Abstract

In-situ gelled acids based on polymer have been used in the field for several years, and were used in the field in stages between the regular acids. Literature survey reveals that certain plugging effects within highly permeable layers may occur and result in the diversion of the injected acid into less permeable zones. Recent studies indicated that sometimes diversion was not achieved with a severe damage that can lead to loss the whole well. There is no agreement on when this system can be successfully applied in the field.

Single-stage and multi stage-stages acid test of well performance were conducted using Indiana limestone cores (1.5 in. diameter & 20 in. long) at 250°F. 15 wt% regular HCl and 5 wt% in-situ gelled acid based on Fe(III) as a crosslinker were the acid that used in this study. Propagation of the acid, polymer, and cross-linker inside the long cores was examined for the first time in detail. Stage volume and injection rate, that were the parameters that affect on the propagating of various chemical species, were examined in details. Samples of the core effluent were collected and the concentrations of calcium, cross-linker, and acid were measured. Material balance was conducted to determine the amounts of polymer, and cross-liker that retained in the core.

Experimental results show that In-situ gelled acid should be pumped at low shear rates. In-situ gelled acid at low shear rate conditions instantaneously plugged the tip of the wormhole and didn't create additional wormhole inside the core. Therefore, when the final regular acid stage bypassed the gel, it started to propagate from nearly the last point that the first stage ended. This is to endure proper acid diversion. In-site gelled acid stage volume should not exceed 0.5 PV. No benefits were gained by increasing the volume of in-situ gelled acids. Retention of total iron in the core increased in multi stage acid treatments, especially at low injection rates.

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