A new in-situ generated acid system for carbonate dissolution in sandstone and carbonate reservoirs

Matrix acidizing has long been used for permeability and productivity enhancement purposes in oil and gas wells. In sandstone reservoirs, a preflush is needed to dissolve carbonate minerals so that HF is not consumed in the main step on carbonate minerals. The preflush step also prevents the potential precipitation of the reaction products of HF. HCl has been an important component in preflush; however, it has high corrosion and reaction rates, especially in high pressure/high-temperature (HP/HT) wells. An acidizing system that is comparable to HCl in terms of availability and price is needed; however, the shortcomings associated with HCl, such as its fast reaction rate and high corrosion rate, must be addressed in the development of similar systems. Therefore, this study investigates the effectiveness of a new in-situ generated HCl acid that offers a slower reaction and corrosion rate in high-temperature reservoirs. The new in-situ generated HCl acid treatment was used to acidize two types of sandstone cores (Grey Berea and Bandera) and two type of carbonate cores (limestone and dolomite). X-Ray Diffraction (XRD) analysis was performed on the sandstone cores to determine the mineralogy to design a better treatment. Coreflood experiments were conducted at 250 and 300°F to study the impact of the treatment on the permeability of sandstone cores. Acid injection rates were set at 1 and 5 cm3/min in the case of the sandstone treatments and 1 and 2 cm3/min for the carbonate stimulation. The effluent samples were collected every quarter pore volume to further analyze the ability of acid to dissolve carbonate and oxide minerals. Porosity profiles and wormhole propagation were determined before and after acid treatment using Computed Tomography (CT) scans. Superior stimulation results were achieved at 250 and 300°F with the in-situ generated acid system. However, 15 wt% regular HCl caused 79% damage to the Berea core at 300. The new acid system resulted in positive stimulation results (kf/ki ranging from 1.7 to 2.7) in both Grey Berea and Bandera sandstone cores without any sign of fines migration or sand deconsolidation. In the case of carbonate stimulation, more PV of acid was needed to reach breakthrough if 15 wt% regular HCl was used as a treatment fluid. The wormhole propagation was more efficient in the case of the new acid system. Face dissolution was observed when using 15 wt% regular HCl in carbonate acidizing. The corrosion rate of low-carbon steel (L-80 and C-95) and chrome-based tubular (Cr-13) was significantly less than that noted with 15 wt% regular HCl at 300. This study will assist in developing a more cost-effective and efficient design of acid treatments with minimal impact to the formation's integrity. This new in-situ generated acid system efficiently removes carbonate and oxide minerals in HP/HT sandstone reservoirs.