NMR Study to Assess Fines Migration Damage and Its Removal Using Regular Mud Acid in Different Sandstone Cores
Authors: Hanafy A. M. , Nasr-El-Din H. A. 
Published: November 2016

The highly sensitive clay presence in both Berea and Kentucky sandstone during various stimulation operations makes them some critical formations when fines migration is addressed. Berea sandstone at 1% total clay content, although its low percentage it is proven to be determinate with double and quadruple permeability reduction upon fresh water exposure. Regular mud acid is known for its dissolving power of the alumino silicate minerals, yet the secondary and tertiary reaction still cast a damaging potential on the sandstone formation especially in presence of fines migration. This work aims to understand how those fines migration interacts with conventional HCl pre-flush and regular mud acid treatment in the Berea sandstone. Also, the impact of the hydrofluoric acid aided with formic acid preflush was examined to assess the clay content on the stimulation outcomes.

This work based on nuclear magnetic resonance (NMR) analysis; it was designed to understand experimentally the fundamentals of fines migration process and further investigate its behavior during acidizing process. Core flood test was conducted on 6 in. by 1.5 in. cores and fresh water was injected at 3 cm3/min to induce fines migration. In total three core flood tests were conducted after the cores were damaged by fines migration and at temperatures of 150 and 250°F. All cores were subjected to a 12 wt% HCl preflush followed by regular mud acid at a flow rate 3 cm3/min. Each core was scanned using NMR, and computed tomography (CT-scan) to assess the fines migration quantity and distribution mechanism before and after their induction; and once more scanned after acidizing process. All of this work was repeated for cores from Kentucky sandstone to investigate the sensitivity to the clay content.

The acidizing process in presence of fines migration was proven to be temperature sensitive. The NMR analysis showed one order of magnitude reduction in the dominant pore size in the last two inches of the core at 150°F for Berea sandstone. At the higher temperature of 250°F the damage was more clear in the last four inches of the core. The CT-scan analysis followed the NMR analysis showing the same pattern in terms of porosity gain percentage that was higher at 150 compared to 250°F. The mud acid failed to dissolve the fines and the damage persist and resulted in a drop in permeability, total porosity, and tighter pore size distribution. On the other hand, although Kentucky formation exhibited high clay content the extent of damage was minor after acidizing in presence of fines migration.

In the end this work will add a further insight on the effect of clay content and nature on the acidizing process in both Berea and Kentucky sandstone. The NMR study showed how fines migration moves in the damaged cores. Furthermore, it will localize and quantify their damage and impact on stimulated formations. Finally, the work will add clearer understanding to how fines migration reacts with the preflush and regular mud acid.