Effect of Transport Variables on Acid Jetting in Carbonates

Acid jetting is a well stimulation method that occurs in limited entry liner completions. Fluid exiting on orifice experiences a significant pressure drop as a high velocity stream impinges against the wellbore wall. Two types of dissolution patterns occur in the process: a cavity formed by surface dissolution and wormholes formed beyond the cavity as a result of a pressure drop between the wellbore and the reservoir. Jetting has been previously used to place acid in horizontal wells, as well as for wellbore cleanup, filter cake removal and stimulation enhancement. These dissolution mechanisms depend on transport variables such as acid injection rate, velocity of the jet, temperature, and permeability of the formation. Jetting changes these variables and as a result the variation in stimulation result between the matrix dissolution mechanism of acid jetting and that of conventional matrix acidizing is most significant at lower interstitial velocities. This study involves the experimental investigation of the comparative wormhole growth of acid jetting versus matrix acidizing, as well as a study of the effect of the abovementioned variables including increased temperature, rock permeability and acid concentration on the efficiency with respect to acid injection rate. The experiments were conducted with a controlled case of 15% HCl and 2-4 mD Indiana limestone and subsequently compared with experiments conducted at 180°F and 28% HCl. The influence of jetting velocity on the jetting outcome was examined by altering the output of the acid injection pump. The results of the experiments were then examined alongside non-jetting cases, which was run in core floods that eliminated the jetting effect of the stream by way of mechanical dispersion. Acid jetting creates a pressure spike point at the face of the wellbore, leading to a heightened interstitial velocity at the contact of the acid and the rock. This results in wormhole propagation occurring at a faster rate than it would in conventional matrix acidizing at an equivalent rate of injection. This phenomena is particularly significant as jetting velocity is increased beyond that of matrix acidizing, and becomes progressively less dramatic at higher jetting velocities. The findings from this study helps to develop the understanding of the mechanism of acid jetting and optimize jetting design