Regular mud acid mixtures have been extensively used to stimulate sandstone formations; however, the use of this acid in a deep well has some major drawbacks, including high and uncontrolled reaction rate and corrosion to well tubulars. A single stage sandstone acid system (HF and a phosphonic acid) has been developed as an alternative to mud acid. Carbonate minerals in sandstone formations can cause formation damage. Therefore, there is a need to remove the carbonates to enhance the performance of this sandstone acid system.
In this study, the single stage sandstone acid systems with different HF concentration were examined to stimulate sandstone formation at high temperature. Berea (5 wt% clays) and Bandera (11 wt% clays) sandstone cores were used in the coreflood experiments. Coreflood experiments on sandstone cores were conducted at 300 °F at a flow rate of 2 cm3/min. The concentrations of Si, Ca, Al, Mg, and Fe in the core effluent samples were analyzed using Inductively Coupled Plasma (ICP). 0.6M GLDA (pH= 3.8), 0.6M HEDTA (pH =3.8), and 9 wt% formic acid solution were evaluated to remove the carbonate minerals from Bandera sandstone cores.
Coreflood experiments showed the sandstone acid systems enhanced the permeability of Berea sandstone cores, and the optimum injection volume is 2.5 PV to keep the integrity of the cores at 2 cm3/min and 300°F. This acid system damaged Bandera sandstone cores severely at 300°F, which contain 16 wt% dolomite, and 10 wt% illite. Analysis of core effluent samples indicated that there was CaF2 precipitate in the core when the acid systems were pumped without preflush. Therefore, calcium carbonate in Bandera sandstone cores should be removed with preflush. GLDA, HEDTA and formic acid (9 wt%) were compatible with Bandera sandstone cores at low pH value, and formic acid was much more effective in remove carbonate in Bandera sandstone cores.