A Radial Scale up of Matrix Acidizing in Carbonate Reservoirs

Abstract

Acidizing is a common technique used in completing wells in carbonate reservoirs to reduce skin factor. Generally, HCl is pumped into the formation where it will react with calcite and dolomite in the carbonate rock. The dissolved volumes are called wormholes because they look like tunnels through the rock. The wormholes must extend past the damage zone to be as effective as possible. At low injection rates, the wormholes are extremely inefficient because they have a large diameter but not a long-propagated distance into the formation. As injection rates increase for the same volume of pumped acid, the diameter decreases, and the wormhole length increases up to an optimal value. The majority of research completed in this area is done on linear core floods due to the simplicity of the experiments. Linear core floods consist of flowing acid in a single direction through a small cylindrical core from one end to the other. The majority of the experiments are done to determine the optimal acid flux for a certain formation or acid type by measuring the amount of acid required to break through the core. More recent studies have shown that linear core floods do not accurately predict optimal values when pumping acid in the field. Radial acidizing floods can bridge the gap to more accurately represent field conditions. This study consisted of radially flowing acid through a block of Indiana limestone that measured 4 feet by 4 feet by 3 feet where the shortest length was the height. A small 2-inch wellbore was drilled through the center of the block with a 1-inch diameter open holecompletion. The acid flux used for this experiment was calculated using a global wormhole model fitted for radial flow. Acid was pumped through the wellbore until breakthrough was seen by the appearance of bubbles exiting the side of the block.