Publications

Produced water from oil and gas wells is typically re-injected back into isolated formations using dedicated water disposal wells. Water is usually injected below aquifer zones containing water usable for domestic or agricultural purposes, into porous interval between impermeable layers of rocks to avoid polluting fresh water. Because of the sensitive environmental issue to protect the groundwater, most countries have set regulations in regard to disposal well construction. As a requirement to be able to drill several water disposal wells in Sumatra, Indonesia, a major operating company needed to be in compliance to regulations set by the Indonesia Ministry of Environment. One of the requirements is the ability to show hydraulic isolation across injection and confinement zone. In this case, hydraulic isolation was to be evaluated using a sonic cement bond log (CBL) tool. Even though the regulation does not clearly state the method of evaluation, this tool is most widely used as the cement evaluation tool. Because of this, since the planning stage of the project, the importance of having good hydraulic isolation, hence a good cement bond log result, across injection zone and confinement zone was well recognized. A slurry system fit-for-purpose was designed for these wells, with excellent fluid loss control and good strength development. Mud removal techniques were optimized by using adequate spacer volume and rheology, proper centralization, optimized pumping rate, and pipe reciprocation. * Schlumberger ** SPE Moreover, the wells were shallow (1000 m or less) and vertical, hence considered less challenging in term of well cleaning. However, despite of the application of good cementing practices and excellent job execution, it was found that the sonic cement bond log result of the first two water disposal wells were considered poor to fair by having high CBL amplitudes and strong casing arrivals on the VDL map of the bond logs . At the same time, it was also observed that after 12 months of injection, neither well showed any sign of fluid migration or annular casing pressure. This paper presents engineering optimization workflow and integrated evaluation process to improve sonic cement bond log results of water disposal wells in Sumatra. The evaluation process includes: analysis of possible factors affecting hydraulic isolation, analysis of factors influencing cement bond log tool response with stress modeling, and verification by field applications. The optimization result was applied to three water disposal wells and the cement bond log result of those wells was excellent. At present day, the water disposal wells have been injected into with no indication of fluid communication in the annulus and no annulus pressure build up.