Location and magnitude of formation damage due to iron precipitation during acidizing carbonate rocks

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Iron precipitation during matrix acidizing treatments continues to thwart production from carbonate reservoirs. However, an extensive literature review highlighted that no systematic study to date has been conducted to determine where this iron precipitates, the factors that affect this precipitation and the magnitude of the resulting damage. Successful iron control can be critical to the outcome of the treatment. Iron (III) precipitation occurs when acids are spent and the pH rises above 1, which can cause severe formation damage. Chelating agents are used during acidizing treatments to minimize iron precipitation, but some of the chelating agents can combine with Ca and Mg.In this work, the effect of iron precipitation during acidizing operations is studied. Hydrochloric acid (HCl) solutions (5–10 wt%) containing 5,000 to 10,000 ppm of Fe3+ served as the main fluid for the experiments described in this paper. Biodegradable GLDA (L-glutamic acid, N, N-diacetic acid) was studied in conjunction with HCl. The effect of varying acid concentration, initial core permeability, core length, temperature, flow rate, and chelant-to-iron mole ratio (CIMR) was studied. Coreflood experiments were conducted on 6 and 20 in. long Indiana limestone cores over a wide range of permeabilities at temperatures up to 300 °F. In these experiments, 0.5 PV of acid solution was injected. The cores were scanned after acid treatments using a computed tomography (CT) scanner and cut to better determine the location of iron deposition. The core effluent samples were analyzed for iron and calcium concentrations using inductively coupled plasma optical emission spectrometry (ICP-OES). A calcium ion-selective electrode measured the concentration of free calcium ions, i.e. calcium ions not complexed by the chelant, in the core effluent samples.Results showed that a significant amount of iron precipitated (up to 89%), especially, on the injection face of the cores and the inner surfaces of wormholes, i.e. where the contact occurs between the acid and the rock. The damage increased as the amount of iron increased in live acid. At higher temperatures and flow rates, the damage was significant. An inverse relation is observed between the wormhole length and extent of recoverable iron in the cases with no breakthrough. The amount of iron recovered depended on both CIMR and the initial permeability of the cores. Calcium is chelated, along with iron, which limits the effectiveness of chelating agents to control iron (III) precipitation. Chelating agents are intended to control iron now that calcium is also chelated; this amount should be accounted for. Acid solutions should be designed considering this important finding for successful treatments.