Reaction rates of hydrochloric acid with chalks

The reaction rate of hydrochloric acid with carbonate rock at acid-fracturing conditions depends on controllable variables such as injection rate and acid concentration, and variables inherent to the carbonate rock (permeability, mineral composition, etc.). In a previous study, the effects of some controllable variables on reaction rates were investigated and the results correlated with theory. This paper presents the results of an investigation of the influences of four rock properties (porosity, permeability, hardness and acid solubility) on reaction rates. Samples of chalk were selected from five North Sea reservoirs. Due care was exercised to insure that the selected samples spanned a wide range of each of the four rock properties investigated, in order to generalize the findings so that they will apply to a variety of conditions. Whenever possible, the four properties were determined for each sample prior to acidification. All reaction rate experiments were performed using an annular flow reactor under the same constant experimental conditions of 93 degrees C, 34.5 MPa, and a flow rate of about 87 ml min(-1). The results revealed that reaction rate is positively correlated with both carbonate content of the chalk and core permeability. The reaction rate also appeared to increase as the porosity increased, and to decrease as the hardness increased. The data, however, scattered so widely that the relationship between reaction rate and porosity or Brinell Hardness Number (BHN) did not appear to be statistically significant. The effects of permeability and acid solubility were incorporated into an overall correlation that combines the effects of controllable and uncontrollable variables. (C) 1998 Elsevier Science B.V. All rights reserved.