Impact of Acid Jetting on Carbonate Stimulation

Abstract Many wells are drilled with long horizontal completions to maximize their contact with reservoirs. In carbonate reservoirs, such long completions present challenges to the even distribution of acid during matrix stimulation. One approach to addressing these challenges involves bullheading acid through a static limited entry liner engineered to choke back acid flow to the higher permeability and/or lower pressure zones through the sparse placement of small nozzles. While the pressure drop across the nozzles is straightforward to calculate, the interactions between the acid jets created by the nozzles and the carbonate formation are not well understood. Specifically, it is unknown how a high velocity acid jet impinging on a carbonate surface might alter the wormholing efficiency. To explore the effects of jetting on carbonate dissolution, laboratory experiments jetted hydrochloric acid onto limestone cores at velocities between 100 and 200 feet per second. Consistently, the acid jet formed a substantial bulb-shaped cavity in the face of the rock. Wormholes formed into the rock from this cavity, even at overall flux rates normally too low to generate wormholes. In the absence of acid flux through the core, bulb-shaped cavities formed without wormholes. Low velocity acid jetting did not form cavities. No cavities or wormholes were observed when jetting with only water. A turbulent jet impinging on a rock will create a localized surface pressure spike near the center of impingement. This pressure spike increases the local flux of acid into the porous rock, and is suspected of initiating the cavity. Once the cavity forms, two mechanisms of carbonate dissolution are active. The first is traditional matrix dissolution, which appears to be enhanced by the increased pressure inside the cavity produced by the kinetic energy of the jet. The second mechanism is surface dissolution, which is accelerated inside the cavity due to turbulent flow along the walls, and results in the enlargement of the cavity throughout the jetting process. Enhancement of the matrix stimulation by the jet is often a benefit. On the other hand, enlargement of the cavity is a disadvantage because it consumes acid that would be better used for wormhole growth into the rock. Research using both experiments and computational fluid dynamics is ongoing to better quantify the influence of the jet on the two dissolution mechanisms. With such knowledge, it should be possible to better design lower completion liners that exploit the jetting effect for both acid distribution and enhanced stimulation.