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Collaborating Authors
Drilling Fluids and Materials
Abstract A kinetic model for the reaction of Hydrochloric acid with limestone bas been determined. Reaction order and rate constant for this model were calculated from experiments where acid reacted with a single calcium carbonate plate. Experiments were performed so that acid flow past the plate and mass transfer rate to the rock surface could be calculated theoretically. The resulting model, therefore, accurately represents the acid reaction process at the rock surface and is independent of mass transfer rate. Combination of this model with existing theory allows prediction of acid reaction during acid fracturing operations. A model for acid reaction in wormholes created during matrix acidization treatments is presented along with data for reaction of hydrochloric, formic and acetic acids in a wormhole. Factors limiting stimulation in acid fracturing or matrix acidizing treatments are then discussed. Introduction To predict the stimulation ratio resulting from acid fracturing or matrix acidizing treatments it is necessary to know the rate of acid reaction under field conditions. In acid fracturing treatments, for example, reaction occurs as acid flows through a narrow fracture. Reaction in a matrix treatment occurs during flow through wormholes (channels of roughly circular cross-section) created by acid reaction. In both treatments, a large amount of mixing occurs during flow through the fracture or channel as a result of tortuosity and wall roughness. Reaction rate can be obtained from experiments, or predicted by theoretical calculations that accurately model field conditions. In general a theoretical procedure is preferred since it can be used without recourse to laboratory testing. Acid-reaction-rate data have been reported from a number of experiments intended to simulate acid reaction in field treatments. Tests most often used are:the static reaction rate test, in which a cube of limestone is contacted with unstirred acid at a known ratio of rock surface area to acid volume; flow experiments, where acid is forced to flow between parallel plates of limestone; and dynamic tests, whine limestone specimens are rotated through an agitated acid solution. In general, these tests model some aspects of the reaction process, such as area to volume ratio, or acid flow velocity, but do not accurately model all field conditions. To obtain an accurate mathematical model for field treatments, assuming fracture or wormhole geometry is known, it is necessary to characterize acid reaction kinetics at the limestone surface, rate of acid transfer to the surface, and rate of fluid loss from the fracture or wormhole. (Each of these processes is shown schematically in Fig. 1.) processes is shown schematically in Fig. 1.) Reaction kinetics are independent of the geometry in which reaction occurs; therefore, once kinetics have been determined for a given acid-rock system field treatments can be simulated by prediction of the rate of acid transfer to the surface and fluid loss to the formation. Unfortunately, experiments reported to dare do not allow determination of a kinetic model. SPEJ P. 406
- Well Completion > Acidizing (1.00)
- Well Drilling > Pressure Management > Well control (0.56)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid management & disposal (0.55)
- (3 more...)
Abstract A procedure is developed for predicting changes in the porosity distribution in a sandstone resulting from reaction with hydrofluoric acid. This procedure is based on a theory for slow heterogeneous reactions in a porous solid where the solid matrix is consumed in the reaction Process. Reaction-rate data for use in this theory are obtained from experiments where acid is injected through short cores and effluent concentration measured using a fluoride specific ion electrode. This rate is found to be first order in hydrofluoric acid concentration. Variations in rate with temperature and quantity of rock dissolved are shown. Introduction Mixtures of hydrofluoric and hydrochloric acid are used to stimulate gas and oil production from sandstone reservoirs by increasing formation porosity and permeability near the wellbore. This porosity and permeability near the wellbore. This acid will react with almost all constituents of naturally occurring sandstones, such as silica, feldspar, clays, and calcareous material. In order to utilize this acid effectively, it is necessary to predict where acid reacts and changes that occur predict where acid reacts and changes that occur with reaction. Chemical reactions between hydrofluoric acid and silica or calcite in the rock matrix are simple, well known reactions. However, reactions with silicates such as clays or feldspars are complex since these minerals occur as three-dimensional lattices with only average empirical formulas. Examples are kaolinite ([A Fe +3 Mg]-Si O1.8 0.1 0.1 2 5 [OH]. Ca), montmorillonite (A Mg Si4 0.05 1.67 0.33 4 O [OH] . NA), and feldspars such as albite10 2 0.33 ([NaSi A ]). In the reactions shown below the3 8 reaction of sodium silicate is used to represent hydrofluoric acid reaction with silicates found in the matrix. REACTION WITH SILICA SiO + 4HF SiF + 2H O2 4 2 SiF + 2HF H SiF4 2 6 REACTION WITH SILICATES (FELDSPAR OR CLAYS) Na SiO + 8HF SiF + 4NaF + 4H O4 4 4 2 2NaF + SiF Na SiF4 2 6 2HF + SiF H SiF4 2 6 REACTION WITH CALCITE CaCO + 2HF CaF + H O + CO3 2 2 2 Reaction of HF-HCl mixtures with silicates and quartz has been the subject of studies by Blumberg, Blumberg and Stavinou, Gatewood et al. and Smith and Hendrickson. These studies indicate that the reaction is first order in HF concentration and that reaction rate with silicates is at least 10 times faster than reaction with silica. To dare, a reliable method for relating reaction data taken on finely ground silica, dispersed clays, or glass slides to the acidization process in a sandstone formation has not been developed Figs. 1 and 2 are photomicrographs of a Berea sandstone core illustrating the system in which acid reaction occurs. In these photomicrographs silica grains are black, and a few feldspar grains are apparent because of their internal porosity, which gives a streaked appearance. Unfortunately, clay or calcite cannot be differentiated from the pore space since all appear as an area shading from pore space since all appear as an area shading from gray to white. It is apparent that the heterogeneous nature of the porous material greatly complicates the reaction problem. For this reason, a theory including mass problem. For this reason, a theory including mass transport, surface kinetics, and a statistical representation for the porous material is required to describe acid reaction. SPEJ P. 306
- North America > United States > West Virginia (0.26)
- North America > United States > Pennsylvania (0.26)
- North America > United States > Ohio (0.26)
- North America > United States > Kentucky (0.26)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Mineral > Silicate > Tectosilicate (1.00)
- Geology > Mineral > Silicate > Phyllosilicate (1.00)