High-temperature laboratory testing of illitic sandstone outcrop cores with HCl-alternative fluids

Illitic sandstone reservoirs are sensitive to hydrochloric-acid (HCl) -based fluids. When HCl contacts illite, it breaks down and causes fines migration and formation damage. The migration of fines through the porous media will block the pores, reduce permeability, and decrease the production rate of oil and gas wells. A thorough literature review showed that all clay minerals are essentially unstable in HCl at temperatures greater than 300°F. In turn, there is a need to search for stimulation fluids other than HCl to stimulate deep sandstone reservoirs. Alternative fluids to HCl/hydrofluoric (HF) mud acids were introduced to stimulate and remove the damage from illitic sandstone reservoirs. These fluids are chelating agents such as hydroxyl ethylene diaminetriacetic acid (HEDTA) and glutamic acid-N,N-diacetic acid (GLDA). In this study, sandstone cores with different illite contents were examined. Illite contents of 1,10,14, and 18 wt% of the sandstone cores were used in the core-flood experiments at 300°F. Different combinations of GLDA/HF were tested to determine the optimum ratio of chelate/HF. Computed tomography scans and permeability measurements before and after the treatment were used to assess the effectiveness of each fluid in removing the damage and in the stimulation of the sandstone cores. The results show that 15 wt% HCl caused severe damage to sandstone cores with different illite contents. GLDA and HEDTA solutions showed a good compatibility with the illitic sandstone cores, with up to 18 wt% at 300°F. Permeability measurements showed that GLDA performed better than HEDTA at a pH of 4 and at the same molar concentration. The optimum ratio of GLDA/HF concentration was found to be 20 wt% GLDA/1 wt% HF, which gives the maximum increase in core permeability. No deconsolidation was noted with the two chelates tested. The results obtained from this study will significantly improve the outcome of acid treatments in illitic sandstone reservoirs at high temperatures.