It is challenging to remove filter cake uniformly and thoroughly from the production interval in horizontal and multi-lateral wells with heterogeneous formation characteristics. The inability to obtain complete filter cake removal is a common occurrence with the currently available reactive fluid systems. Another instance where conventional reactive systems are not adequate is when it’s desirable to spot the treatment fluid prior to running final completion. In some instances, this kind of operation can take several days.

Conventional practice to remove filter cake formed by water-based drill-in fluid usually includes strong mineral acids, buffered organic acids, oxidizers, enzymes, and chelating agents. These systems all have limitations that can result in non-uniform clean-up of the filter cake. Certain esters of organic acid have been investigated and used for filter cake removal to achieve better zonal coverage by delaying acid release. However, esters of formic acid generally hydrolyze too quickly, while acetic and lactic acid precursors hydrolyze more slowly but are not strong enough to effectively remove filter cake.

This paper presents a new delayed-release acid system that can generate an organic acid in situ. The system hydrolyzes more slowly than formates and lactates in the medium-to-high temperature ranges, and the generated organic acid is stronger than formic, lactic and acetic acid. Laboratory studies with a static HT/HP filter press demonstrated this new system can clean up filter cake effectively. Laboratory evaluation also demonstrated that polymer-specific enzymes can be added to the system to enhance clean-up efficiency because of its long delay time. Compared to conventional chemical methods of filter cake removal, this new delayed-release acid system provides a more efficient alternative, especially when the completion activity requires a long system delay time before the clean-up acid will be released across the entire production interval.

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