Acid Etching Patterns in Naturally-Fractured Formations

Abstract When a naturally fractured carbonate formation is treated with acid at pressures below the fracturing pressure, the acidizing process will likely be different from either a true matrix treatment or an acid fracturing treatment. In a matrix treatment of a relatively homogeneous, un-fractured carbonate, the acid initially penetrates into the pore structure and usually creates wormholes. In an acid fracturing treatment, the acid flows through the relatively wide fracture, resulting in more uniform etching of the fracture walls. In naturally fractured formations, acid could either mainly generate wormholes, as in matrix acidizing of carbonates without natural fractures, or etch the fracture surfaces, as in acid fracturing. Determining the etching pattern is essential for modeling acidizing in carbonates with natural fractures. We studied the influence of natural fractures on the acid etching pattern. We used two closely contacted rock samples to simulate natural fractures. The gap between the samples was in the range of average natural fracture widths indicated in the literature. The samples were then acidized with 13.5% HCl. Numerous experiments were conducted to determine the acid etching pattern. Our experimental results show that the acid etching pattern of naturally fractured formations depends strongly on the width of the natural fracture. For typical natural fracture widths, the pattern tends to be wormholing, rather than etching of the fracture surfaces. In most experiments, one broad channel was created at the area near the inlet of the samples, and as acid flows down the fracture, the channel narrows quickly and finally becomes a deep narrow wormhole. The effect of natural fractures on acid etching pattern vanishes as acid moves away from the inlet. As the width of the natural fracture increases, the channel created near the inlet becomes broader and longer, and as the width of natural fracture decreases, the channel becomes narrower and shorter.