Many Khuff gas wells have been experiencing inorganic scale deposits: iron sulfide, iron oxides and calcium carbonate, which are causing weak production and poor wellbore accessibility. The scale problem requires a prompt solution to meet Saudi Aramco gas production objectives.
The history of scale removal trials in Khuff gas wells in Saudi Arabia included the mechanical removal with Coiled Tubing (CT), and the chemical dissolution with acid and/or other scale dissolvers pumped through CT or bullheaded. All these methods failed to provide the expected outcome and some jobs resulted in major operational failures like stuck pipe, pinholes, H2S release at surface, and formation damage. As a result of these failures the descaling jobs were put on hold by Saudi Aramco many times between 2007 and 2010 with no significant progress made until the 2nd half of 2010.
A project was launched to establish an innovative descaling procedure aimed at a safe and effective scale removal. The integrated solution starts with the temporary isolation of the reservoir to eliminate gas and H2S returns at surface, enable a "closed system" cleanout with safe solid handling at low choke pressure, and minimum surface testing equipment. The second step is based on mechanical removal that eliminates long soaking time and/or H2S generation during chemical dissolution. For that, a high pressure jetting tool designed and proved to remove hard scale is deployed on CT. The critical step of scale cleanout to surface is optimized with the use of large outside diameter CT string and special high temperature gel to ensure efficient lift and eliminate the risk of stuck pipes. The major enhancement to this project is the ability to monitor in real-time bottom-hole pressure and temperature, annular solid concentration, and the high pressure jetting tool efficiency with a fiber optic enabled coiled tubing string. The whole process resulted in a controlled and optimized scale removal treatment.
Multiple case histories are presented in this paper to demonstrate the successful implementation of the integrated solution, which is then selected as the only approved method for scale removal in Khuff gas wells.