh water cement slurries after having set, the addition of sodium sulfate to the cement at the time of mixing was not at first considered as a satisfactory and practical means of increasing cement expansion. Considerable expansion was realized with no visible sign of deterioration of the cement in a period of nine months. Laboratory tests have shown that bonding of cement between concentric sections of pipe was considerably improved when expanding cements were used. Measurements of the thermal expansion of cement have been made and coefficients of expansion calculated for several temperatures. A moderate decrease in cement expansion was noted with an increase in curing pressure with time. Introduction Since as early as 1920, numerous investigators have been searching for a means of counteracting the shrinkage of concrete that occurs when it loses moisture under dry curing conditions. For some applications such as pre-stressed concrete, improved bonding in confined spaces and for inducing tensile stress in restraining steel, it has been more desirable to develop an expansive cement designed to more than compensate for any potential shrinkage. Expansive cements have been reviewed by Lafuma1 who discussed the factors affecting volume changes in concrete and cited references to earlier work dealing with this problem. More recently, the composition of expanding cements and the chemical reactions involved, particularly the role played by calcium sulfoaluminate admixtures and related compounds, have been studied by Klein and Troxell.2 The preparation of anhydrous calcium aluminosulfate to produce expanding cement was reported by them, and in subsequent work Halstead and Moore3 were able to determine the crystal structure of this compound. Typical compositions of these expansive cements and methods of manufacture are revealed in two recently issued patents.4,5 The mechanism by which expansion can occur in Portland cement concrete as a result of the development of pressure by salt crystals was described in a paper by Hansen.6 The physical properties of expansive cement were recently investigated by Monfore7 who states: "If the expansion can be controlled so that it takes place when the concrete has developed some strength but is still extensible, the concrete may accommodate the expansion with a minimum of cracking. Such a cement might be termed a usefully expansive cement. But if the expansion occurs after the concrete has lost extensibility and has become so brittle that it can no longer accommodate the expansion without serious cracking or disintegration, the cement is properly termed unsound." The principal concern of this investigation has been to determine the effect of some expansive agents upon cementing compositions for use in oil and gas wells, predicated on the improved bonding of cement to both casing and formation. Although good placement techniques are of the utmost importance in accomplishing a satisfactory oil well cement job, there are occasions when under even the most ideal conditions microflow channels may result from pressure differential between completion and production fluids in the casing, or because of thermal changes during the setting of cement. In such instances there is a probability that fluid or gas migration in the pipe-formation annulus can be reduced by the use of an expanding cement. With regard to oil well cement slurries, it has been determined that a slight expansion will occur when the cement is cured under moist conditions. These same slurries, if air cured, will exhibit shrinkage due to loss of moisture. Since cement slurries are not generally subjected to a drying environment under down-hole conditions, they consequently would be expected to expand. However, the degree of expansion will vary somewhat with various classes, brands or batches of cement due to differences in chemical compositions, particularly tricalcium aluminate content.