Influence of Transport Conditions on Optimal Injection Rate for Acid Jetting in Carbonate Reservoirs

Abstract

Summary Acid jetting occurs as a result of pumping acid through limited-entry liner completions, causing high-velocity streams to impinge against the wellbore wall. The dissolution effect of jetting differs significantly from conventional matrix acidizing. Acid jetting causes cavities to be formed at the points of contact of the jet with the rock, with wormholes forming beyond the cavity. Jetting has been shown to be an effective technique for placing acid along extended-reach laterals, removing filter cake, and enhancing wormhole propagation. The velocity of the impinging jet and its standoff distance from the rock cause some of the acid to penetrate the formation and some to flow back in the annular space of the liner. Two types of dissolution mechanisms occur: surface dissolution forming the cavity and matrix dissolution forming the wormholes. These dissolution mechanisms are highly dependent on the acid-injection rate, velocity of the jet, temperature, and permeability of the formation. The differences between the matrix dissolution mechanism of acid jetting and that of conventional matrix acidizing are most obvious at low acid-injection rates. Experiments were performed with the intention of quantifying the difference in pore volume (PV) to breakthrough between acid jetting and matrix acidizing, as well as determining the effect of increased temperature, rock permeability, and acid concentration on this value with respect to the acid-injection rate. The baseline parameters of room temperature, 15% hydrochloric (HCl) acid, and 2- to 4-md Indiana limestone were individually compared with experiments run at 180°F, 28% HCl, and Indiana limestone cores of 30, 60, and 140 md. The effect of jetting velocity was also investigated. A direct comparison with conventional matrix acidizing was made by eliminating the jetting effect of the stream through mechanical dispersion. Acid jetting creates a point of heightened interstitial velocity at the contact of the acid and the rock, causing wormhole propagation to occur at a faster rate than it would in conventional matrix acidizing at that injection rate. This effect is especially pronounced as the jetting velocity is increased above that of matrix acidizing, and it tapers off at progressively higher jetting velocities.