Stimulation of Multilayered High-Carbonate-Content Sandstone Formations in West Africa Using Chelant-Based Fluids and Mechanical Diversion

摘要

Matrix stimulation of high-temperature sandstones using hydrochloric acid (HC1) is difficult to achieve due to its fast reaction, possible sand deconsolidation, clays destabilization, and tubular corrosion. These problems are common in stimulating wells completed across the Pinda formation in West Africa. This formation is a multilayered formation with a wide range of carbonate content (varying from 2% to nearly 100%) and bottomhole temperatures in the range of 300°F. In addition, most of the wells have up to 1,500 ft of perforated intervals producing together from different layers. Stimulation treatments in the area historically have been performed using 7.5% HC1 pumped through coiled tubing and using foam diversion. In 2008 a different approach was taken to stimulate producing zones across this formation, using a low-pH chelant (pH 4) as the main stimulation fluid and straddle or inflatable packers for mechanical diversion, whenever applicable. Six wells were treated in a stimulation campaign using the chelant solution. Mechanical diversion was used in three of the six wells treated; two were treated with a mechanical straddle packer and one with an inflatable packer. Low bottomhole pressure (BHP) or wellbore configuration precluded the use of mechanical diversion for the other three wells; foam diversion was used instead. The results of these stimulations were encouraging, with the combined production of all six wells almost doubling. The good post-job results confirm the effectiveness of low-pH chelant in stimulating carbonate and carbonate-rich sandstones at high temperature, with the added value of low corrosion rates and reduced risk of sand deconsolidation and clays destabilization. This stimulation campaign also tested current technological limits of mechanical and inflatable packers. The combination of high expansion ratio, low BHP, and high temperature requirements precluded the stimulation of three of the six wells with mechanical diversion. With the increasing need to stimulate depleted high-temperature formations, these challenges must be addressed in the future.