Residual oil saturation is determined by injecting water containing dissolved CO.sub.2 and subsequently active base generating reactant into an oil and water containing reservoir and chromatographically analyzing the patterns of the concentrations of in situ reaction-depleted CO.sub.2 and reaction-increased bicarbonate salt in fluid produced from the reservoir.

Residual oil saturation is determined by injecting water containing or contacting dissolved halocarboxylic acid salt and carbonate salt into an oil and water containing reservoir and chromatographically analyzing the patterns of the concentrations of in situ generated CO.sub.2 and water soluble acid salt in fluid produced from the reservoir.

The oil saturation within a subterranean reservoir is determined by injecting water containing dissolved reactants which are substantially oil-insoluble and which convert the injected solution into a tracer-containing solution in which there is at least one each of oil and water tracers, with the oil tracers being gaseous, and chromatographically analyzing the patterns of the concentrations of the tracers within fluid produced from the reservoir for determining the oil saturation.

A hydraulic cement slurry which forms a gas-locked cement which contains microsized clefts that are filled with nitrogen gas and is both impermeable and non-shrinking is formed by dissolving in the slurry mix water a correlated amount of gas-forming reactants having a delayed rate of reaction such that the gas production occurs mainly at the time the volume of the slurry mix water is being reduced to a low value during the setting and hardening of the cement.

Materials in and around a well are acidized with chemically heated, hot acid by injecting a solution of both nitrogen and heat generating reactants and a mixture of weak acid and weak acid salt that both buffers the relation and provides a weak acid acidizing solution.

The volume change of a hydraulic cement slurry can be measured by measuring the amount of pressurized gas which flows into or out of the volume initially occupied by the slurry during the setting of the cement, for example, where the slurry is one formed by dissolving in the slurry mix water a correlated amount of gas-forming reactants having a delayed rate of reaction such that the gas production occurs mainly at the time the volume of the slurry mix water is being reduced to a low value during...

A testing apparatus is provided for measuring changes in the bulk volume and gas permeability of a cement slurry, such as one formed by dissolving in the slurry mix water a correlated amount of gas-forming reactants having a delayed rate of reaction such that the gas production occurs mainly at the time the volume of the slurry mix water is being reduced to a low value during the setting and hardening of the cement.

In reacting a siliceous material with an acidic solution containing hydrogen fluoride, the reaction rate is kept relatively low by contacting the material with an aqueous solution of a fluoride salt, a weak acid and a weak acid salt in proportions that maintain a significant but low concentration of hydrogen fluoride in the solution.

The rate of acidizing a material such as a carbonate reservoir is reduced by treating it with an aqueous solution that contains enough weak acid to react with a significant amount of the material and enough weak acid salt to provide a weak acid to weak salt ratio that maintains the concentration of ionized weak acid at a relatively low level at which its rate of reaction is relatively low.

The precipitation time of a precipitate-forming aqueous solution containing a polyvalent metal that precipitates as a gelatinous metal hydroxide and a reactant that raises the pH to cause the precipitation is controlled, and/or precipitation at temperatures above about 130.degree.F is inhibited, by dissolving in the aqueous solution a reactive mixture of halide ions and an epoxide.