Acid Jetting in Carbonate Rocks: An Experimental Study

Abstract

Summary Acid 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 by means of 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. The hypotheses for the design of more-efficient stimulation treatments are currently being investigated. We have conducted an experimental study to investigate the effect of jetting on wormhole efficiency. Each jetting experiment was conducted as a constant-pressure (equivalent to a desired initial flux through the core) linear coreflood test, in which a standoff 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 limestone cores. The combination of mechanical and chemical reaction stimulates limestone rocks better than matrix acidizing without the jetting nozzle. When the jetting velocity increased, the dissolution pattern changed. An isolated local 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. A 15% hydrochloric acid (HCl) (by weight) at ambient temperature was used, and the core dimensions were 4 in. in diameter and 16 in. 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. The experimental results from the two different rock samples are compared. Overall, the experimental results indicate that acid jetting follows the same trend as matrix acidizing, regarding wormhole propagation after 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 the cores from Indiana limestone and Winterset limestone. The observations from this work highlight the importance of understanding the dynamic physical and chemical process of jetting in the design of successful acid-stimulation jobs.