Global Application of Coiled-Tubing Acid Tunneling Yields Effective Carbonate Stimulation

Abstract

Abstract Since 2005, a stimulation technique termed "coiled tubing acid tunneling" has been successfully applied on limestone and dolomite reservoirs in several regions around the world (South East Asia, USA, South America, Europe and Middle East). The technique is applicable on carbonate wells with barefoot (open hole) completions, with either relatively thick pay zones, or with thinner pay sections, for example with close water contacts. It consists of creating multiple drainage holes (tunnels) out into the reservoir pay zone. The tunnels are created by pumping hydrochloric acid (HCl) through a specially designed jetting bottom assembly (which includes a special kick off tool and procedure) and uses conventional coiled tubing and acid pumping equipment. Typical acid tunneling is performed on complete losses, resulting in a negative skin completion. The purpose of these drainage holes (with its own network of worm holes) is to improve the connectivity between the reservoir and the wellbore and positively impact well production. The process can create as many drainage holes as required at different depths (production simulators are used to estimate optimum number of tunnels). The depth selections are based on key well information such as available logs (image, resistivity, caliper, drilling), past reservoir knowledge of the zone and observed lost circulation zones while drilling. Results of this new stimulation technique on carbonate wells have been excellent as compared against conventional acid work (matrix acidizing, conventional acid jetting with coiled tubing and acid fracing). Numerous case histories from each region of the world are included in this paper and will contain comparisons of acid tunneling to conventional stimulation methods, stimulation design criteria, operational procedures, and production results. In one specific Indonesia field the pretotal average production on 12 wells was 955 bopd while the post acid tunneling result was 1,670 bopd, sustained production (the early 5 wells now have between 850 to 950 days post tunneling production history and all wells have at least 300 days post tunneling history).