Abstract

A gel-based, crosslink-diverter system has been the fluid of choice for a wide range of temperature applications for carbonate acidizing treatments in Qatar. Recent field experiences with acidizing low-temperature wells (100 to 140 °F) indicated the need for a chemical diverter system that develops viscosity rapidly enough to divert acid in a continuous pumping stream.

A viscoelastic surfactant- (VES) based acid-diverter system was selected for a trial treatment because of its attributed phenomena of viscosity generation. Viscosity development for this system is a function of divalent cations generated as a result of HCl acid reaction with carbonates. Laboratory tests were performed to evaluate a viscosity profile for viscosity development versus acid spending. It was established that using 15% HCl mixed with the VES system would provide sufficient reaction rates at these low temperatures to generate an adequate amount of the divalent cations for rapid viscosity development. Formation fluids act as breakers for the VES gel. Treatment guidelines were developed based on the sensitivity of VES systems to iron content, corrosion-inhibitor concentration, and incompatibility with emulsified acids.

Field application of the system showed promising results by providing better acid diversion for these low-temperature wells compared to the gel-based crosslink diverter. Downhole-pressure plots confirmed the rapid viscosity development as predicted from the lab tests. Field mixing and pumping guidelines were developed for further application of the system based on the learning from these field applications.

This paper presents various guidelines for successfully using the VES-based diverter system in low-temperature wells for effective acid diversion. Lab-test procedures and results to serve as a guideline for such applications are discussed. Case studies are shown to demonstrate the effectiveness and success of carbonate acidizing treatments using VES systems for diversion in low-temperature wells.

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