Abstract

The objectives of this study were; (1) selection of size distribution of bridging particles of polymer-based drill-in fluid based on the actual reservoir porosity distribution in order to minimize the internal filtration damage, and (2) to develop a non-corrosive wellbore cleanup fluid comprising of a specially designed bio-enzyme and in-situ organic acid precursors to remove the external filter cakes and bring the skin close to zero prior to well completion. Drill-in-fluids were formulated and developed based on Ideal Packing Theory (IPT). The calcite particle size simulation was conducted using a particle size simulator based on IPT using FMI log information from a carbonate reservoir. Xanthan-starch specific bio-enzymes were screened and optimized to achieve complete dissolution of the polymers present in the mud-cakes. Acid precursors were formulated and developed to dissolve the calcite particles and at the same time keeping the corrosion level below the maximum acceptable limit. Compatibility tests between the acid precursors and the bio-enzymes were performed. The final cleanup formulation was subjected to corrosion studies at dynamic conditions. The entire development was verified at reservoir conditions through core flooding experiments.

The formulated fluid was tested successfully against ceramic disks and carbonate core plugs by performing core-flood tests. The polymer based drill-in-fluid resulted in a very low permeability filter cake (<0.1 md), preventing internal filtrate damage significantly. Deposition of a very low permeability filter cake was achieved on carbonate core plugs which results in reduced damaging invasion. Several ester hydrolysis reaction kinetics were studied and finally one combination was chosen as the suitable acid precursor, because of their ability to generate required concentration of acid within three hours of hydrolysis at downhole condition. The best acid precursor satisfied the requirements by generating 3% of acid needed after 3 hours and maintained low acid concentrations for 12 hrs. The corrosion rates were found to be significantly below the industry limits and use of acid corrosion inhibitor is not necessary. Return-permeability of mud damaged carbonate core plugs was as high as 96% after exposing to clean-up solution.

A new drill-in fluid has been developed and its filter-cake deposition was completely cleaned up with enzyme-acid precursor combination. Normally wellbore cleanup fluids are highly acidic and need corrosion inhibitor. The new formulation was effective in minimizing corrosion of downhole tools without the use of corrosion inhibitors.

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