Post-Placement Crossflow was Prevented in 20 Wells by Using a New Approach

Across the West African region, 40 wells were drilled through multiple potential flow zones. Half of the wells were cemented using conventional slurries that met American Petroleum Institute (API) Standard 65-2 requirements; 18 of these wells developed zonal isolation issues, resulting in water production as high as 90% in certain wells. The remaining 20 wells were cemented using a proprietary slurry system with a shortened time to the transition period (the time between potential fluid influx to no fluid influx into setting cement) within 10 minutes of placement. All wells achieved outstanding cement bond log results with less than 1% water production. This paper describes how the short time to transition approach was used to successfully prevent post-placement crossflow. Initially, a conventional class G cement was deployed containing additives to meet the requirements of API Standard 65-2. However, cement evaluation mapping showed localized fluid invasion channels, suggesting post-placement crossflow, which was subsequently confirmed based on excessive water production of up to 90% in some wells. External casing packers and 2-stage cementing were contemplated but increased equivalent circulation density, leading to potential losses, and logistics were prohibitive. To improve the resilience of the cement against crossflow, a proprietary additive was incorporated into the design to reach the commencement of the transition period within 10 minutes of cement job cessation. This characteristic was verified in a laboratory using a static gel strength analyzer. The 40 wells selected were drilled with their cemented intervals crossing multiple formation layers containing water, oil, or gas at different pressures. For each project, the cement slurry was batch-mixed and pumped with a two-plug wiper plug configuration. In Congo, the operator did not permit slurry design modification beyond API Standard 65-2 requirements. As a result, all three wells cemented initially produced about 90% water from the outset. In Mozambique, two wells with very shallow gas layers were identified. One well was cemented using the novel approach and showed better isolation compared to the well cemented with conventional cement slurry. The remaining 35 wells were cemented in Gabon, of which 19 used the novel cement approach and yielded excellent cement bond logs with no crossflow. Cementing a section through interbedded zones containing different fluids can be challenging. Crossflow can occur when the cement column starts setting, thus transmitting less hydrostatic pressure on the formation. The phenomenon can result in formation fluid invasion into the cement matrix, compromising zonal isolation. Only aiming to meet API Standard 65-2 requirements might be insufficient. By reducing the time to the start of the transition period to 10 minutes or less, the resulting slurry was found to be effective in preventing crossflow in 20 wells.