An Experimental Investigation of Drilling-Fluid Filter-Cake Removal by Acid Jetting

Abstract

Abstract 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 ?p, 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. Introduction High pressure jetting is used in many applications, mainly for cleaning surfaces and cutting materials. A liquid is forced through a small orifice by high pressure, causing a high velocity jet to impinge on the surface to be cleaned or cut. Acid or brine solutions are used as the jetting fluid in oil and gas well completions to erode and dissolve the filter cake created by the leakoff of drilling and completion fluids. A schematic of an acid jetting operation for filter cake removal in an openhole well in a carbonate formation is shown in Figure 1. Acid is injected from the jet nozzles as the pipe is moved along the wellbore, the filter cake is degraded over the treated section by the mechanical action of the jets, and injectivity into the formation at this location is restored. If the pressure in the wellbore is higher than the reservoir pressure (overbalanced pressure), acid will flow into the formation, creating wormholes which further enhance the near-well permeability. If the jetting tool is conveyed on coiled tubing, it can be continuously moved along the well by withdrawing the coiled tubing. When drill pipe is used, acid is jetted while a few joints of pipe are withdrawn, then acid injection is stopped while pipe sections are disconnected and laid down on the surface. This process is repeated numerous times to treat the entire productive interval. The process of filter cake removal by acid or water jetting jetting has been the topic of several recent studies, including those of Aslam and Al-Salat (1998), Aslam and Al-Salat (2000), Dahroug et al. (2001), and Johnson et al. (1996). These studies concluded that the cake removal effectiveness depends strongly on standoff distance (the distance from the nozzle to the filter cake surface), jet velocity, duration of jetting, and rotation of the jets. Based on these studies, we set the standoff distance in our experiments to about 10 times the nozzle diameter. We varied the velocity and duration of jetting to study these effects. Our apparatus did not allow for movement of the jet nozzle.