Abstract: Cementitious slurries capable of controlling and/or preventing strength retrogression during the cementing of a formation contain a bismuth salt or derivative thereof. Bismuth trioxide is preferred. The cementitious slurry may further contain a density modifying agent and/or strength enhancer. The cementitious slurries of the invention are suitable for use over a broad temperature range of from about 45° F. to about 500° F.

Abstract: A cementitious composition for cementing an oil or gas well and which exhibits when cured, increased flexural strength and a flexural strength to compressive strength ratio between from about 0.29 to about 0.80, contains a hydraulically-active cementitious material, such as Portland cement, and substantially spherical zeolite. Representative zeolites include natrolite, heulandite, analcime, chabazite, stilbite, and clinoptilolite. The weight percent of zeolite in the cement composition is generally less than or equal to 15 percent. In practice, a well bore may be cemented by pumping the activated slurry and pumping it within the well bore to a pre-selected location and allowing it to solidify.

Abstract: Sandstone formations of oil and gas and geothermal wells are effectively stimulated when a buffered HF-sandstone acidizing solution is employed without the prior introduction of an acid containing preflush solution. By not using a preflush solution, buffered HF-sandstone acidizing solutions are highly effective in dissolving and removing siliceous material while minimizing the formation of calcium fluoride.

Abstract: A method of stimulating subterranean formations in oil and gas wells using a high pH buffered acid system. The method may be used to treat sandstone formations wherein the pH buffered acid system is used as the preflush or overflush or both. The method consists of introducing into the well a hydrofluoric acid containing sandstone acidizing solution either prior to or subsequent to introduction of the pH buffered acid system. The pH buffered acid system may further be used in the treatment of carbonate reservoirs. The pH of the buffered acid solution is substantially equivalent to that of the pH of the acidizing solution. The buffered acid solution is void of either hydrofluoric acid or hydrofluoric acid precursors.

Abstract: A cementitious composition for cementing an oil or gas well and which exhibits, when cured, increased flexural strength and a flexural strength to compressive strength ratio between from about 0.29 to about 0.80, contains a hydraulically-active cementitious material, such as Portland cement, and substantially spherical zeolite. Representative zeolites include natrolite, heulandite, analcime, chabazite, stilbite, and clinoptilolite. The weight percent of zeolite in the cement composition is generally less than or equal to 15 percent. In practice, a well bore may be cemented by pumping the activated slurry and pumping it within the well bore to a pre-selected location and allowing it to solidify.

Abstract: A storable, hydraulically-active, cementitious slurry is made and used to cement within subterranean foundations for oil and gas wells. The slurry is made from a hydraulically-active cementitious material, a suspending agent, and, as a retarder, boric acid. The boric acid is present in the cementitious slurry in an amount sufficient to lower the pH of the storable cementitious slurry to at least 12.0, preferably to at least 11.0 or preferably, for reasons of longevity, to a pH of at least 9.0. Generally the amount of boric acid in the cementitious slurry is between from about 1 to about 6 percent by weight. Preferred as suspending agent are iota carrageenan as well as poly (methyl vinyl ether/maleic anhydride) decadiene copolymer. The slurry remains liquid during storage. The slurry is activated when needed for cementing and is pumped into the subterranean formation where it is allowed to set.