Efficient Bull-Head Stimulation of High-Temperature, Ultra-Sour Gas Reservoir with 10,000 ft Open-Hole Drains

Abstract Reservoir A onshore Abu Dhabi is a carbonate reservoir with permeability in the range 1-5mD and a reservoir temperature close to 300 °F. With an average H2S content above 20 mol%, the reservoir fluid is categorized as an ultra-sour wet gas. A proven cost-effective development of this resource calls for long horizontal wells completed barefoot, as the metallurgy of a lower completion liner would prove prohibitively expensive. Ensuring the best possible well deliverability requires effective stimulation, which is a challenge in long, open-hole drains. In this paper we describe how high-rate bull-heading of a novel acid recipe improved well productivity for the same acid volume used with legacy emulsified systems. Due to the ultra-sour nature of the fluids, the acid had to be bullheaded from surface at a high rate while staying below the fracturing pressure. To delay the acid-rock reaction, a single-phase polymer-free delayed acid system (PFDA) was selected, which included a very efficient friction reducer. The injectivity improvement was tracked during the stimulation job by converting rate and surface pressure into an injectivity improvement (relative to the pre-stim conditions) as a function of acid coverage (defined as volume of acid pumped divided by the drain length). The actual friction reduction efficiency was calibrated to downhole gauge measurements and compared against flow loop measurements provided by the chemical supplier. The acid was delivered at a rate of 100 bpm, the highest rate ever pumped in the country. Because of the open-hole drain, the wellbore surface area is very high, resulting in an average acid velocity of only 0.8 m/s, which would be too low for wormhole propagation using conventional HCl. The new acid system achieved a substantial transient injectivity improvement with an acid dosage of only 0.3 bbl/ft. Friction reduction during acid stimulation was 70% and approached the theoretical limit of 80% during pre- and post-flush with water. Gas was initially present in the production tubing and slight gas ingress was observed during post-stimulation shut-in. The acid system has so far delivered superior production performance in six wells with drain lengths of 10,000 ft or more.