Formation Damage Due To Iron Precipitation In Carbonate Rocks

Abstract

Iron precipitation during matrix acidizing treatments is a well-known problem. However, extensive literature review highlighted that no systematic study was conducted to determine where this iron precipitates, the factors that affect this precipitation and the magnitude of the resulting damage. In this paper, the effect of iron precipitation in the acidizing operations is studied. HCl solutions (5 – 10 wt%) containing 5,000 to 10,000 ppm of Fe3+ were used for these experiments. The effect of varying acid concentration, initial core permeability, core length, temperature, and flow rate was studied. Coreflood experiments were conducted on 6 and 20 in. long Indiana limestone cores over a wide range of permeabilities and up to 300°F. In these experiments, 0.5 PV of acid solution was injected. The cores were scanned after treatments using a CT scanner and cut to better determine the location of iron deposition. The core effluent samples were analyzed for iron and calcium concentrations using ICP-OES. Results showed a significant amount of iron precipitated on the injection face of the cores and the sides of wormholes, i.e. where the contact occurs between the acid and the rock, producing a minimal or no gain in the final permeability, which indicated severe formation damage. The damage increased with the increase of the amount of iron in solution. At higher temperatures and flow rates, the damage was significant. Core length didn't affect the degree of damage. This paper will discuss the results obtained and give recommendations on whether to use iron control agents in the field or not.

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