A Modified Low-Strength Cement

Abstract

Abstract The need for a low-strength cementing composition for use in well cementingis reviewed and results are presented of laboratory and experimental fieldtests of a modified cement having a controlled ultimate tensile strength ofapproximately 200 psi. The modified cementing composition may be prepared fromeither high early strength or normal portland cements by the addition ofbentonite clay and a suitable agent for dispersing and controlling the set ofthe slurry. Substitution of the modified cement for conventional slow-setcements may give better completion results in many wells because the modifiedcementing composition has lower set strength, lower slurry density, and greaterslurry stability than conventional cement slurries. The lower ultimate strengthallows greater penetration with less shattering of the set cement whenperforating casing and cement. The lower slurry density allows the placement oflonger columns of cement slurry, and the greater slurry stability reduces thepossibility of having an uncemented section caused by the settling of cementparticles before the cement sets. Introduction High strength has always been one of the accepted criteria of a good cement.During the early use of portland cement in well cementing, emphasis was placedupon securing cements with higher strengths. In 1931, Barkis reported that,?Normal oil well cements have been improved to develop greater strengths anduniformity of product, which has aided in producing successful jobs incementing the deeper strings.? As long as most wells were completed by the open-hole method, the use ofcements having high strengths seemed desirable, and there was no objectionraised against high-strength cementing compositions. For a number of years, however, the industry has been completing a large number of wells by settingand cementing casing through productive horizons and then obtaining productionby gun-perforating the section of casing opposite the desired interval.Although this method has been generally successful, difficulties have beenexperienced in some cases in completing or recompleting wells because ofapparent lack of adequate penetration by the bullets through the casing andsurrounding sheath of cement and into producing formation. In addition to thepenetration trouble, there have been indications that fracturing and shatteringof the set cement by perforating might be a contributing factor in causing thefailure of some jobs to exclude water or gas from oil producing zones. T.P. 3062