Summary

Sandstone stimulation remains challenging because of formation heterogeneity and the sensitivity of clay minerals to acids such as hydrochloric acid (HCl) and mud acid [HCL/hydrofluoric acid (HF)]. Fines migration complicates the well-stimulation process and reduces formation productivity. Multiple field studies show that some stimulation methods can result in permanent damage to the rock matrix near the wellbore because of fines migration. This study aims to locate, quantify, and describe the damage resulting from fines migration after the stimulation of sandstone formations, and examine the composition of clay content in the formation and its effects on the stimulation process and subsequent fines migration.

This work evaluates the fines-migration damage during well stimulation in Bandera, Gray Berea, and Kentucky Sandstones. Fines migration was induced by injecting deionized water between brine stages to trigger the mobilization of the clay minerals in sample cores. HCl, formic acid (HCOOH), and HF stimulation stages were then injected after the fines-migration induction. The new formation-damage-evaluation method proposed in this work uses computed-tomography (CT) scanning and nuclear-magnetic-resonance (NMR) measurements before and after the fines-migration induction and experimental stimulation. The CT and NMR data were then combined and processed to generate a 3D representation of the pore structure throughout the core samples, which yields insight on how the clay composition affects the stimulation process and changes the pore system.

The developed technique exhibited an excellent ability to visualize the pore-size distribution and the changes in the pore structure after the fines-migration damage and the acid treatment. The mapping of the pore-size distribution using CT and its comparison with the rock mineralogy of Bandera, Gray Berea, and Kentucky Sandstones successfully predicted the changes in the pore structure of these formations upon induction of fines-migration damage using deionized water. These changes in pore structure prevailed as a controlling variable of the acidizing process. The stimulation of the damaged cores at 150 and 250°F resulted in aluminosilicate deposition toward the core outlet. These deposits are attributed to the acid leaching of aluminum (Al) and iron (Fe) ions from the aluminosilicate structures. The higher temperature resulted in the deposition of aluminosilicates closer to the injection point. However, an enhancement in permeability was noticed in all of the sampled formations, which was because of the propagation of narrow channels between heavily deformed pore structures.

This work adds to the understanding of sandstone-stimulation technology and contributes a new process to assess the effects of acid stimulation on fines-migration damage. The high level of resolution in visualizing the changes in the pore structure facilitates the optimization of treatments to reduce costs while improving production from clay-rich sandstone formations. This technique offers further potential as a formation-evaluation tool for real-time assessment of a variety of formation-damage mechanisms, such as fracturing fluids and water blockage.

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