The Impact of Rock Properties on Acid Jetting in Carbonate Rocks: An Experimental Study
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AbstractAcid jetting, as a well stimulation method for carbonate reservoirs, has shown optimistic results in the production enhancement of some extended reach horizontal wells. It was used initially to promote damage removal along a wellbore via multiple strategically-located injection nozzles. It has the potential to place the injecting fluid at the locations that need stimulation. Jetting may also enhance wormhole propagation compared with conventional matrix acidizing. These hypotheses are currently being investigated to improve the design for more efficient stimulation treatments.We have conducted an experimental study to investigate the effect of jetting on wormhole efficiency. The jetting experiments are conducted at a constant pressure with linear core-flood tests, where a stand-off distance is maintained between the injection nozzle tip and the core. At low-velocity acid injection, jetting effectively removes mud filter-cake by mechanical actions. Jetting also creates wormholes in carbonate cores. The combination of mechanical and chemical reactions stimulates carbonate rocks better than matrix acidizing without the jetting nozzle. When the jetting velocity increases, the dissolution pattern changes. An isolated locally compact dissolution results in a cavity at the entries of core samples by jetting, followed by a wormhole structure. With the known dissolution pattern, sensitivity studies are carried out to investigate the effect of various parameters on the experimental outcome.We used Indiana and Winterset limestone rocks in the experiments. 15% HCl (by weight) at ambient temperature was used and the core dimensions were 4 inches in diameter and 16 inches in length. Various combinations of acid jetting velocities and acid fluxes were considered. The Winterset limestone cores are more heterogeneous, with higher porosity and lower permeability than the Indiana limestone cores. From the experimental results, the results of acid jetting from the two different rock samples are compared.The experimental results indicate that acid jetting follows the same trend as matrix acidizing regarding wormhole propagation once cavities are created. Jetting velocity and acid flux are the critical parameters in jetting design for optimal stimulation results. Acid jetting tends to create different dissolution patterns for Indiana limestone and Winterset limestone cores. The observations from this work highlight the importance of understanding the dynamic chemical process of jetting in the design of successful acid stimulation jobs.
