Accurate Horizontal Well Placement in Waterflooded Field's Drilling Project: A Case Study from Central Sumatra Basin, Indonesia

Abstract

Central Sumatra Basin is recognized as one of the most important lacustrine oil systems in Southeast Asia. Marine sandstones, are the principal reservoirs of this system, in production since 1950s using their water drive nature as the primary recovery mechanism. However, the reservoir pressure has declined over the years, reaching the limit of the aquifer expansion into the reservoir and impacting production. Series of enhancements through secondary recovery mechanisms has been applied, one of which is waterflooding project. High water content resulting from water-injection has compelled the optimization of production from selective sand intervals. Therefore, implementing geosteering services with horizontal well designs has been chosen as the most effective subsurface strategies for identifying and placing the trajectory in unswept areas. This paper analyses the planning, execution, and evaluation of horizontal well geosteered in the Central Sumatra Basin's onshore infill. Field- DI4A2F has been producing from 6 major sand bodies throughout the years. Subsurface simulation determined that specifically targeting the 2nd lobe of upper-sand should result with initial target of 14 ft net pay of clean oil. Pre-well modeling and drilling simulations play a significant role in determining the main strategy to reach the desired sand interval in this waterflooded low-resistivity-low-contrast environment. During the execution of Well-A, various geological and drilling challenges have been surpassed. As the start of the 12.25 in. landing section, the obstacle of well-pad construction had propelled a complex trajectory, turning from 200deg towards 300deg of Azimuth in the span of +/- 1600 ft MD whilst increasing inclination from 0 to 50deg., effective drilling parameters and practices allowed to achieve the planned well trajectory, and to land with 87deg Inclination at the bottom of the shale formation, immediately above the the targeted sand. Such well design minimizes the length of open hole in known unstable shale. During Drilling the lateral 8.5 in. section required multiple steering adjustments to maintain the hole within the targeted sand layer. The constant well path adjustments were due to a combination of geological changes and a natural dropping tendency of the Bottom-Hole Assembly (BHA) in such soft formation. By using geosteering technologies, the well was successfully placed, with 89deg of inclination while maintaining 600ft length of lateral inside the 8ft TVT upper-sand sweetspot zone. Smooth drilling was accomplished by optimizing the trajectory placement within 8 feet below and 1 feet left from the initial well plan. Target well was successfully placed in the thinnest reservoir layer in Field- DI4A2F, with the indication of low water cut result. The accuracy of the geosteering performance and pro-active directional drilling action has increased confidence in the growth of this basin's production. This success, in both operation and safety, has pioneered future development planning on infill wells to improve waterflooded fields drilling project.