Acid Etching Patterns in Naturally-Fractured Carbonate Reservoirs

by
Chengli Dong, MSE

University of Texas at Austin, 1999
Supervisor: A. Daniel Hill

When a naturally fractured carbonate formation is treated with acid at a pressure lower than the formation fracture pressure, the acid etching patterns will be different from either a true matrix acidizing which results in wormholes, or acid fracturing which results in etching of the fracture surfaces.

We conducted series of experiments to determine the acid etching patterns after acidizing naturally fractured carbonate formations. The natural fracture is represented by two closely contacted core samples. Different fracture widths, fracture surface roughness and leakoff rates are used in the experiments.

The experimental results show that acid etching patterns in naturally fractured formations depend on the fracture width and fracture surface roughness. For the typical natural widths, acid creates channels along the fracture, which are broad and shallow near the inlet and narrow and deep father away from the inlet. When the fracture width decreases, the channel becomes broader and shallower and finally the entire fracture surface will be etched by acid. When the fracture width decreases, the channel becomes narrower and deeper and finally converges to the wormhole. Wormholes are more pronounced in rough-surfaced fractures than in smooth-surfaced fractures. The leakoff acid does not change the etching pattern on the fracture surface but creates wormholes perpendicular to the fracture surface whose diameters depend on the leakoff rate. The fracture width and the fracture surface roughness are the important properties to determine the acid etching patterns in naturally fractured carbonate formations.

Back to theses index