A New Model of the Physical and Chemical Changes in Sandstone During Acidizing
Authors: McCune C.C. , Fogler H.S. , Cunningham J.R. 
Published: December 2007

McCune, C.C., Member SPE-AIME, Chevron Oil Field Research Co., La Habra, Calif., Fogler, H.S., Lund, K.,* and Cunningham, J.R., U. of Michigan, Ann Arbor, Mich., Ault, J.W., Member SPE-AIME, Chevron Oil Field Research Co., La Habra, Calif.

Abstract
This paper demonstrates a practical method for applying laboratory results with cylindrical cores to the design of acid jobs. As mud acid is injected into the matrix of a sandstone. an acid-mineral reaction zone is developed that moves at a velocity much less than that of the acid itself. As a result, the rock volume stimulated is much smaller than the volume filled by the acid.Laboratory stimulation tests with undamaged sandstone cores showed that, as mud acid was injected at a constant rate, the permeability rose rapidly after a time delay that was correlated with injection rate, HF concentration, and core length.The physical and chemical changes occurring during the acid injection were modeled mathematically to describe the movement of the acid and reaction fronts and to determine the rock properties and test conditions that control the fronts. The model was developed for both the linear test cores and the radial geometry around the wellbore. The model can be readily applied to field acid jobs in a specific sandstone by conducting a limited number of linear core tests in the laboratory. The tests and their use in field designs are described.

Introduction
The productivity of oil and gas sandstone reservoirs may be increased by injecting mud acid, hydrochloric/hydrofluoric acid mixtures (HCl/HF), into the matrix in the vicinity of the wellbore to dissolve portions of the rock minerals, thereby increasing the formation permeability and well productivity. productivity. This paper describes, in sequence, how the acid reaction front was identified through laboratory tests and, in turn, how a mathematical model describing the front in terms of rock properties and test conditions was developed. The extension of the model to the radial geometry around the wellbore was then carried out. Finally, a method is described whereby data obtained from laboratory tests with cores from a sandstone of interest are used to determine the necessary parameters for utilizing the radial model for designing acid jobs in that sandstone.

EXPERIMENTAL
Phacoides sandstone samples from an outcrop in Chico Martinez Creek, Kern County, Calif., were used in the laboratory studies. The mineral composition by X-ray diffraction was 78.4 to 85.2 percent quartz, 4.8 to 6.6 percent sodium feldspar, percent quartz, 4.8 to 6.6 percent sodium feldspar, 9.6 to 12.-5 percent potassium feldspar, 0 to 0.8 percent kaolinite, and 0.5 to 1.7 percent illite. The percent kaolinite, and 0.5 to 1.7 percent illite. The permeability ranged from 0.5 to 1.0 md and the permeability ranged from 0.5 to 1.0 md and the porosity was 10 to 12 percent. Acidizing tests by porosity was 10 to 12 percent. Acidizing tests by the method previously described were made with 1-in.-diameter, 1/2- to 4-in.-long core samples. The cores, initially saturated with distilled water, were acidized at a constant flow rate with mixtures of 1.4 to 10.5 percent HCl and 0.8 to 5 percent HF. Flow rates ranged from 1 to 13 cc/min (0.05 to 0.6 gal/min ft 2) and the test temperature was 120 degrees to 130 degrees F. Details of the tests were reported earlier and only part of the results are reviewed here.The results of two acidizing experiments are illustrated in Figs. 1 and 2. During the first part of the acidization, the average permeability of the core remains essentially constant, or it may even decrease somewhat. The loss in permeability has been noted by several investigators and is attributed to (1) interaction of the acid with clays and other fines, causing them to move and resulting in partial plugging, and (2) the reprecipitation of acid-mineral plugging, and (2) the reprecipitation of acid-mineral reaction products. The time required for the permeability to rise rapidly is referred to as the permeability to rise rapidly is referred to as the breakthrough time, tb. This corresponds to the breakthrough of a zone of rapidly increasing permeability and not of the injected fluid. permeability and not of the injected fluid.
SPEJ
p. 361