Investigating the Effects of Core Length on Pore Volume to Breakthrough (PVBT) Behavior in Carbonate Core Samples during Matrix Acidizing with Hydrochloric Acid

Abstract

Most literature contains Hydrochloric acid (HCl) carbonate acidizing experiments performed on short (2 - 6 inch) cores. These cores do not accurately represent reservoir conditions, as spent acid is not propagated for any appreciable distance along the length of the sample. In this work, HCl injection experiments are performed on both short (6 inch) and long (20 inch) calcite cores to investigate the pore volume to breakthrough (PVBT) behavior.

PVBT is defined as the volume of acid necessary to propagate the wormhole network from the inlet to the outlet of the core sample, divided by the pore volume of the core. HCl (5 and 15 percent by weight) injection core flood experiments were performed on 6 inch and 20 inch calcite (Indiana Limestone) cores. The cores were CAT scanned before and after acid injection to observe wormhole propagation. Core outlet effluent samples were collected and their calcium concentration was measured using Inductively- Coupled Plasma.

Results from core flood experiments show an increased PVBT for 20 inch cores compared to the 6 inch samples. Results from CAT scan experiments show enlarged worm-holing and face dissolution on the 20 inch cores compared to the 6 inch cores, due to increased acid spending at the same acid concentration, flow rate, and injection temperature. Results from experiments performed at various flowrates indicate the existence of an optimum injection rate for 20 inch cores, just as in 6 inch cores. This study summarizes and explains the results obtained from the aforementioned experiments.