New Insights on Coiled Tubing Acid Tunneling Stimulation in Carbonate Reservoirs

Abstract

Abstract A coiled tubing (CT) acid tunneling stimulation technique has been successfully applied in the last 15 years on limestone and dolomite reservoirs around the world (Middle East, South East Asia, North America, South America, and Europe). Several case histories were presented in the past showing that this technique brings significant benefits over other carbonate stimulation methods in open-hole wells. In this paper, the parameters influencing the predicted and achieved tunnel lengths are discussed. The acid tunneling technique consists of pumping hydrochloric (HCl) acid through conventional CT and a bottom-hole assembly (BHA) with jetting nozzles to create (without drilling) stable drainage holes (tunnels) out into the reservoir pay zone. The BHA also includes a special kick off tool with two pressure-activated bending joints that controls the tunnel initiation direction. The acid that is not consumed during the main tunneling process leaks into the reservoir rock creating wormholes that improve the connectivity between the reservoir and the wellbore and positively impact well production. This acid tunneling technique can create as many tunnels as required at different depths. The optimization of the tunnel initiation depth selection is made by production software simulation using such critical information as the well parameters (trajectory, size, etc.) available logs (image, resistivity, caliper, drilling), past reservoir information, etc. The CT acid tunneling technique on carbonate wells has yielded excellent results when compared to conventional acid jetting with CT, matrix acidizing and acid fracturing. Many case histories from regions around the world were presented before (Stanley et al. 2010). However, many questions are unanswered about what actually happens downhole in terms of tunnel initiation and creation. Most importantly, a detailed discussion regarding the acid tunneling modeling software is included. The parameters influencing the predicted tunnel lengths and the parameters that could be monitored or adjusted in order to protect the CT from any damage and to smoothly create the tunnels are discussed. The paper describes the CT acid tunneling technology and discusses some of the most important questions regarding the downhole CT acid tunneling. The acid tunneling technique performance and benefits confirmed during the recent field operations are presented.