Overcoming Challenges and Increasing Efficiency for Coiled Tubing Wellbore Cleanout and Perforation in a Subhydrostatic Gas Injector Well with Real-Time Downhole Measurement
Authors: Keong Azwan , Hansen Anders , Hansen Bjarne , Corak Djurdjica , Hamdani Merouane 
Published: November 2020

Abstract
Coiled tubing (CT) intervention in a subhydrostatic gas injector well carries several challenges. For example, excessive fluid leakoff during a CT sand cleanout operation requires nitrifying the cleanout fluids to achieve a stable solid return rate, which necessitates a huge volume of liquid nitrogen. For perforation with CT, methods of activating the firing head are limited because the fluid column in the well cannot be sustained if a hydraulically activated firing head is used.
Alternatively, cleanout operations for such wells can be done by pushing all the solids into the formation instead of taking returns through surface solids-handling equipment. For this case describing such an operation, real-time downhole measurements were used to accurately determine the top of fill, monitor for signs of formation plugging during the cleanout, and detect sudden increases in downhole overpull indicating solids starting to plug around the bottomhole assembly (BHA). The fiber optic telemetry within the real-time CT system also enabled use of an electrical firing head powered by downhole batteries, which allows CT perforation without the need to pump fluids.
The method of cleanout by injecting into the formation through a sacrificial open perforation can be considered an unconventional CT intervention. Solvent was used with a high-pressure jetting tool to break down asphaltene residues across the open perforation before injecting into the formation at high flow rate. Real-time downhole weight measurements on the BHA provided valuable information to continuously locate the top of fill because the depth changes as the solids are pushed into the formation. This cleanout method reduced the equipment footprint on the offshore installation since no surface solids-handling equipment was needed. Early detection of formation plugging through real-time monitoring of downhole pressure eliminates the need to take solid returns to the surface production separator because the injection flow rate was adjusted in real time before gaining back the desired leakoff as pressure built in the wellbore to create a breakthrough. The ability to add new perforations with the same CT setup and using a fiber-optic–enabled electrical firing head avoiding the need to replace CT with wireline equipment, increasing the overall efficiency and saving more than 48 hours of operational time. Downhole measurements were also used to correlate depth and indicate perforation gun firing.
The newly perforated zone contributed 13% of the injected gas into the targeted formation and no further intervention was required after the CT cleanout and perforation. The underbalanced perforation for the new zone was critical to minimize near-wellbore damage for this gas injector well. Two surrounding producers that were not producing before have benefited from this CT intervention, producing hydrocarbon at 500 Sm3/d and 350 Sm3/d post-intervention.