Acid jetting from ports near the end of a tubing string is a common method being applied to remove drilling or completion fluid filter cakes and to stimulate near-wellbore permeability. To be effective, the acid jet must remove enough of the filter cake to eliminate any significant filter cake Δp, and should stimulate some distance into the formation if internal damage has occurred. In this study, we experimentally evaluated the efficiency of acid jetting at conditions similar to those being applied in the field. The results obtained with acid jetting were compared with those obtained jetting with water alone to determine the relative importance of the mechanical removal of the filter cake by the liquid jet and the dissolution of filter cake and formation solids by the acid.
We first created a drilling fluid filter cake and internal damage by circulating a typical drill-in fluid containing sized CaCO3 particles past the face of a 4-inch diameter by 18-inch long core for up to 18 hours. A pressure differential of 500 psi was held across the core to drive filtrate and possibly solids invasion into the rock. Acid (15% HCl) was then jetted from a small port on to the center of the core face at velocities typical of field applications. The velocity and duration of jetting was varied systematically in the experiments. In each experiment, we measured the evolving permeability of the filter cake/core system by measuring the pressure drop across the system.
In this paper, we show how the key parameters of jet velocity and duration of jetting affect jetting efficiency. Comparison of the acid jetting results with those obtained jetting with water alone show how much of the cleanup is accomplished by the mechanical action of the liquid jet compared with the chemical dissolution of the solids by the acid. In many cases, the jetting action provides the primary restoration of productivity with less effect caused by acid dissolution.