A Novel In-Situ Generated Acid System on Carbonate Mineral Dissolution in Sandstone Reservoirs

Abstract

Matrix acidizing has been used in oil and gas well stimulation for permeability and productivity enhancement purposes. HCl has been repeatedly used as a stand-alone stimulating fluid to decrease skin damage, create highly conductive wormholes in carbonate reservoirs, and stimulate sandstone reservoirs as a part of mud acid treatment. However, HCl in high-pressure/high-temperature (HP/HT) wells is a concern because of its rapid reactivity resulting in face dissolution, corrosion, and associated increased inhibition costs. This study investigates the effectiveness of a novel insitu-generated acid formulation with slower reaction and corrosion rates in stimulation operations in high temperature reservoirs.

The new insitu-generated acid treatment was applied to stimulate two types of sandstone cores (Grey Berea and Bandera). X-Ray Diffraction (XRD) was performed on the sandstone cores to analyze their carbonate and clay content. Coreflood studies were conducted to investigate the impact of the treatment fluid on the permeability of outcrop and reservoir cores. Different flow rates of acid injection were set at 1 and 5 cm^3/min.

The influence of temperature of 250 and 300˚F (121-148˚C) was investigated using the resulting effluent samples. These samples were analyzed using Inductively Coupled Plasma (ICP) for elemental analysis of key cations. Porosity profiles were determined before and after treatment using CT scans.

Experiments were conducted on both the regular HCl acid system and the insitu- generated acid system for comparative purposes. Superior stimulation results were achieved at 121˚C (250) with the insitu-generated acid system. Although plugging problems caused by clay instability in reaction with HCl remains a major concern, positive stimulation results occurred in both Grey Berea and Bandera sandstone cores. This insitu-generated acid system efficiently removes carbonate and oxide minerals in HPHT sandstone reservoirs. No fines migration was observed with Grey Berea. A stimulation Kf/Ki (% increase) ranging from 10 to 30% was achieved with Grey Berea sandstone.

The outcomes of this study will assist in a more cost-effective and efficient design of acid treatments with minimal jeopardy to the formation integrity.