Successful Acid Treatment of a Production Well in the Tauhara Field, New Zealand

Abstract

A two-phase production well at Tauhara being used for industrial heat supply has been subject to calcite scaling, requiring regular workovers to remove the scale and maintain production flows. These workovers have been performed on an annual basis to fit in with the process plant maintenance programme. The original productivity was more than 100 t/h per bar, but in 2010 the well was unable to sustain flow at the normal production wellhead pressure and subsequent downhole surveys showed the productivity had declined to 20 t/h per bar.

After reviewing well performance, the cause of the productivity decline was assessed to be calcite scale in the feedzone fractures near the wellbore. It was decided the best option to restore the productivity was to acidize the well following the “normal” annual mechanical scale removal.

The well has multiple feedzones and even when “quenched” with cold water there is a strong interzonal flow with wellbore temperatures around 100°C. The challenge of the acidizing process was to achieve a successful acid treatment of the fractures near the wellbore, without attacking and damaging the perforated liner and other downhole equipment.

A treatment program using 15 % HCl followed by a soda ash mix to neutralise any remaining acid, together with a corrosion inhibitor to protect the liner, was designed. This was successful in restoring the well productivity to the original value without causing any damage to the well casing or perforated liner.